diff --git a/Docs/SettingUpPSQL.md b/Docs/SettingUpPSQL.md
index c83ddb662..dfa9989c6 100644
--- a/Docs/SettingUpPSQL.md
+++ b/Docs/SettingUpPSQL.md
@@ -10,26 +10,37 @@ Then activate following extensions for this database (right click > Create > Ext
 - Postgis activeren (rechtsklikken > Create > extension)
 - HStore activeren
 
-Install osm2pgsql (hint: compile from source is painless)
+Increase the max number of connections. osm2pgsql needs connection one per table (and a few more), and since we are making one table per layer in MapComplete, this amounts to a lot.
 
-pg_tileserv kan hier gedownload worden: https://github.com/CrunchyData/pg_tileserv
-
-DATABASE_URL=postgresql://user:none@localhost:5444/osm-poi ./pg_tileserv 
+- Open PGAdmin, open the PGSQL-tool (CLI-button at the top)
+- Run `max_connections = 2000;` and `show config_file;` to get the config file location (in docker). This is probably `/var/lib/postgresql/data/postgresql.conf`
+- In a terminal, run `sudo docker exec -i <docker-container-id> bash` (run `sudo docker ps` to get the container id)
+- `sed -i "s/max_connections = 100/max_connections = 5000/" /var/lib/postgresql/data/postgresql.conf`
+- Validate with `cat /var/lib/postgresql/data/postgresql.conf | grep "max_connections"`
+- `sudo docker restart <ID>`
 
 ## Create export scripts for every layer
 
-Use scripts/osm2pgsl
+Use `vite-node ./scripts/osm2pgsql/generateBuildDbScript.ts`
 
 ## Importing data
 
+Install osm2pgsql (hint: compile from source is painless)
 To seed the database:
 
 ````
-osm2pgsql -O flex -E 4326 -S build_db.lua -s  --flat-nodes=import-help-file -d postgresql://user:password@localhost:5444/osm-poi andorra-latest.osm.pbf 
+osm2pgsql -O flex -E 4326 -S build_db.lua -s  --flat-nodes=import-help-file -d postgresql://user:password@localhost:5444/osm-poi <file>.osm.pbf 
 ````
+Storing properties to table '"public"."osm2pgsql_properties" takes about 25 minutes with planet.osm
+
+Belgium (~555mb) takes 15m
+World (80GB) should take 15m*160 = 2400m = 40hr
+
 
 ## Deploying a tile server
 
+pg_tileserv kan hier gedownload worden: https://github.com/CrunchyData/pg_tileserv
+
 ````
 export DATABASE_URL=postgresql://user:password@localhost:5444/osm-poi
 ./pg_tileserv
diff --git a/scripts/Script.ts b/scripts/Script.ts
index 48a727b22..25b0b14e7 100644
--- a/scripts/Script.ts
+++ b/scripts/Script.ts
@@ -13,8 +13,15 @@ export default abstract class Script {
         ScriptUtils.fixUtils()
         const args = [...process.argv]
         args.splice(0, 2)
+        const start = new Date()
         this.main(args)
-            .then((_) => console.log("All done"))
+            .then((_) =>{
+                const end = new Date()
+                const millisNeeded = end.getTime() - start.getTime()
+
+                const green = (s) => "\x1b[92m" + s + "\x1b[0m"
+                console.log(green("All done! (" + millisNeeded + " ms)"))
+            })
             .catch((e) => console.log("ERROR:", e))
     }
 
diff --git a/scripts/generateLayerOverview.ts b/scripts/generateLayerOverview.ts
index 39e8c6424..f60f96d10 100644
--- a/scripts/generateLayerOverview.ts
+++ b/scripts/generateLayerOverview.ts
@@ -9,7 +9,7 @@ import {
     DoesImageExist,
     PrevalidateTheme,
     ValidateLayer,
-    ValidateThemeAndLayers,
+    ValidateThemeAndLayers, ValidateThemeEnsemble,
 } from "../src/Models/ThemeConfig/Conversion/Validation"
 import { Translation } from "../src/UI/i18n/Translation"
 import { PrepareLayer } from "../src/Models/ThemeConfig/Conversion/PrepareLayer"
@@ -25,6 +25,8 @@ import LayerConfig from "../src/Models/ThemeConfig/LayerConfig"
 import PointRenderingConfig from "../src/Models/ThemeConfig/PointRenderingConfig"
 import { ConversionContext } from "../src/Models/ThemeConfig/Conversion/ConversionContext"
 import { GenerateFavouritesLayer } from "./generateFavouritesLayer"
+import LayoutConfig from "../src/Models/ThemeConfig/LayoutConfig"
+import { TagsFilter } from "../src/Logic/Tags/TagsFilter"
 
 // This scripts scans 'src/assets/layers/*.json' for layer definition files and 'src/assets/themes/*.json' for theme definition files.
 // It spits out an overview of those to be used to load them
@@ -123,6 +125,7 @@ class AddIconSummary extends DesugaringStep<{ raw: LayerConfigJson; parsed: Laye
     }
 }
 
+
 class LayerOverviewUtils extends Script {
     public static readonly layerPath = "./src/assets/generated/layers/"
     public static readonly themePath = "./src/assets/generated/themes/"
@@ -355,7 +358,6 @@ class LayerOverviewUtils extends Script {
         const layerWhitelist = new Set(args.find(a => a.startsWith("--layers="))
             ?.substring("--layers=".length)?.split(",") ?? [])
 
-        const start = new Date()
         const forceReload = args.some((a) => a == "--force")
 
         const licensePaths = new Set<string>()
@@ -382,17 +384,21 @@ class LayerOverviewUtils extends Script {
             sharedLayers,
             recompiledThemes,
             forceReload,
-            themeWhitelist
+            themeWhitelist,
         )
 
-        if (recompiledThemes.length > 0){
+        new ValidateThemeEnsemble().convertStrict(
+            Array.from(sharedThemes.values()).map(th => new LayoutConfig(th, true)))
+
+
+        if (recompiledThemes.length > 0) {
             writeFileSync(
                 "./src/assets/generated/known_layers.json",
                 JSON.stringify({
                     layers: Array.from(sharedLayers.values()).filter((l) => l.id !== "favourite"),
                 }),
             )
-         }
+        }
 
         const mcChangesPath = "./assets/themes/mapcomplete-changes/mapcomplete-changes.json"
         if (
@@ -437,7 +443,7 @@ class LayerOverviewUtils extends Script {
             )
         }
 
-        if(recompiledThemes.length > 0) {
+        if (recompiledThemes.length > 0) {
             writeFileSync(
                 "./src/assets/generated/known_themes.json",
                 JSON.stringify({
@@ -446,17 +452,10 @@ class LayerOverviewUtils extends Script {
             )
         }
 
-        const end = new Date()
-        const millisNeeded = end.getTime() - start.getTime()
         if (AllSharedLayers.getSharedLayersConfigs().size == 0) {
             console.error(
-                "This was a bootstrapping-run. Run generate layeroverview again!(" +
-                millisNeeded +
-                " ms)",
+                "This was a bootstrapping-run. Run generate layeroverview again!"
