fix(cascade): suppress floor heat loss for "another dwelling below" (code 6)

A floor lodged API floor_heat_loss=6 ("another dwelling below") sits over
another heated dwelling, so it is a party floor with no heat loss (RdSAP
10 §3). The mapper mapped code 6 → None and the heat-transmission step
drove floor exposure solely from the dwelling-level `has_exposed_floor`
flag — which is keyed only on the dwelling_type label and defaults a
"Ground-floor flat" to an exposed floor. So a ground-floor flat above a
basement dwelling kept its full ground-floor heat-loss area.

Map code 6 → "(another dwelling below)" (still != "Ground floor", so the
§5 (12) suspended-timber rule stays inert) and have the cascade suppress
that BP's floor when its floor_type carries the signal, mirroring the
roof's existing "another dwelling above" per-BP party override.

Cert 2115-4121-4711-9361-3686 (ground-floor flat, floor_heat_loss=6):
floor_w_per_k 47.85 → 0; SAP -23.44 → -4.41. Cert 0350-…-6435 -12.38 →
-0.55; 0926-…-9024 -2.35 → -0.82. Eval mean |err| 1.982 → 1.944.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

datatypes/epc/domain/mapper.py

@@ -2631,16 +2631,21 @@ def _api_floor_construction_str(value: Optional[int]) -> Optional[str]:
 #                                  the same top-level floors[] description
 #                                  as code 2; route to the same cascade
 #                                  signal until a fixture forces them apart
-#   6 = "(another dwelling below)" — top-floor flat over a party floor;
-#                                  cert 9501 lodges this. The cascade's
-#                                  floor-as-party-floor dispatch already
-#                                  handles this via `property_type=Flat` +
-#                                  cert.floors[].description, so the
-#                                  floor_type string from this helper is
-#                                  not consumed for the (12) spec rule
-#                                  in that path — explicit None preserves
-#                                  the cert 9501 cascade match without
-#                                  silently letting unknown codes through.
+#   6 = "(another dwelling below)" — the floor sits over another heated
+#                                  dwelling (e.g. an upper-floor flat, or a
+#                                  ground-floor flat above a basement flat),
+#                                  so it is a party floor with no heat loss
+#                                  (RdSAP 10 §3). The heat-transmission step
+#                                  reads this string to suppress the BP's
+#                                  floor area, mirroring the roof's "another
+#                                  dwelling above" party override — the
+#                                  dwelling-level exposure heuristic (keyed
+#                                  only on the dwelling_type label) defaults
+#                                  has_exposed_floor=True for a ground-floor
+#                                  flat, so the per-BP lodgement is needed to
+#                                  override it. It is != "Ground floor", so
+#                                  the §5 (12) suspended-timber rule stays
+#                                  inert (short-circuits exactly as None did).
 #   7 = "Ground floor"           — typical ground-floor heat loss
 #
 # Codes 4/5/8+ are not yet observed in any fixture; the strict-raise
@@ -2650,7 +2655,7 @@ _API_FLOOR_HEAT_LOSS_TO_FLOOR_TYPE: Dict[int, Optional[str]] = {
     1: "To external air",
     2: "To unheated space",
     3: "To unheated space",
-    6: None,
+    6: "(another dwelling below)",
     7: "Ground floor",
 }
 

datatypes/epc/domain/tests/test_from_rdsap_schema.py

@@ -887,6 +887,34 @@ class TestElmhurstGlazingTypeWrappedGap:
         assert code == 2
 
 
+class TestApiFloorTypeCode:
+    """`_api_floor_type_str` maps the GOV.UK API integer floor_heat_loss
+    code to the floor-position string the cascade reads. Code 6 ("another
+    dwelling below") must surface "(another dwelling below)" so the
+    heat-transmission step can suppress that BP's floor as a party floor
+    (RdSAP 10 §3) — it previously mapped to None and the floor leaked
+    heat-loss area. Cert 2115-4121-4711-9361-3686 (ground-floor flat over
+    another dwelling) under-rated ~23 SAP from the over-counted floor."""
+
+    def test_code_6_maps_to_another_dwelling_below(self) -> None:
+        # Arrange
+        from datatypes.epc.domain.mapper import _api_floor_type_str  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_floor_type_str(6)
+
+        # Assert — a party-floor signal the cascade consumes (not None).
+        assert result == "(another dwelling below)"
+
+    def test_code_7_still_maps_to_ground_floor(self) -> None:
+        # Arrange — regression guard: the ground-floor signal the §5 (12)
+        # suspended-timber rule keys on is unchanged.
+        from datatypes.epc.domain.mapper import _api_floor_type_str  # pyright: ignore[reportPrivateUsage]
+
+        # Act / Assert
+        assert _api_floor_type_str(7) == "Ground floor"
+
+
 class TestApiFloorConstructionCode:
     """`_api_floor_construction_str` maps the GOV.UK API integer
     floor_construction code to the description string the cascade's

