Stop a System-built landlord override being mis-read as a basement 🟥

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

domain/modelling/simulation.py

@@ -36,6 +36,14 @@ class BuildingPartOverlay:
     construction_age_band: Optional[str] = None
     wall_construction: Optional[int] = None
     wall_insulation_type: Optional[int] = None
+    # Disambiguates the RdSAP `wall_construction == 6` code collision: gov-EPC
+    # code 6 = "Basement wall" but RdSAP `WALL_SYSTEM_BUILT` is also 6, and
+    # `SapBuildingPart.main_wall_is_basement` falls back to the code-6 heuristic
+    # when the flag is `None`. A Landlord Override that sets a System-built wall
+    # (construction 6) must therefore set this `False` so the override isn't
+    # mis-read as a basement — the overlay-path mirror of the gov-API mapper's
+    # `_clear_basement_flag_when_system_built`.
+    wall_is_basement: Optional[bool] = None
     # Added solid-wall insulation depth (mm) — drives the calculator's Table 6
     # bucket / §5.8 documentary U-value for EWI (`wall_insulation_type=1`) and
     # IWI (`wall_insulation_type=3`); λ defaults to 0.04 W/m·K in the calculator.

tests/domain/epc/test_wall_type_overlay.py

@@ -25,6 +25,30 @@ def test_solid_brick_with_internal_insulation_overlays_main_wall() -> None:
     assert overlay.wall_insulation_type == 3
 
 
+def test_system_built_override_is_not_mis_read_as_a_basement() -> None:
+    # A System-built wall is RdSAP code 6, which collides with the gov-EPC
+    # code-6 basement sentinel. The overlay must set wall_is_basement=False so
+    # main_wall_is_basement doesn't fire the code-6 heuristic (phantom basement).
+    simulation = wall_overlay_for(
+        "System built, as built, no insulation (assumed)", 0
+    )
+
+    assert simulation is not None
+    overlay = simulation.building_parts[BuildingPartIdentifier.MAIN]
+    assert overlay.wall_construction == 6
+    assert overlay.wall_is_basement is False
+
+
+def test_non_system_built_override_leaves_basement_flag_untouched() -> None:
+    # Cavity (code 4) can't collide with the basement sentinel, so the overlay
+    # must not assert a basement verdict either way — leave the flag None.
+    simulation = wall_overlay_for("Cavity wall, with internal insulation", 0)
+
+    assert simulation is not None
+    overlay = simulation.building_parts[BuildingPartIdentifier.MAIN]
+    assert overlay.wall_is_basement is None
+
+
 @pytest.mark.parametrize(
     ("wall_type_value", "construction", "insulation"),
     [