             )
-        } else {
-            const green = (s) => "\x1b[92m" + s + "\x1b[0m"
-            console.log(green("All done! (" + millisNeeded + " ms)"))
         }
     }
 
@@ -482,7 +481,7 @@ class LayerOverviewUtils extends Script {
     private buildLayerIndex(
         doesImageExist: DoesImageExist,
         forceReload: boolean,
-        whitelist: Set<string>
+        whitelist: Set<string>,
     ): Map<string, LayerConfigJson> {
         // First, we expand and validate all builtin layers. These are written to src/assets/generated/layers
         // At the same time, an index of available layers is built.
@@ -500,9 +499,9 @@ class LayerOverviewUtils extends Script {
         const recompiledLayers: string[] = []
         let warningCount = 0
         for (const sharedLayerPath of ScriptUtils.getLayerPaths()) {
-            if(whitelist.size > 0){
-            const idByPath = sharedLayerPath.split("/").at(-1).split(".")[0]
-                if(Constants.priviliged_layers.indexOf(<any> idByPath) < 0 && !whitelist.has(idByPath)){
+            if (whitelist.size > 0) {
+                const idByPath = sharedLayerPath.split("/").at(-1).split(".")[0]
+                if (Constants.priviliged_layers.indexOf(<any>idByPath) < 0 && !whitelist.has(idByPath)) {
                     continue
                 }
             }
@@ -672,7 +671,7 @@ class LayerOverviewUtils extends Script {
         sharedLayers: Map<string, LayerConfigJson>,
         recompiledThemes: string[],
         forceReload: boolean,
-        whitelist: Set<string>
+        whitelist: Set<string>,
     ): Map<string, LayoutConfigJson> {
         console.log("   ---------- VALIDATING BUILTIN THEMES ---------")
         const themeFiles = ScriptUtils.getThemeFiles()
@@ -710,7 +709,7 @@ class LayerOverviewUtils extends Script {
             const themeInfo = themeFiles[i]
             const themePath = themeInfo.path
             let themeFile = themeInfo.parsed
-            if(whitelist.size > 0 && !whitelist.has(themeFile.id)){
+            if (whitelist.size > 0 && !whitelist.has(themeFile.id)) {
                 continue
             }
 
@@ -795,21 +794,21 @@ class LayerOverviewUtils extends Script {
             }
         }
 
-        if(whitelist.size == 0){
-        this.writeSmallOverview(
-            Array.from(fixed.values()).map((t) => {
-                return {
-                    ...t,
-                    hideFromOverview: t.hideFromOverview ?? false,
-                    shortDescription:
-                        t.shortDescription ??
-                        new Translation(t.description)
-                            .FirstSentence()
-                            .OnEveryLanguage((s) => parse_html(s).textContent).translations,
-                    mustHaveLanguage: t.mustHaveLanguage?.length > 0,
-                }
-            }),
-        )
+        if (whitelist.size == 0) {
+            this.writeSmallOverview(
+                Array.from(fixed.values()).map((t) => {
+                    return {
+                        ...t,
+                        hideFromOverview: t.hideFromOverview ?? false,
+                        shortDescription:
+                            t.shortDescription ??
+                            new Translation(t.description)
+                                .FirstSentence()
+                                .OnEveryLanguage((s) => parse_html(s).textContent).translations,
+                        mustHaveLanguage: t.mustHaveLanguage?.length > 0,
+                    }
+                }),
+            )
         }
 
         console.log(
diff --git a/scripts/osm2pgsql/generateBuildDbScript.ts b/scripts/osm2pgsql/generateBuildDbScript.ts
index a2c9363f2..0fce50b06 100644
--- a/scripts/osm2pgsql/generateBuildDbScript.ts
+++ b/scripts/osm2pgsql/generateBuildDbScript.ts
@@ -1,13 +1,13 @@
-import LayerConfig from "../../src/Models/ThemeConfig/LayerConfig"
 import { TagsFilter } from "../../src/Logic/Tags/TagsFilter"
 import { Tag } from "../../src/Logic/Tags/Tag"
 import { And } from "../../src/Logic/Tags/And"
 import Script from "../Script"
-import { AllSharedLayers } from "../../src/Customizations/AllSharedLayers"
 import fs from "fs"
 import { Or } from "../../src/Logic/Tags/Or"
 import { RegexTag } from "../../src/Logic/Tags/RegexTag"
 import { Utils } from "../../src/Utils"
+import { ValidateThemeEnsemble } from "../../src/Models/ThemeConfig/Conversion/Validation"
+import { AllKnownLayouts } from "../../src/Customizations/AllKnownLayouts"
 
 class LuaSnippets {
     /**
@@ -35,28 +35,31 @@ class LuaSnippets {
 }
 
 class GenerateLayerLua {
-    private readonly _layer: LayerConfig
+    private readonly _id: string
+    private readonly _tags: TagsFilter
+    private readonly _foundInThemes: string[]
 
-    constructor(layer: LayerConfig) {
-        this._layer = layer
+    constructor(id: string, tags: TagsFilter, foundInThemes: string[] = []) {
+        this._tags = tags
+        this._id = id
+        this._foundInThemes = foundInThemes
     }
 
     public functionName() {
-        const l = this._layer
-        if (!l.source?.osmTags) {
+        if (!this._tags) {
             return undefined
         }
-        return `process_poi_${l.id}`
+        return `process_poi_${this._id}`
     }
 
     public generateFunction(): string {
-        const l = this._layer
-        if (!l.source?.osmTags) {
+        if (!this._tags) {
             return undefined
         }
         return [
-            `local pois_${l.id} = osm2pgsql.define_table({`,
-            `  name = '${l.id}',`,
+            `local pois_${this._id} = osm2pgsql.define_table({`,
+            this._foundInThemes ? "-- used in themes: " + this._foundInThemes.join(", ") : "",
+            `  name = '${this._id}',`,
             "  ids = { type = 'any', type_column = 'osm_type', id_column = 'osm_id' },",
             "  columns = {",
             "    { column = 'tags', type = 'jsonb' },",
@@ -66,7 +69,7 @@ class GenerateLayerLua {
             "",
             "",
             `function ${this.functionName()}(object, geom)`,
-            "  local matches_filter = " + this.toLuaFilter(l.source.osmTags),
+            "  local matches_filter = " + this.toLuaFilter(this._tags),
             "  if( not matches_filter) then",
             "    return",
             "  end",
@@ -75,7 +78,7 @@ class GenerateLayerLua {
             "    tags = object.tags",
             "  }",
             "  ",
-            `  pois_${l.id}:insert(a)`,
+            `  pois_${this._id}:insert(a)`,
             "end",
             "",
         ].join("\n")
@@ -86,6 +89,8 @@ class GenerateLayerLua {
             return `object.tags["${tag.key}"] ~= "${tag.value}"`
         }
 
+        const v = (<RegExp> tag.value).source.replace(/\\\//g, "/")
+
         if ("" + tag.value === "/.+/is" && !tag.invert) {
             return `object.tags["${tag.key}"] ~= nil`
         }
@@ -104,10 +109,10 @@ class GenerateLayerLua {
         }
 
         if (tag.invert) {
-            return `object.tags["${tag.key}"] == nil or not string.find(object.tags["${tag.key}"], "${tag.value}")`
+            return `object.tags["${tag.key}"] == nil or not string.find(object.tags["${tag.key}"], "${v}")`
         }
 
-        return `(object.tags["${tag.key}"] ~= nil and string.find(object.tags["${tag.key}"], "${tag.value}"))`
+        return `(object.tags["${tag.key}"] ~= nil and string.find(object.tags["${tag.key}"], "${v}"))`
     }
 
     private toLuaFilter(tag: TagsFilter, useParens: boolean = false): string {
@@ -141,15 +146,21 @@ class GenerateLayerLua {
     }
 }
 
-class GenerateLayerFile extends Script {
