diff --git a/frontend/src/lib/overlays.ts b/frontend/src/lib/overlays.ts
index 72d4d44..fb192cc 100644
--- a/frontend/src/lib/overlays.ts
+++ b/frontend/src/lib/overlays.ts
@@ -12,6 +12,8 @@ export interface OverlayDefinition {
   label: string;
   description: string;
   detail: string;
+  /** Enabled on a fresh visit (no `overlay` URL params). */
+  defaultEnabled?: boolean;
 }
 
 export const OVERLAYS: OverlayDefinition[] = [
@@ -28,6 +30,7 @@ export const OVERLAYS: OverlayDefinition[] = [
     description: 'Approximate police.uk street-crime heatmap',
     detail:
       'Client-side heatmap of street-level crimes published by police.uk over the most recent months. Police.uk coordinates are anonymised snap-to-grid points, not exact offence locations, so the heatmap should be read as an approximation of relative density rather than a precise map of incidents.',
+    defaultEnabled: true,
   },
   {
     id: 'trees-outside-woodlands',
@@ -35,6 +38,7 @@ export const OVERLAYS: OverlayDefinition[] = [
     description: 'Tree canopy and woodland polygons',
     detail:
       'Forest Research Trees Outside Woodland (TOW) v1 canopy polygons — lone trees and groups of trees — unioned with National Forest Inventory (NFI) woodland blocks (≥0.5 ha) that TOW deliberately excludes. Together they cover both street trees and actual woods. Polygon opacity scales with canopy area.',
+    defaultEnabled: true,
   },
   {
     id: 'property-borders',
@@ -45,6 +49,11 @@ export const OVERLAYS: OverlayDefinition[] = [
   },
 ];
 
+/** Overlays shown on a fresh visit, before any `overlay` URL params apply. */
+export const DEFAULT_OVERLAY_IDS: OverlayId[] = OVERLAYS.filter((o) => o.defaultEnabled).map(
+  (o) => o.id
+);
+
 const OVERLAY_ID_SET = new Set<string>(OVERLAY_IDS);
 
 export function isOverlayId(value: string): value is OverlayId {