fix(floor): exposed floor on a flat carries heat loss (RdSAP §3.12)

A mid-/top-floor flat whose lowest floor is lodged as an exposed floor
(API floor_heat_loss=1) had its floor area zeroed by the dwelling-level
exposure heuristic, which keys only on the flat label and defaults
has_exposed_floor=False (assuming the floor sits over another *heated*
dwelling). RdSAP 10 §3.12 (PDF p.25) is explicit:

  "Otherwise the floor area of the flat ... is:
     - an exposed floor if there is an open space below"

i.e. a flat cantilevered over a passageway IS a heat-loss floor on
Table 20. The per-BP `is_exposed_floor` lodgement is authoritative and
now overrides the dwelling-level suppression upward, mirroring the
existing "another dwelling below" party override (which suppresses
downward). The code-1↔"E To external air" enum is confirmed by the
paired API+Summary worksheet certs (0350, 3800).

Eval: 45.1% → 45.3% within 0.5 (909 computed); cert 3836 +6.79 → +0.77,
5717 +1.31 → -0.07 and 0997 +0.76 → +0.05 cross into <0.5. Two
already-failing under-rated certs (7636, 2241) shift further — both are
dominated by independent cost-side over-counts the exposed floor merely
unmasks (7636 walls = 8.98 W/K for 33.87 m² is the real defect).

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

domain/sap10_calculator/worksheet/heat_transmission.py

@@ -972,7 +972,17 @@ 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()
-        part_has_exposed_floor = exposure.has_exposed_floor and not part_floor_is_party
+        # A floor lodged as an *exposed* floor (API floor_heat_loss=1 →
+        # `is_exposed_floor`, "an exposed floor if there is an open space
+        # below" per RdSAP 10 §3.12, PDF p.25) carries heat loss even when
+        # the dwelling-level flat heuristic (`_dwelling_exposure`) defaults
+        # a mid-/top-floor flat to has_exposed_floor=False on the assumption
+        # its floor sits over another *heated* dwelling. The per-BP lodgement
+        # is authoritative: it overrides the suppression upward, mirroring
+        # how the "another dwelling below" party signal overrides it down.
+        part_has_exposed_floor = (
+            exposure.has_exposed_floor or is_exposed_floor
+        ) 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,
             _AREA_ROUND_DP,

tests/domain/sap10_calculator/worksheet/test_heat_transmission.py

@@ -996,6 +996,43 @@ def test_floor_over_another_dwelling_below_zeroes_floor_despite_exposed_flag() -
     assert result.walls_w_per_k > 0
 
 
+def test_exposed_floor_on_flat_carries_heat_loss_despite_unexposed_flag() -> None:
+    # Arrange — a top-/mid-floor flat whose lowest floor is lodged as an
+    # exposed floor (API floor_heat_loss=1, "an exposed floor if there is
+    # an open space below" per RdSAP 10 §3.12, PDF p.25 — e.g. a flat
+    # cantilevered over a passageway) IS a heat-loss floor on Table 20.
+    # The dwelling-level exposure heuristic, keyed only on the flat label,
+    # defaults has_exposed_floor=False on the assumption the floor sits over
+    # another heated dwelling; the per-BP `is_exposed_floor` lodgement is
+    # authoritative and must override that suppression upward, mirroring the
+    # "another dwelling below" party override (which suppresses downward).
+    main = make_building_part(
+        construction_age_band="B",
+        wall_construction=4, wall_insulation_type=4,
+        party_wall_construction=1, roof_construction=4,
+        floor_type="To external air",
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=18.0, room_height_m=2.88,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=8.68, floor=0,
+            ),
+        ],
+    )
+    main.sap_floor_dimensions[0].is_exposed_floor = True
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=18.0, country_code="ENG", sap_building_parts=[main],
+    )
+
+    # Act — dwelling-level exposure flags the floor as NOT exposed (flat).
+    result = heat_transmission_from_cert(
+        epc, exposure=DwellingExposure(has_exposed_floor=False, has_exposed_roof=True),
+    )
+
+    # Assert — the per-BP exposed-floor lodgement wins → Table 20 floor loss
+    # (1.20 W/m²K × 18 m² = 21.6 W/K), not the suppressed 0.0.
+    assert result.floor_w_per_k == pytest.approx(21.6, abs=0.1)
+
+
 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-