+class GenerateBuildDbScript extends Script {
     constructor() {
         super("Generates a .lua-file to use with osm2pgsql")
     }
 
     async main(args: string[]) {
-        const layers = Array.from(AllSharedLayers.sharedLayers.values())
+        const allNeededLayers = new ValidateThemeEnsemble().convertStrict(
+            AllKnownLayouts.allKnownLayouts.values(),
+        )
 
-        const generators = layers.filter(l => l.source.geojsonSource === undefined).map(l => new GenerateLayerLua(l))
+        const generators: GenerateLayerLua[] = []
+
+        allNeededLayers.forEach(({ tags, foundInTheme }, layerId) => {
+            generators.push(new GenerateLayerLua(layerId, tags, foundInTheme))
+        })
 
         const script = [
             ...generators.map(g => g.generateFunction()),
@@ -159,7 +170,8 @@ class GenerateLayerFile extends Script {
         const path = "build_db.lua"
         fs.writeFileSync(path, script, "utf-8")
         console.log("Written", path)
+        console.log(allNeededLayers.size+" layers will be created. Make sure to set 'max_connections' to at least  "+(10 + allNeededLayers.size) )
     }
 }
 
-new GenerateLayerFile().run()
+new GenerateBuildDbScript().run()
diff --git a/src/Models/ThemeConfig/Conversion/Validation.ts b/src/Models/ThemeConfig/Conversion/Validation.ts
index ed4c79098..6380df2e5 100644
--- a/src/Models/ThemeConfig/Conversion/Validation.ts
+++ b/src/Models/ThemeConfig/Conversion/Validation.ts
@@ -21,9 +21,7 @@ import PresetConfig from "../PresetConfig"
 import { TagsFilter } from "../../../Logic/Tags/TagsFilter"
 import { Translatable } from "../Json/Translatable"
 import { ConversionContext } from "./ConversionContext"
-import * as eli from "../../../assets/editor-layer-index.json"
 import { AvailableRasterLayers } from "../../RasterLayers"
-import Back from "../../../assets/svg/Back.svelte"
 import PointRenderingConfigJson from "../Json/PointRenderingConfigJson"
 
 class ValidateLanguageCompleteness extends DesugaringStep<LayoutConfig> {
@@ -33,7 +31,7 @@ class ValidateLanguageCompleteness extends DesugaringStep<LayoutConfig> {
         super(
             "Checks that the given object is fully translated in the specified languages",
             [],
-            "ValidateLanguageCompleteness"
+            "ValidateLanguageCompleteness",
         )
         this._languages = languages ?? ["en"]
     }
@@ -47,18 +45,18 @@ class ValidateLanguageCompleteness extends DesugaringStep<LayoutConfig> {
                 .filter(
                     (t) =>
                         t.tr.translations[neededLanguage] === undefined &&
-                        t.tr.translations["*"] === undefined
+                        t.tr.translations["*"] === undefined,
                 )
                 .forEach((missing) => {
                     context
                         .enter(missing.context.split("."))
                         .err(
                             `The theme ${obj.id} should be translation-complete for ` +
-                                neededLanguage +
-                                ", but it lacks a translation for " +
-                                missing.context +
-                                ".\n\tThe known translation is " +
-                                missing.tr.textFor("en")
+                            neededLanguage +
+                            ", but it lacks a translation for " +
+                            missing.context +
+                            ".\n\tThe known translation is " +
+                            missing.tr.textFor("en"),
                         )
                 })
         }
@@ -75,7 +73,7 @@ export class DoesImageExist extends DesugaringStep<string> {
     constructor(
         knownImagePaths: Set<string>,
         checkExistsSync: (path: string) => boolean = undefined,
-        ignore?: Set<string>
+        ignore?: Set<string>,
     ) {
         super("Checks if an image exists", [], "DoesImageExist")
         this._ignore = ignore
@@ -111,15 +109,15 @@ export class DoesImageExist extends DesugaringStep<string> {
         if (!this._knownImagePaths.has(image)) {
             if (this.doesPathExist === undefined) {
                 context.err(
-                    `Image with path ${image} not found or not attributed; it is used in ${context}`
+                    `Image with path ${image} not found or not attributed; it is used in ${context}`,
                 )
             } else if (!this.doesPathExist(image)) {
                 context.err(
-                    `Image with path ${image} does not exist.\n     Check for typo's and missing directories in the path.`
+                    `Image with path ${image} does not exist.\n     Check for typo's and missing directories in the path.`,
                 )
             } else {
                 context.err(
-                    `Image with path ${image} is not attributed (but it exists); execute 'npm run query:licenses' to add the license information and/or run 'npm run generate:licenses' to compile all the license info`
+                    `Image with path ${image} is not attributed (but it exists); execute 'npm run query:licenses' to add the license information and/or run 'npm run generate:licenses' to compile all the license info`,
                 )
             }
         }
@@ -143,7 +141,7 @@ export class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
         doesImageExist: DoesImageExist,
         path: string,
         isBuiltin: boolean,
-        sharedTagRenderings?: Set<string>
+        sharedTagRenderings?: Set<string>,
     ) {
         super("Doesn't change anything, but emits warnings and errors", [], "ValidateTheme")
         this._validateImage = doesImageExist
@@ -162,15 +160,15 @@ export class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
                 if (json["units"] !== undefined) {
                     context.err(
                         "The theme " +
-                            json.id +
-                            " has units defined - these should be defined on the layer instead. (Hint: use overrideAll: { '+units': ... }) "
+                        json.id +
+                        " has units defined - these should be defined on the layer instead. (Hint: use overrideAll: { '+units': ... }) ",
                     )
                 }
                 if (json["roamingRenderings"] !== undefined) {
                     context.err(
                         "Theme " +
-                            json.id +
-                            " contains an old 'roamingRenderings'. Use an 'overrideAll' instead"
+                        json.id +
+                        " contains an old 'roamingRenderings'. Use an 'overrideAll' instead",
                     )
                 }
             }
@@ -178,7 +176,7 @@ export class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
         if (!json.title) {
             context.enter("title").err(`The theme ${json.id} does not have a title defined.`)
         }
-        if(!json.icon){
+        if (!json.icon) {
             context.enter("icon").err("A theme should have an icon")
         }
         if (this._isBuiltin && this._extractImages !== undefined) {
@@ -188,10 +186,10 @@ export class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
             for (const remoteImage of remoteImages) {
                 context.err(
                     "Found a remote image: " +
-                        remoteImage.path +
-                        " in theme " +
-                        json.id +
-                        ", please download it."