domain/sap10_calculator/worksheet/heat_transmission.py

@@ -937,8 +937,18 @@ def heat_transmission_from_cert(
             rw_area_part if _bp_rr_roof_absorbs_rooflight(part, geom) else 0.0
         )
         roof_area = max(0.0, gross_roof_area - (rw_area_part - rw_area_on_rr))
+        # Per-BP floor exposure: a floor lodged "(another dwelling below)"
+        # (API floor_heat_loss=6) sits over another heated dwelling, so it
+        # is a party floor with no heat loss (RdSAP 10 §3) — suppress that
+        # BP's floor even when the dwelling-level `has_exposed_floor` flag
+        # is True. The flag is keyed only on the dwelling_type label, which
+        # defaults a "Ground-floor flat" to an exposed floor; the per-BP
+        # lodgement is authoritative. Mirrors the roof's "another dwelling
+        # above" override above. Cert 2115-4121-4711-9361-3686.
+        part_floor_is_party = "another dwelling below" in (part.floor_type or "").lower()
+        part_has_exposed_floor = exposure.has_exposed_floor and not part_floor_is_party
         floor_area_total = _round_half_up(
-            geom["ground_floor_area_m2"] if exposure.has_exposed_floor else 0.0,
+            geom["ground_floor_area_m2"] if part_has_exposed_floor else 0.0,
             _AREA_ROUND_DP,
         )
 

domain/sap10_ml/tests/_fixtures.py

@@ -155,6 +155,7 @@ def make_building_part(
     roof_construction: Optional[int] = 4,
     floor_dimensions: Optional[list[SapFloorDimension]] = None,
     sap_room_in_roof: Optional[SapRoomInRoof] = None,
+    floor_type: Optional[str] = None,
 ) -> SapBuildingPart:
     """Build a SapBuildingPart with sensible SAP10 defaults."""
     return SapBuildingPart(
@@ -169,6 +170,7 @@ def make_building_part(
         if floor_dimensions is not None
         else [make_floor_dimension()],
         sap_room_in_roof=sap_room_in_roof,
+        floor_type=floor_type,
     )
 
 

tests/domain/sap10_calculator/worksheet/test_heat_transmission.py

@@ -921,6 +921,42 @@ def test_ground_floor_flat_exposure_keeps_floor_drops_roof() -> None:
     assert ground.roof_w_per_k == 0.0
 
 
+def test_floor_over_another_dwelling_below_zeroes_floor_despite_exposed_flag() -> None:
+    # Arrange — a "Ground-floor flat" lodged with floor_heat_loss=6
+    # ("another dwelling below") sits over a heated dwelling (e.g. a
+    # basement flat), so its floor is a party floor (U=0, no heat loss)
+    # even though the dwelling-level exposure heuristic — keyed only on
+    # the "Ground-floor flat" label — defaults has_exposed_floor=True.
+    # The per-BP `floor_type` lodgement is authoritative and must
+    # suppress that BP's floor, mirroring the roof's "another dwelling
+    # above" party override. RdSAP 10 §3 — party floors between dwellings
+    # are not heat-loss elements. Cert 2115-4121-4711-9361-3686.
+    main = make_building_part(
+        construction_age_band="G",
+        wall_construction=4, wall_insulation_type=4,
+        party_wall_construction=1, roof_construction=4,
+        floor_type="(another dwelling below)",
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=60.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=30.0, floor=0,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=60.0, country_code="ENG", sap_building_parts=[main],
+    )
+
+    # Act — dwelling-level exposure still flags the floor as exposed.
+    result = heat_transmission_from_cert(
+        epc, exposure=DwellingExposure(has_exposed_floor=True, has_exposed_roof=False),
+    )
+
+    # Assert — the per-BP "another dwelling below" override wins → no floor loss.
+    assert result.floor_w_per_k == 0.0
+    assert result.walls_w_per_k > 0
+
+
 def test_ground_floor_flat_extension_with_flat_roof_exposes_extension_roof_only() -> None:
     """Per-BP roof exposure: an extension on a ground-floor flat can have
     its own external (e.g. single-storey) roof even though the dwelling-