fix(heat-transmission): bill a ground-floor flat's ground floor (RdSAP 10 §3.12)

The flat floor-exposure heuristic keys on dwelling_type: a flat defaults
to has_exposed_floor=False (assuming a heated dwelling below). The
Elmhurst Summary path lodges a ground-floor flat's vertical position as a
"Ground floor" floor_type rather than the API floor_heat_loss=1 exposed
code, and the mapper can label such a flat "Top-floor flat" — so the
cascade dropped the ground floor entirely (a ground floor is in contact
with the ground and carries heat loss).

Treat a "ground floor" floor_type as a heat-loss floor, overriding the
dwelling-level suppression upward — mirroring the existing "another
dwelling below" party override downward.

Worksheet-validated to 1e-4 on simulated case 45 (a ground-floor flat
the mapper labelled "Top-floor flat"): floor (28a) 0 -> 25.38 W/K,
fabric (33) 75.63 -> 101.0104, HTC (39) 112.93 -> 145.3579, all matching
the P960 exactly; SAP 67.81 -> 62.52. RdSAP-21.0.1 corpus within-0.5
69.5% -> 69.7% (MAE 0.859 -> 0.854). Floors ratcheted. Pinned in
test_heat_transmission (ground-floor billed + party-floor suppressed).

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

domain/sap10_calculator/worksheet/heat_transmission.py

@@ -1105,6 +1105,16 @@ def heat_transmission_from_cert(
         # 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()
+        # A part whose floor_type is a GROUND floor sits in contact with the
+        # ground (RdSAP 10 §3.12) and is therefore a heat-loss floor, even when
+        # the dwelling-level flat heuristic (`_dwelling_exposure`) defaults a
+        # flat to has_exposed_floor=False. The Elmhurst Summary path lodges a
+        # ground-floor flat's position as a "Ground floor" floor_type (not the
+        # API floor_heat_loss=1 exposed code), so without this signal the
+        # cascade dropped its ground floor entirely — simulated case 45 (a
+        # ground-floor flat the mapper labelled "Top-floor flat"): worksheet
+        # (28a) = 47.0 × 0.54 = 25.38 W/K billed as 0, over-rating by +7 SAP.
+        part_floor_is_ground = "ground floor" in (part.floor_type or "").lower()
         # A floor lodged as a heat-loss floor — *exposed* (API
         # floor_heat_loss=1 → `is_exposed_floor`, "an exposed floor if there
         # is an open space below") or *above a partially heated space* (API
@@ -1117,6 +1127,7 @@ def heat_transmission_from_cert(
         # the "another dwelling below" party signal overrides it downward.
         part_has_exposed_floor = (
             exposure.has_exposed_floor or is_exposed_floor or is_above_partial
+            or part_floor_is_ground
         ) and not part_floor_is_party
         floor_area_total = _round_half_up(
             geom["ground_floor_area_m2"] if part_has_exposed_floor else 0.0,

tests/domain/sap10_calculator/worksheet/test_heat_transmission.py

@@ -157,6 +157,70 @@ def test_mixed_flat_pitched_roof_does_not_contaminate_pitched_u_value() -> None:
     assert abs(result.roof_w_per_k - 44.6) <= 2.0
 
 
+def test_ground_floor_flat_bills_floor_despite_flat_dwelling_type() -> None:
+    # Arrange — a ground-floor flat whose dwelling_type the mapper labelled
+    # "Top-floor flat" (so the dwelling-level exposure heuristic
+    # `_dwelling_exposure` suppresses the floor on the assumption a heated
+    # dwelling sits below), but whose building part lodges a "Ground floor"
+    # floor_type. A ground floor is in contact with the ground (RdSAP 10
+    # §3.12) -> heat-loss floor. The Elmhurst Summary path lodges this as a
+    # "Ground floor" floor_type (not the API floor_heat_loss=1 exposed code),
+    # so without the per-part ground signal the cascade dropped the floor.
+    # Worksheet-validated by simulated case 45: (28a) = 47.0 × U=0.54 = 25.38
+    # W/K, billed as 0 before this fix (+7 SAP).
+    ground = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        construction_age_band="C",
+        floor_type="Ground floor",
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=47.0, room_height_m=2.4,
+                heat_loss_perimeter_m=15.8, party_wall_length_m=0.0, floor=0,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=47.0, country_code="ENG",
+        dwelling_type="Top-floor flat", property_type="Flat",
+        sap_building_parts=[ground],
+    )
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert — the ground floor carries heat loss (≈ 47 × 0.54), not 0.
+    assert result.floor_w_per_k > 20.0
+
+
+def test_top_floor_flat_with_party_floor_stays_suppressed() -> None:
+    # Arrange — the contrast: a flat lodging "(another dwelling below)" sits
+    # over a heated dwelling, so its floor is a party floor with no heat loss
+    # (RdSAP 10 §3). The ground-floor override must NOT fire — proving the
+    # discriminator is the floor_type, not the flat label.
+    party = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        construction_age_band="C",
+        floor_type="To another dwelling below",
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=47.0, room_height_m=2.4,
+                heat_loss_perimeter_m=15.8, party_wall_length_m=0.0, floor=0,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=47.0, country_code="ENG",
+        dwelling_type="Top-floor flat", property_type="Flat",
+        sap_building_parts=[party],
+    )
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert — party floor, no heat loss.
+    assert result.floor_w_per_k == 0.0
+
+
 def test_part_geometry_floorless_part_honours_full_key_contract() -> None:
     # Arrange — a building part lodged with NO sap_floor_dimensions (e.g.
     # a party-wall-only or RR-only extension; observed on 5 certs in a

tests/infrastructure/epc_client/test_sap_accuracy_corpus.py

@@ -119,7 +119,15 @@ _CORPUS = Path(
 # 100010129331 (roof 110.5 -> 31.3 W/K, +13.1 -> -0.05 SAP). within-0.5
 # 68.8% -> 69.5% (MAE 0.888 -> 0.859; PE 13.9 -> 13.6); 3-part cohort 56% ->
 # 61%. Pinned in test_heat_transmission (by_kind split + no-contamination).
-_MIN_WITHIN_HALF_SAP = 0.69
+# GROUND-FLOOR FLAT FLOOR EXPOSURE (RdSAP 10 §3.12): a ground-floor flat whose
+# dwelling_type the mapper labelled "Top-floor flat" had its ground floor (in
+# contact with the ground -> heat loss) dropped, because the flat exposure
+# heuristic keys on dwelling_type and the Summary path lodges the position as a
+# "Ground floor" floor_type (not the API floor_heat_loss=1 code). Treating a
+# "ground floor" floor_type as exposed (worksheet-validated to 1e-4 on simulated
+# case 45: floor (28a) 0 -> 25.38 W/K, fabric (33) 75.6 -> 101.01) -> 69.5% ->
+# 69.7% (MAE 0.859 -> 0.854). Pinned in test_heat_transmission.
+_MIN_WITHIN_HALF_SAP = 0.695
 _MAX_SAP_MAE = 0.86
 _MAX_CO2_MAE_TONNES = 0.30      # t CO2 / yr vs co2_emissions_current
 _MAX_PE_PER_M2_MAE = 14.0       # kWh / m2 / yr vs energy_consumption_current