diff --git a/domain/epc/property_overlays/wall_type_overlay.py b/domain/epc/property_overlays/wall_type_overlay.py
index 0620819c..848ced77 100644
--- a/domain/epc/property_overlays/wall_type_overlay.py
+++ b/domain/epc/property_overlays/wall_type_overlay.py
@@ -31,6 +31,9 @@ _MATERIAL_CONSTRUCTION: dict[str, int] = {
     "Curtain Wall": 9,
 }
 
+# RdSAP `WALL_SYSTEM_BUILT` — shares code 6 with the gov-EPC basement sentinel.
+_WALL_SYSTEM_BUILT = 6
+
 # RdSAP `wall_insulation_type` codes by insulation-state suffix
 # (domain/sap10_ml/rdsap_uvalues.py): external 1, filled-cavity 2, internal 3,
 # as-built/uninsulated 4, cavity+external 6, cavity+internal 7.
@@ -66,11 +69,18 @@ def wall_overlay_for(
         if building_part == 0
         else BuildingPartIdentifier.extension(building_part)
     )
+    # System-built is RdSAP code 6, which collides with the gov-EPC code-6
+    # basement sentinel that `main_wall_is_basement` falls back to. A landlord
+    # naming a System-built wall is asserting the material, not a basement — pin
+    # the flag False so the override isn't mis-read as one (ADR-0033 / the
+    # overlay mirror of the mapper's `_clear_basement_flag_when_system_built`).
+    wall_is_basement = False if construction == _WALL_SYSTEM_BUILT else None
     return EpcSimulation(
         building_parts={
             identifier: BuildingPartOverlay(
                 wall_construction=construction,
                 wall_insulation_type=insulation,
+                wall_is_basement=wall_is_basement,
             )
         }
     )