+                    remoteImage.path +
+                    " in theme " +
+                    json.id +
+                    ", please download it.",
                 )
             }
             for (const image of images) {
@@ -207,17 +205,17 @@ export class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
 
                 const filename = this._path.substring(
                     this._path.lastIndexOf("/") + 1,
-                    this._path.length - 5
+                    this._path.length - 5,
                 )
                 if (theme.id !== filename) {
                     context.err(
                         "Theme ids should be the same as the name.json, but we got id: " +
-                            theme.id +
-                            " and filename " +
-                            filename +
-                            " (" +
-                            this._path +
-                            ")"
+                        theme.id +
+                        " and filename " +
+                        filename +
+                        " (" +
+                        this._path +
+                        ")",
                     )
                 }
                 this._validateImage.convert(theme.icon, context.enter("icon"))
@@ -225,13 +223,13 @@ export class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
             const dups = Utils.Duplicates(json.layers.map((layer) => layer["id"]))
             if (dups.length > 0) {
                 context.err(
-                    `The theme ${json.id} defines multiple layers with id ${dups.join(", ")}`
+                    `The theme ${json.id} defines multiple layers with id ${dups.join(", ")}`,
                 )
             }
             if (json["mustHaveLanguage"] !== undefined) {
                 new ValidateLanguageCompleteness(...json["mustHaveLanguage"]).convert(
                     theme,
-                    context
+                    context,
                 )
             }
             if (!json.hideFromOverview && theme.id !== "personal" && this._isBuiltin) {
@@ -239,7 +237,7 @@ export class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
                 const targetLanguage = theme.title.SupportedLanguages()[0]
                 if (targetLanguage !== "en") {
                     context.err(
-                        `TargetLanguage is not 'en' for public theme ${theme.id}, it is ${targetLanguage}. Move 'en' up in the title of the theme and set it as the first key`
+                        `TargetLanguage is not 'en' for public theme ${theme.id}, it is ${targetLanguage}. Move 'en' up in the title of the theme and set it as the first key`,
                     )
                 }
 
@@ -282,6 +280,13 @@ export class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
             }
         }
 
+        for (let i = 0; i < theme.layers.length; i++) {
+            const layer = theme.layers[i]
+            if (!layer.id.match("[a-z][a-z0-9_]*")) {
+                context.enters("layers", i, "id").err("Invalid ID:" + layer.id + "should match [a-z][a-z0-9_]*")
+            }
+        }
+
         return json
     }
 }
@@ -291,7 +296,7 @@ export class ValidateThemeAndLayers extends Fuse<LayoutConfigJson> {
         doesImageExist: DoesImageExist,
         path: string,
         isBuiltin: boolean,
-        sharedTagRenderings?: Set<string>
+        sharedTagRenderings?: Set<string>,
     ) {
         super(
             "Validates a theme and the contained layers",
@@ -301,10 +306,10 @@ export class ValidateThemeAndLayers extends Fuse<LayoutConfigJson> {
                 new Each(
                     new Bypass(
                         (layer) => Constants.added_by_default.indexOf(<any>layer.id) < 0,
-                        new ValidateLayerConfig(undefined, isBuiltin, doesImageExist, false, true)
-                    )
-                )
-            )
+                        new ValidateLayerConfig(undefined, isBuiltin, doesImageExist, false, true),
+                    ),
+                ),
+            ),
         )
     }
 }
@@ -314,7 +319,7 @@ class OverrideShadowingCheck extends DesugaringStep<LayoutConfigJson> {
         super(
             "Checks that an 'overrideAll' does not override a single override",
             [],
-            "OverrideShadowingCheck"
+            "OverrideShadowingCheck",
         )
     }
 
@@ -363,6 +368,22 @@ class MiscThemeChecks extends DesugaringStep<LayoutConfigJson> {
         if (json.socialImage === "") {
             context.warn("Social image for theme " + json.id + " is the emtpy string")
         }
+        {
+            for (let i = 0; i < json.layers.length; i++) {
+                const l = json.layers[i]
+                if (l["override"]?.["source"] === undefined) {
+                    continue
+                }
+                if (l["override"]?.["source"]?.["geoJson"]) {
+                    continue // We don't care about external data as we won't cache it anyway
+                }
+                if (l["override"]["id"] !== undefined) {
+                    continue
+                }
+                context.enters("layers", i).err("A layer which changes the source-tags must also change the ID")
+            }
+        }
+
         return json
     }
 }
@@ -372,7 +393,7 @@ export class PrevalidateTheme extends Fuse<LayoutConfigJson> {
         super(
             "Various consistency checks on the raw JSON",
             new MiscThemeChecks(),
-            new OverrideShadowingCheck()
+            new OverrideShadowingCheck(),
         )
     }
 }
@@ -382,7 +403,7 @@ export class DetectConflictingAddExtraTags extends DesugaringStep<TagRenderingCo
         super(
             "The `if`-part in a mapping might set some keys. Those keys are not allowed to be set in the `addExtraTags`, as this might result in conflicting values",
             [],
-            "DetectConflictingAddExtraTags"
+            "DetectConflictingAddExtraTags",
         )
     }
 
@@ -409,7 +430,7 @@ export class DetectConflictingAddExtraTags extends DesugaringStep<TagRenderingCo
                         .enters("mappings", i)
                         .err(
                             "AddExtraTags overrides a key that is set in the `if`-clause of this mapping. Selecting this answer might thus first set one value (needed to match as answer) and then override it with a different value, resulting in an unsaveable question. The offending `addExtraTags` is " +
-                                duplicateKeys.join(", ")
+                            duplicateKeys.join(", "),
                         )
                 }
             }
@@ -427,13 +448,13 @@ export class DetectNonErasedKeysInMappings extends DesugaringStep<QuestionableTa
         super(
             "A tagRendering might set a freeform key (e.g. `name` and have an option that _should_ erase this name, e.g. `noname=yes`). Under normal circumstances, every mapping/freeform should affect all touched keys",
             [],
-            "DetectNonErasedKeysInMappings"
+            "DetectNonErasedKeysInMappings",
         )
     }
 
     convert(
         json: QuestionableTagRenderingConfigJson,
-        context: ConversionContext
+        context: ConversionContext,
     ): QuestionableTagRenderingConfigJson {
         if (json.multiAnswer) {
             // No need to check this here, this has its own validation
@@ -487,8 +508,8 @@ export class DetectNonErasedKeysInMappings extends DesugaringStep<QuestionableTa
                         .enters("freeform")
                         .warn(
                             "The freeform block does not modify the key `" +
-                                neededKey +
-                                "` which is set in a mapping. Use `addExtraTags` to overwrite it"
+                            neededKey +
+                            "` which is set in a mapping. Use `addExtraTags` to overwrite it",
                         )
                 }
             }
@@ -506,8 +527,8 @@ export class DetectNonErasedKeysInMappings extends DesugaringStep<QuestionableTa
                         .enters("mappings", i)
                         .warn(
                             "This mapping does not modify the key `" +
-                                neededKey +
-                                "` which is set in a mapping or by the freeform block. Use `addExtraTags` to overwrite it"
+                            neededKey +
+                            "` which is set in a mapping or by the freeform block. Use `addExtraTags` to overwrite it",
                         )
                 }
             }
@@ -531,7 +552,7 @@ export class DetectShadowedMappings extends DesugaringStep<TagRenderingConfigJso
      * DetectShadowedMappings.extractCalculatedTagNames({calculatedTags: ["_abc=js()"]}) // => ["_abc"]
      */
     private static extractCalculatedTagNames(
-        layerConfig?: LayerConfigJson | { calculatedTags: string[] }
+        layerConfig?: LayerConfigJson | { calculatedTags: string[] },
     ) {
         return (
             layerConfig?.calculatedTags?.map((ct) => {
@@ -617,16 +638,16 @@ export class DetectShadowedMappings extends DesugaringStep<TagRenderingConfigJso
                     json.mappings[i]["hideInAnswer"] !== true
                 ) {
                     context.warn(
-                        `Mapping ${i} is shadowed by mapping ${j}. However, mapping ${j} has 'hideInAnswer' set, which will result in a different rendering in question-mode.`
+                        `Mapping ${i} is shadowed by mapping ${j}. However, mapping ${j} has 'hideInAnswer' set, which will result in a different rendering in question-mode.`,
                     )
                 } else if (doesMatch) {
                     // The current mapping is shadowed!
