diff --git a/website/src/lib/components/map/layer-control/CustomLayers.svelte b/website/src/lib/components/map/layer-control/CustomLayers.svelte
index d9bb06329..8d9825382 100644
--- a/website/src/lib/components/map/layer-control/CustomLayers.svelte
+++ b/website/src/lib/components/map/layer-control/CustomLayers.svelte
@@ -18,7 +18,7 @@
         Layers2,
     } from '@lucide/svelte';
     import { i18n } from '$lib/i18n/i18n.svelte';
-    import { defaultBasemap, type CustomLayer } from '$lib/data/layers';
+    import { defaultBasemap, layerGroup, type CustomLayer } from '$lib/data/layers';
     import { onMount } from 'svelte';
     import { remove } from './utils';
     import { settings } from '$lib/state/settings';
@@ -128,8 +128,9 @@
                 ],
             };
         }
-        addLayer(layerId);
+
         $customLayers[layerId] = layer;
+        addLayer(layerId);
         selectedLayerId = undefined;
         setDataFromSelectedLayer();
     }
@@ -255,13 +256,12 @@
                 onfinalize={(e) => {
                     customBasemapItems = e.detail.items;
                     $customBasemapOrder = customBasemapItems.map((item) => item.id);
-                    $selectedBasemapTree.basemaps['custom'] = customBasemapItems.reduce(
-                        (acc, item) => {
-                            acc[item.id] = true;
-                            return acc;
-                        },
-                        {}
-                    );
+                    selectedBasemapTree.update(($tree) => {
+                        layerGroup($tree.basemaps)['custom'] = Object.fromEntries(
+                            customBasemapItems.map((item) => [item.id, true])
+                        );
+                        return $tree;
+                    });
                 }}
             >
                 {#each customBasemapItems as item (item.id)}
@@ -315,13 +315,12 @@
                 onfinalize={(e) => {
                     customOverlayItems = e.detail.items;
                     $customOverlayOrder = customOverlayItems.map((item) => item.id);
-                    $selectedOverlayTree.overlays['custom'] = customOverlayItems.reduce(
-                        (acc, item) => {
-                            acc[item.id] = true;
-                            return acc;
-                        },
-                        {}
-                    );
+                    selectedOverlayTree.update(($tree) => {
+                        layerGroup($tree.overlays)['custom'] = Object.fromEntries(
+                            customOverlayItems.map((item) => [item.id, true])
+                        );
+                        return $tree;
+                    });
                 }}
             >
                 {#each customOverlayItems as item (item.id)}
diff --git a/website/src/lib/components/map/layer-control/LayerTreeNode.svelte b/website/src/lib/components/map/layer-control/LayerTreeNode.svelte
index 815e7956e..76908c314 100644
--- a/website/src/lib/components/map/layer-control/LayerTreeNode.svelte
+++ b/website/src/lib/components/map/layer-control/LayerTreeNode.svelte
@@ -22,7 +22,7 @@
         selected?: string;
         onselect?: (value: string) => void;
         multiple: boolean;
-        checked: LayerTreeType;
+        checked?: LayerTreeType;
     } = $props();
 
     const { customLayers } = settings;
@@ -53,7 +53,9 @@
                             id="{name}-{id}"
                             {name}
                             value={id}
-                            bind:checked={checked[id]}
+                            bind:checked={
+                                () => checked[id] === true, (value) => (checked[id] = value)
+                            }
                             class="scale-90"
                             aria-label={i18n._(`layers.label.${id}`)}
                         />
@@ -90,12 +92,18 @@
                 {#snippet content()}
                     <div class="ml-2">
                         <Self
-                            node={node[id]}
+                            node={node[id] as LayerTreeType}
                             {name}
                             {selected}
                             {onselect}
                             {multiple}
-                            bind:checked={checked[id]}
+                            bind:checked={
+                                () => {
+                                    const group = checked[id];
+                                    return typeof group === 'object' ? group : undefined;
+                                },
+                                (value) => (checked[id] = value ?? {})
+                            }
                         />
                     </div>
                 {/snippet}
diff --git a/website/src/lib/components/map/layer-control/extension-api.ts b/website/src/lib/components/map/layer-control/extension-api.ts
index 50441b593..b20285099 100644
--- a/website/src/lib/components/map/layer-control/extension-api.ts
+++ b/website/src/lib/components/map/layer-control/extension-api.ts
@@ -1,7 +1,7 @@
 import { settings } from '$lib/state/settings';
 import { derived, get, writable, type Writable } from 'svelte/store';
 import { isSelected, remove, removeAll } from './utils';
-import { overlays, overlayTree, type LayerTreeType } from '$lib/data/layers';
+import { layerGroup, overlays, overlayTree, type LayerTreeType } from '$lib/data/layers';
 import { browser } from '$app/environment';
 import { map } from '$lib/components/map/map';
 
@@ -84,11 +84,12 @@ export class ExtensionAPI {
             ],
         };
 
-        if (!Object.hasOwn(overlayTree.overlays as LayerTreeType, overlay.extensionName)) {
-            overlayTree.overlays[overlay.extensionName] = {};
+        const extensions = layerGroup(overlayTree.overlays);
+        if (!Object.hasOwn(extensions, overlay.extensionName)) {
+            extensions[overlay.extensionName] = {};
         }
 
-        overlayTree.overlays[overlay.extensionName][overlay.id] = true;
+        layerGroup(extensions[overlay.extensionName])[overlay.id] = true;
 
         selectedOverlayTree.update((selected) => {
             if (!Object.hasOwn(selected.overlays, overlay.extensionName)) {
diff --git a/website/src/lib/data/layers.ts b/website/src/lib/data/layers.ts
index 18eea4ed5..83d1aeded 100644
--- a/website/src/lib/data/layers.ts
+++ b/website/src/lib/data/layers.ts
@@ -794,6 +794,14 @@ export const defaultOpacities: { [key: string]: number } = {
 
 export type LayerTreeType = { [key: string]: LayerTreeType | boolean };
 
+/** A node of a layer tree that is a group of layers, not a layer (which is selected or not). */
+export function layerGroup(node: LayerTreeType | boolean | undefined): LayerTreeType {
+    if (typeof node !== 'object') {
+        throw new Error('Not a group of layers');
+    }
+    return node;
+}
+
 // Hierarchy containing all basemaps
 export const basemapTree: LayerTreeType = {
     basemaps: {