                     context.err(`Mapping ${i} is shadowed by mapping ${j} and will thus never be shown:
     The mapping ${parsedConditions[i].asHumanString(
-        false,
-        false,
-        {}
-    )} is fully matched by a previous mapping (namely ${j}), which matches:
+                        false,
+                        false,
+                        {},
+                    )} is fully matched by a previous mapping (namely ${j}), which matches:
     ${parsedConditions[j].asHumanString(false, false, {})}.
 
     To fix this problem, you can try to:
@@ -653,7 +674,7 @@ export class DetectMappingsWithImages extends DesugaringStep<TagRenderingConfigJ
         super(
             "Checks that 'then'clauses in mappings don't have images, but use 'icon' instead",
             [],
-            "DetectMappingsWithImages"
+            "DetectMappingsWithImages",
         )
         this._doesImageExist = doesImageExist
     }
@@ -693,14 +714,14 @@ export class DetectMappingsWithImages extends DesugaringStep<TagRenderingConfigJ
                 if (!ignore) {
                     ctx.err(
                         `A mapping has an image in the 'then'-clause. Remove the image there and use \`"icon": <your-image>\` instead. The images found are ${images.join(
-                            ", "
-                        )}. (This check can be turned of by adding "#": "${ignoreToken}" in the mapping, but this is discouraged`
+                            ", ",
+                        )}. (This check can be turned of by adding "#": "${ignoreToken}" in the mapping, but this is discouraged`,
                     )
                 } else {
                     ctx.info(
                         `Ignored image ${images.join(
-                            ", "
-                        )} in 'then'-clause of a mapping as this check has been disabled`
+                            ", ",
+                        )} in 'then'-clause of a mapping as this check has been disabled`,
                     )
 
                     for (const image of images) {
@@ -721,7 +742,7 @@ class ValidatePossibleLinks extends DesugaringStep<string | Record<string, strin
         super(
             "Given a possible set of translations, validates that <a href=... target='_blank'> does have `rel='noopener'` set",
             [],
-            "ValidatePossibleLinks"
+            "ValidatePossibleLinks",
         )
     }
 
@@ -751,21 +772,21 @@ class ValidatePossibleLinks extends DesugaringStep<string | Record<string, strin
 
     convert(
         json: string | Record<string, string>,
-        context: ConversionContext
+        context: ConversionContext,
     ): string | Record<string, string> {
         if (typeof json === "string") {
             if (this.isTabnabbingProne(json)) {
                 context.err(
                     "The string " +
-                        json +
-                        " has a link targeting `_blank`, but it doesn't have `rel='noopener'` set. This gives rise to reverse tabnapping"
+                    json +
+                    " has a link targeting `_blank`, but it doesn't have `rel='noopener'` set. This gives rise to reverse tabnapping",
                 )
             }
         } else {
             for (const k in json) {
                 if (this.isTabnabbingProne(json[k])) {
                     context.err(
-                        `The translation for ${k} '${json[k]}' has a link targeting \`_blank\`, but it doesn't have \`rel='noopener'\` set. This gives rise to reverse tabnapping`
+                        `The translation for ${k} '${json[k]}' has a link targeting \`_blank\`, but it doesn't have \`rel='noopener'\` set. This gives rise to reverse tabnapping`,
                     )
                 }
             }
@@ -783,7 +804,7 @@ class CheckTranslation extends DesugaringStep<Translatable> {
         super(
             "Checks that a translation is valid and internally consistent",
             ["*"],
-            "CheckTranslation"
+            "CheckTranslation",
         )
         this._allowUndefined = allowUndefined
     }
@@ -829,17 +850,17 @@ class MiscTagRenderingChecks extends DesugaringStep<TagRenderingConfigJson> {
 
     convert(
         json: TagRenderingConfigJson | QuestionableTagRenderingConfigJson,
-        context: ConversionContext
+        context: ConversionContext,
     ): TagRenderingConfigJson {
         if (json["special"] !== undefined) {
             context.err(
-                'Detected `special` on the top level. Did you mean `{"render":{ "special": ... }}`'
+                "Detected `special` on the top level. Did you mean `{\"render\":{ \"special\": ... }}`",
             )
         }
 
         if (Object.keys(json).length === 1 && typeof json["render"] === "string") {
             context.warn(
-                `use the content directly instead of {render: ${JSON.stringify(json["render"])}}`
+                `use the content directly instead of {render: ${JSON.stringify(json["render"])}}`,
             )
         }
 
@@ -851,7 +872,7 @@ class MiscTagRenderingChecks extends DesugaringStep<TagRenderingConfigJson> {
                 const mapping = json.mappings[i]
                 CheckTranslation.noUndefined.convert(
                     mapping.then,
-                    context.enters("mappings", i, "then")
+                    context.enters("mappings", i, "then"),
                 )
                 if (!mapping.if) {
                     context.enters("mappings", i).err("No `if` is defined")
@@ -862,18 +883,18 @@ class MiscTagRenderingChecks extends DesugaringStep<TagRenderingConfigJson> {
                     context
                         .enters("mappings", i, "then")
                         .warn(
-                            "A mapping should not start with 'yes' or 'no'. If the attribute is known, it will only show 'yes' or 'no' <i>without</i> the question, resulting in a weird phrasing in the information box"
+                            "A mapping should not start with 'yes' or 'no'. If the attribute is known, it will only show 'yes' or 'no' <i>without</i> the question, resulting in a weird phrasing in the information box",
                         )
                 }
             }
         }
         if (json["group"]) {
-            context.err('Groups are deprecated, use `"label": ["' + json["group"] + '"]` instead')
+            context.err("Groups are deprecated, use `\"label\": [\"" + json["group"] + "\"]` instead")
         }
 
         if (json["question"] && json.freeform?.key === undefined && json.mappings === undefined) {
             context.err(
-                "A question is defined, but no mappings nor freeform (key) are. Add at least one of them"
+                "A question is defined, but no mappings nor freeform (key) are. Add at least one of them",
             )
         }
         if (json["question"] && !json.freeform && (json.mappings?.length ?? 0) == 1) {
@@ -883,7 +904,7 @@ class MiscTagRenderingChecks extends DesugaringStep<TagRenderingConfigJson> {
             context
                 .enter("questionHint")
                 .err(
-                    "A questionHint is defined, but no question is given. As such, the questionHint will never be shown"
+                    "A questionHint is defined, but no question is given. As such, the questionHint will never be shown",
                 )
         }
 
@@ -893,10 +914,10 @@ class MiscTagRenderingChecks extends DesugaringStep<TagRenderingConfigJson> {
                     .enter("render")
                     .err(
                         "This tagRendering allows to set a value to key " +
-                            json.freeform.key +
-                            ", but does not define a `render`. Please, add a value here which contains `{" +
-                            json.freeform.key +
-                            "}`"
+                        json.freeform.key +
+                        ", but does not define a `render`. Please, add a value here which contains `{" +
+                        json.freeform.key +
+                        "}`",
                     )
             } else {
                 const render = new Translation(<any>json.render)
@@ -927,7 +948,7 @@ class MiscTagRenderingChecks extends DesugaringStep<TagRenderingConfigJson> {
                     const keyFirstArg = ["canonical", "fediverse_link", "translated"]
                     if (
                         keyFirstArg.some(
-                            (funcName) => txt.indexOf(`{${funcName}(${json.freeform.key}`) >= 0
+                            (funcName) => txt.indexOf(`{${funcName}(${json.freeform.key}`) >= 0,
                         )
                     ) {
                         continue
@@ -950,7 +971,7 @@ class MiscTagRenderingChecks extends DesugaringStep<TagRenderingConfigJson> {
                     context
                         .enter("render")
                         .err(
-                            `The rendering for language ${ln} does not contain \`{${json.freeform.key}}\`. This is a bug, as this rendering should show exactly this freeform key!`
+                            `The rendering for language ${ln} does not contain \`{${json.freeform.key}}\`. This is a bug, as this rendering should show exactly this freeform key!`,
                         )
                 }
             }
@@ -958,8 +979,8 @@ class MiscTagRenderingChecks extends DesugaringStep<TagRenderingConfigJson> {
         if (json.render && json["question"] && json.freeform === undefined) {
             context.err(
                 `Detected a tagrendering which takes input without freeform key in ${context}; the question is ${new Translation(
-                    json["question"]
-                ).textFor("en")}`
+                    json["question"],
+                ).textFor("en")}`,
             )
         }
 
@@ -970,9 +991,9 @@ class MiscTagRenderingChecks extends DesugaringStep<TagRenderingConfigJson> {
                     .enters("freeform", "type")
                     .err(
                         "Unknown type: " +
-                            freeformType +
-                            "; try one of " +
-                            Validators.availableTypes.join(", ")
+                        freeformType +
+                        "; try one of " +
+                        Validators.availableTypes.join(", "),
                     )
             }
         }
@@ -1004,7 +1025,7 @@ export class ValidateTagRenderings extends Fuse<TagRenderingConfigJson> {
             new On("question", new ValidatePossibleLinks()),
             new On("questionHint", new ValidatePossibleLinks()),
             new On("mappings", new Each(new On("then", new ValidatePossibleLinks()))),
-            new MiscTagRenderingChecks()
+            new MiscTagRenderingChecks(),
         )
     }
 }
@@ -1034,8 +1055,9 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
             if (json.id?.toLowerCase() !== json.id) {
                 context.enter("id").err(`The id of a layer should be lowercase: ${json.id}`)
             }
-            if (json.id?.match(/[a-z0-9-_]/) == null) {
-                context.enter("id").err(`The id of a layer should match [a-z0-9-_]*: ${json.id}`)
+            const layerRegex = /[a-zA-Z][a-zA-Z_0-9]+/
+            if (json.id.match(layerRegex) === null) {
+                context.enter("id").err("Invalid ID. A layer ID should match " + layerRegex.source)
             }
         }
 
@@ -1043,7 +1065,7 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
             context
                 .enter("source")
                 .err(
-                    "No source section is defined; please define one as data is not loaded otherwise"
+                    "No source section is defined; please define one as data is not loaded otherwise",
                 )
         } else {
             if (json.source === "special" || json.source === "special:library") {
@@ -1051,7 +1073,7 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
                 context
                     .enters("source", "osmTags")
                     .err(
-                        "No osmTags defined in the source section - these should always be present, even for geojson layer"
+                        "No osmTags defined in the source section - these should always be present, even for geojson layer",
                     )
             } else {
                 const osmTags = TagUtils.Tag(json.source["osmTags"], context + "source.osmTags")
@@ -1060,7 +1082,7 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
                         .enters("source", "osmTags")
                         .err(
                             "The source states tags which give a very wide selection: it only uses negative expressions, which will result in too much and unexpected data. Add at least one required tag. The tags are:\n\t" +
-                                osmTags.asHumanString(false, false, {})
+                            osmTags.asHumanString(false, false, {}),
                         )
                 }
             }
@@ -1086,10 +1108,10 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
                 .enter("syncSelection")
                 .err(
                     "Invalid sync-selection: must be one of " +
-                        LayerConfig.syncSelectionAllowed.map((v) => `'${v}'`).join(", ") +
-                        " but got '" +
-                        json.syncSelection +
-                        "'"
+                    LayerConfig.syncSelectionAllowed.map((v) => `'${v}'`).join(", ") +
+                    " but got '" +
+                    json.syncSelection +
+                    "'",
                 )
         }
         if (json["pointRenderings"]?.length > 0) {
@@ -1107,7 +1129,7 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
             context.enter("pointRendering").err("There are no pointRenderings at all...")
         }
 
-        json.pointRendering?.forEach((pr,i) => this._validatePointRendering.convert(pr, context.enters("pointeRendering", i)))
+        json.pointRendering?.forEach((pr, i) => this._validatePointRendering.convert(pr, context.enters("pointeRendering", i)))
 
         if (json["mapRendering"]) {
             context.enter("mapRendering").err("This layer has a legacy 'mapRendering'")
@@ -1123,8 +1145,8 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
             if (!Constants.priviliged_layers.find((x) => x == json.id)) {
                 context.err(
                     "Layer " +
-                        json.id +
-                        " uses 'special' as source.osmTags. However, this layer is not a priviliged layer"
+                    json.id +
+                    " uses 'special' as source.osmTags. However, this layer is not a priviliged layer",
                 )
             }
         }
@@ -1139,19 +1161,19 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
                 context
                     .enter("title")
                     .err(
-                        "This layer does not have a title defined but it does have tagRenderings. Not having a title will disable the popups, resulting in an unclickable element. Please add a title. If not having a popup is intended and the tagrenderings need to be kept (e.g. in a library layer), set `title: null` to disable this error."
+                        "This layer does not have a title defined but it does have tagRenderings. Not having a title will disable the popups, resulting in an unclickable element. Please add a title. If not having a popup is intended and the tagrenderings need to be kept (e.g. in a library layer), set `title: null` to disable this error.",
                     )
             }
             if (json.title === null) {
                 context.info(
-                    "Title is `null`. This results in an element that cannot be clicked - even though tagRenderings is set."
+                    "Title is `null`. This results in an element that cannot be clicked - even though tagRenderings is set.",
                 )
             }
 
             {
                 // Check for multiple, identical builtin questions - usability for studio users
                 const duplicates = Utils.Duplicates(
-                    <string[]>json.tagRenderings.filter((tr) => typeof tr === "string")
+                    <string[]>json.tagRenderings.filter((tr) => typeof tr === "string"),
                 )
                 for (let i = 0; i < json.tagRenderings.length; i++) {
                     const tagRendering = json.tagRenderings[i]
@@ -1181,7 +1203,7 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
         {
             // duplicate ids in tagrenderings check
             const duplicates = Utils.NoNull(
-                Utils.Duplicates(Utils.NoNull((json.tagRenderings ?? []).map((tr) => tr["id"])))
+                Utils.Duplicates(Utils.NoNull((json.tagRenderings ?? []).map((tr) => tr["id"]))),
             )
             if (duplicates.length > 0) {
                 // It is tempting to add an index to this warning; however, due to labels the indices here might be different from the index in the tagRendering list
@@ -1219,8 +1241,8 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
             if (json["overpassTags"] !== undefined) {
                 context.err(
                     "Layer " +
-                        json.id +
-                        'still uses the old \'overpassTags\'-format. Please use "source": {"osmTags": <tags>}\' instead of "overpassTags": <tags> (note: this isn\'t your fault, the custom theme generator still spits out the old format)'
+                    json.id +
+                    "still uses the old 'overpassTags'-format. Please use \"source\": {\"osmTags\": <tags>}' instead of \"overpassTags\": <tags> (note: this isn't your fault, the custom theme generator still spits out the old format)",
                 )
             }
             const forbiddenTopLevel = [
@@ -1240,7 +1262,7 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
             }
             if (json["hideUnderlayingFeaturesMinPercentage"] !== undefined) {
                 context.err(
-                    "Layer " + json.id + " contains an old 'hideUnderlayingFeaturesMinPercentage'"
+                    "Layer " + json.id + " contains an old 'hideUnderlayingFeaturesMinPercentage'",
                 )
             }
 
@@ -1257,9 +1279,9 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
             if (this._path != undefined && this._path.indexOf(expected) < 0) {
                 context.err(
                     "Layer is in an incorrect place. The path is " +
-                        this._path +
-                        ", but expected " +
-                        expected
+                    this._path +
+                    ", but expected " +
+                    expected,
                 )
             }
         }
@@ -1277,13 +1299,13 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
                     .enter(["tagRenderings", ...emptyIndexes])
                     .err(
                         `Some tagrendering-ids are empty or have an emtpy string; this is not allowed (at ${emptyIndexes.join(
-                            ","
-                        )}])`
+                            ",",
+                        )}])`,
                     )
             }
 
             const duplicateIds = Utils.Duplicates(
-                (json.tagRenderings ?? [])?.map((f) => f["id"]).filter((id) => id !== "questions")
+                (json.tagRenderings ?? [])?.map((f) => f["id"]).filter((id) => id !== "questions"),
             )
             if (duplicateIds.length > 0 && !Utils.runningFromConsole) {
                 context
@@ -1307,7 +1329,7 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
         if (json.tagRenderings !== undefined) {
             new On(
                 "tagRenderings",
-                new Each(new ValidateTagRenderings(json, this._doesImageExist))
+                new Each(new ValidateTagRenderings(json, this._doesImageExist)),
             ).convert(json, context)
         }
 
@@ -1334,7 +1356,7 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
                         context
                             .enters("pointRendering", i, "marker", indexM, "icon", "condition")
                             .err(
-                                "Don't set a condition in a marker as this will result in an invisible but clickable element. Use extra filters in the source instead."
+                                "Don't set a condition in a marker as this will result in an invisible but clickable element. Use extra filters in the source instead.",
                             )
                     }
                 }
@@ -1372,9 +1394,9 @@ export class PrevalidateLayer extends DesugaringStep<LayerConfigJson> {
                         .enters("presets", i, "tags")
                         .err(
                             "This preset does not match the required tags of this layer. This implies that a newly added point will not show up.\n    A newly created point will have properties: " +
-                                tags.asHumanString(false, false, {}) +
-                                "\n    The required tags are: " +
-                                baseTags.asHumanString(false, false, {})
+                            tags.asHumanString(false, false, {}) +
+                            "\n    The required tags are: " +
+                            baseTags.asHumanString(false, false, {}),
                         )
                 }
             }
@@ -1391,7 +1413,7 @@ export class ValidateLayerConfig extends DesugaringStep<LayerConfigJson> {
         isBuiltin: boolean,
         doesImageExist: DoesImageExist,
         studioValidations: boolean = false,
-        skipDefaultLayers: boolean = false
+        skipDefaultLayers: boolean = false,
     ) {
         super("Thin wrapper around 'ValidateLayer", [], "ValidateLayerConfig")
         this.validator = new ValidateLayer(
@@ -1399,7 +1421,7 @@ export class ValidateLayerConfig extends DesugaringStep<LayerConfigJson> {
             isBuiltin,
             doesImageExist,
             studioValidations,
-            skipDefaultLayers
+            skipDefaultLayers,
         )
     }
 
@@ -1428,12 +1450,12 @@ class ValidatePointRendering extends DesugaringStep<PointRenderingConfigJson> {
         }
         if (json.marker && !Array.isArray(json.marker)) {
             context.enter("marker").err(
-                "The marker in a pointRendering should be an array"
+                "The marker in a pointRendering should be an array",
             )
         }
         if (json.location.length == 0) {
-            context.enter("location").err (
-                "A pointRendering should have at least one 'location' to defined where it should be rendered. "
+            context.enter("location").err(
+                "A pointRendering should have at least one 'location' to defined where it should be rendered. ",
             )
         }
         return json
@@ -1441,41 +1463,44 @@ class ValidatePointRendering extends DesugaringStep<PointRenderingConfigJson> {
 
     }
 }
+
 export class ValidateLayer extends Conversion<
     LayerConfigJson,
     { parsed: LayerConfig; raw: LayerConfigJson }
 > {
     private readonly _skipDefaultLayers: boolean
     private readonly _prevalidation: PrevalidateLayer
+
     constructor(
         path: string,
         isBuiltin: boolean,
         doesImageExist: DoesImageExist,
         studioValidations: boolean = false,
-        skipDefaultLayers: boolean = false
+        skipDefaultLayers: boolean = false,
     ) {
         super("Doesn't change anything, but emits warnings and errors", [], "ValidateLayer")
         this._prevalidation = new PrevalidateLayer(
             path,
             isBuiltin,
             doesImageExist,
-            studioValidations
+            studioValidations,
         )
         this._skipDefaultLayers = skipDefaultLayers
     }
 
     convert(
         json: LayerConfigJson,
-        context: ConversionContext
+        context: ConversionContext,
     ): { parsed: LayerConfig; raw: LayerConfigJson } {
         context = context.inOperation(this.name)
         if (typeof json === "string") {
             context.err(
-                `Not a valid layer: the layerConfig is a string. 'npm run generate:layeroverview' might be needed`
+                `Not a valid layer: the layerConfig is a string. 'npm run generate:layeroverview' might be needed`,
             )
             return undefined
         }
 
+
         if (this._skipDefaultLayers && Constants.added_by_default.indexOf(<any>json.id) >= 0) {
             return { parsed: undefined, raw: json }
         }
@@ -1502,7 +1527,7 @@ export class ValidateLayer extends Conversion<
                 context
                     .enters("calculatedTags", i)
                     .err(
-                        `Invalid function definition: the custom javascript is invalid:${e}. The offending javascript code is:\n    ${code}`
+                        `Invalid function definition: the custom javascript is invalid:${e}. The offending javascript code is:\n    ${code}`,
                     )
             }
         }
@@ -1553,8 +1578,8 @@ export class ValidateFilter extends DesugaringStep<FilterConfigJson> {
                         .enters("fields", i)
                         .err(
                             `Invalid filter: ${type} is not a valid textfield type.\n\tTry one of ${Array.from(
-                                Validators.availableTypes
-                            ).join(",")}`
+                                Validators.availableTypes,
+                            ).join(",")}`,
                         )
                 }
             }
@@ -1571,13 +1596,13 @@ export class DetectDuplicateFilters extends DesugaringStep<{
         super(
             "Tries to detect layers where a shared filter can be used (or where similar filters occur)",
             [],
-            "DetectDuplicateFilters"
+            "DetectDuplicateFilters",
         )
     }
 
     convert(
         json: { layers: LayerConfigJson[]; themes: LayoutConfigJson[] },
-        context: ConversionContext
+        context: ConversionContext,
     ): { layers: LayerConfigJson[]; themes: LayoutConfigJson[] } {
         const { layers, themes } = json
         const perOsmTag = new Map<
@@ -1641,7 +1666,7 @@ export class DetectDuplicateFilters extends DesugaringStep<{
                 filter: FilterConfigJson
             }[]
         >,
-        layout?: LayoutConfigJson | undefined
+        layout?: LayoutConfigJson | undefined,
     ): void {
         if (layer.filter === undefined || layer.filter === null) {
             return
@@ -1681,7 +1706,7 @@ export class DetectDuplicatePresets extends DesugaringStep<LayoutConfig> {
         super(
             "Detects mappings which have identical (english) names or identical mappings.",
             ["presets"],
-            "DetectDuplicatePresets"
+            "DetectDuplicatePresets",
         )
     }
 
@@ -1692,13 +1717,13 @@ export class DetectDuplicatePresets extends DesugaringStep<LayoutConfig> {
         if (new Set(enNames).size != enNames.length) {
             const dups = Utils.Duplicates(enNames)
             const layersWithDup = json.layers.filter((l) =>
-                l.presets.some((p) => dups.indexOf(p.title.textFor("en")) >= 0)
+                l.presets.some((p) => dups.indexOf(p.title.textFor("en")) >= 0),
             )
             const layerIds = layersWithDup.map((l) => l.id)
             context.err(
                 `This themes has multiple presets which are named:${dups}, namely layers ${layerIds.join(
-                    ", "
-                )} this is confusing for contributors and is probably the result of reusing the same layer multiple times. Use \`{"override": {"=presets": []}}\` to remove some presets`
+                    ", ",
+                )} this is confusing for contributors and is probably the result of reusing the same layer multiple times. Use \`{"override": {"=presets": []}}\` to remove some presets`,
             )
         }
 
@@ -1713,17 +1738,17 @@ export class DetectDuplicatePresets extends DesugaringStep<LayoutConfig> {
                     Utils.SameObject(presetATags, presetBTags) &&
                     Utils.sameList(
                         presetA.preciseInput.snapToLayers,
-                        presetB.preciseInput.snapToLayers
+                        presetB.preciseInput.snapToLayers,
                     )
                 ) {
                     context.err(
                         `This themes has multiple presets with the same tags: ${presetATags.asHumanString(
                             false,
                             false,
-                            {}
+                            {},
                         )}, namely the preset '${presets[i].title.textFor("en")}' and '${presets[
                             j
-                        ].title.textFor("en")}'`
+                            ].title.textFor("en")}'`,
                     )
                 }
             }
@@ -1732,3 +1757,63 @@ export class DetectDuplicatePresets extends DesugaringStep<LayoutConfig> {
         return json
     }
 }
+
+export class ValidateThemeEnsemble extends Conversion<LayoutConfig[], Map<string, {
+    tags: TagsFilter,
+    foundInTheme: string[]
+}>> {
+    constructor() {
+        super("Validates that all themes together are logical, i.e. no duplicate ids exists within (overriden) themes", [], "ValidateThemeEnsemble")
+    }
+
+    convert(json: LayoutConfig[], context: ConversionContext): Map<string, {
+        tags: TagsFilter,
+        foundInTheme: string[]
+    }> {
+
+
+        const idToSource = new Map<string, { tags: TagsFilter, foundInTheme: string[] }>()
+
+        for (const theme of json) {
+            for (const layer of theme.layers) {
+                if (typeof layer.source === "string") {
+                    continue
+                }
+                if (Constants.priviliged_layers.indexOf(<any>layer.id) >= 0) {
+                    continue
+                }
+                if (!layer.source) {
+                    console.log(theme, layer, layer.source)
+                    context.enters(theme.id, "layers", "source", layer.id).err("No source defined")
+                    continue
+                }
+                if (layer.source.geojsonSource) {
+                    continue
+                }
+                const id = layer.id
+                const tags = layer.source.osmTags
+                if (!idToSource.has(id)) {
+                    idToSource.set(id, { tags, foundInTheme: [theme.id] })
+                    continue
+                }
+
+                const oldTags = idToSource.get(id).tags
+                const oldTheme = idToSource.get(id).foundInTheme
+                if (oldTags.shadows(tags) && tags.shadows(oldTags)) {
+                    // All is good, all is well
+                    oldTheme.push(theme.id)
+                    continue
+                }
+                context.err(["The layer with id '" + id + "' is found in multiple themes with different tag definitions:",
+                    "\t In theme " + oldTheme + ":\t" + oldTags.asHumanString(false, false, {}),
+                    "\tIn theme " + theme.id + ":\t" + tags.asHumanString(false, false, {}),
+
+
+                ].join("\n"))
+            }
+        }
+
+
+        return idToSource
+    }
+}