Wall + party-wall area = Σ (perim_i × height_i), not ground × avg × count

SAP §3 wall heat-loss area sums each storey individually:
`Σ (heat_loss_perimeter_i × room_height_i)`. Pre-fix used the short-cut
`ground_perimeter × avg_height × storey_count`, which over-counts upper
storeys whenever they have a smaller perimeter than the ground (set-back
top floors, ground-floor additions, etc.). RdSAP §5.10 party-wall area
follows the same per-storey-sum convention.

Surfaced by Elmhurst 000474 Main (ground perim 7.07, first 5.27): our
gross-wall over-counted by ~10 m², the (29a) W/K downstream by ~15 W/K
on this cert. Documented at the time as follow-up #2; this slice closes
it. The §3 partial-conformance test's gap-#2 entry is removed; gap #1
(RR sub-areas) remains.

Fix lives in two parallel code paths:
- dimensions.py: per-storey accumulation inside the existing fd loop
- heat_transmission.py: _part_geometry now emits gross_wall_area_m2 and
  party_wall_area_m2 directly, dropping the avg_height + storey_count
  intermediate fields (no other consumer)

Tests:
- New: gross_wall_area_sums_per_storey_perimeter_times_height_…
  (2-storey main, ground 10 m / first 6 m, same height — expects
  Σ=40 m² not ground×avg×count=50)
- New: party_wall_area_sums_per_storey_party_length_… (same shape,
  ground party 5 / first party 3 → Σ=20 not 25)
- New: walls_w_per_k_uses_sum_of_per_storey_perimeter_… (heat-
  transmission counterpart: 0.6 × 40 = 24 W/K not 30)

829 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/domain/src/domain/sap/worksheet/dimensions.py

@@ -112,13 +112,17 @@ def dimensions_from_cert(epc: EpcPropertyData) -> Dimensions:
         ground_area += ground.total_floor_area_m2 or 0.0
         ground_perim += ground.heat_loss_perimeter_m or 0.0
         top_area += top.total_floor_area_m2 or 0.0
-        gross_wall += (ground.heat_loss_perimeter_m or 0.0) * part_height * part_floor_count
+        # SAP §3 wall area: Σ (heat_loss_perimeter_i × height_i) across each
-        party_wall += (ground.party_wall_length_m or 0.0) * part_height * part_floor_count
+        # storey of the part. Pre-fix `ground_perim × avg_height × count`
+        # over-counts upper storeys whenever they have a different
+        # perimeter (e.g. set-back top floor, Elmhurst 000474 Main).
         for fd in part.sap_floor_dimensions:
             fa = fd.total_floor_area_m2 or 0.0
             fh = fd.room_height_m or _DEFAULT_STOREY_HEIGHT_M
             sum_per_storey_area_m2 += fa
             sum_per_storey_volume_m3 += fa * fh
+            gross_wall += (fd.heat_loss_perimeter_m or 0.0) * fh
+            party_wall += (fd.party_wall_length_m or 0.0) * fh
 
         # Room-in-roof: counts as one additional storey per RdSAP §1.8 +
         # §3.9. Both failing certs in the golden suite are Simplified

packages/domain/src/domain/sap/worksheet/heat_transmission.py

@@ -143,29 +143,32 @@ def _part_geometry(part: SapBuildingPart) -> dict[str, float]:
     if not part.sap_floor_dimensions:
         return {
             "ground_floor_area_m2": 0.0,
-            "ground_perimeter_m": 0.0,
             "top_floor_area_m2": 0.0,
-            "party_wall_length_m": 0.0,
+            "gross_wall_area_m2": 0.0,
-            "avg_room_height_m": _DEFAULT_STOREY_HEIGHT_M,
+            "party_wall_area_m2": 0.0,
-            "storey_count": 1.0,
         }
     fds = list(part.sap_floor_dimensions)
     ground = next((fd for fd in fds if fd.floor == 0), fds[0])
     indexed = [(fd.floor if fd.floor is not None else 0, fd) for fd in fds]
     top = max(indexed, key=lambda kv: kv[0])[1]
-    total_area = sum(fd.total_floor_area_m2 or 0.0 for fd in fds)
+    # SAP §3 wall area is Σ (perimeter_i × height_i) across each storey of
-    weighted_height = sum(
+    # the part — same convention as dimensions.gross_wall_area_m2. The
-        (fd.total_floor_area_m2 or 0.0) * (fd.room_height_m or _DEFAULT_STOREY_HEIGHT_M)
+    # ground-perim × avg × count short-cut over-counts upper storeys when
+    # the perimeter shrinks (e.g. Elmhurst 000474 Main: ground 7.07, first
+    # 5.27).
+    gross_wall = sum(
+        (fd.heat_loss_perimeter_m or 0.0) * (fd.room_height_m or _DEFAULT_STOREY_HEIGHT_M)
+        for fd in fds
+    )
+    party_wall = sum(
+        (fd.party_wall_length_m or 0.0) * (fd.room_height_m or _DEFAULT_STOREY_HEIGHT_M)
         for fd in fds
     )
-    avg_height = (weighted_height / total_area) if total_area > 0 else _DEFAULT_STOREY_HEIGHT_M
     return {
         "ground_floor_area_m2": ground.total_floor_area_m2 or 0.0,
-        "ground_perimeter_m": ground.heat_loss_perimeter_m or 0.0,
         "top_floor_area_m2": top.total_floor_area_m2 or 0.0,
-        "party_wall_length_m": ground.party_wall_length_m or 0.0,
+        "gross_wall_area_m2": gross_wall,
-        "avg_room_height_m": avg_height,
+        "party_wall_area_m2": party_wall,
-        "storey_count": float(len(fds)),
     }
 
 
@@ -286,13 +289,11 @@ def heat_transmission_from_cert(
         upw = u_party_wall(party_wall_construction=party_construction)
         y = thermal_bridging_y(age_band=age_band)
 
-        storey_count = geom["storey_count"]
+        gross_wall_area = geom["gross_wall_area_m2"]
-        storey_height = geom["avg_room_height_m"]
-        gross_wall_area = geom["ground_perimeter_m"] * storey_height * storey_count
         w_area = window_total_area_m2 if i == 0 else 0.0
         d_area = door_area if i == 0 else 0.0
         net_wall_area = max(0.0, gross_wall_area - w_area - d_area)
-        party_area = geom["party_wall_length_m"] * storey_height * storey_count
+        party_area = geom["party_wall_area_m2"]
         roof_area = geom["top_floor_area_m2"] if exposure.has_exposed_roof else 0.0
         floor_area_total = geom["ground_floor_area_m2"] if exposure.has_exposed_floor else 0.0
 

packages/domain/src/domain/sap/worksheet/tests/test_dimensions.py

@@ -180,6 +180,65 @@ def test_dwelling_storey_count_is_max_across_parts_not_sum() -> None:
     assert result.storey_count == 2
 
 
+def test_gross_wall_area_sums_per_storey_perimeter_times_height_not_ground_perim_times_avg() -> None:
+    # Arrange — 2-storey terrace where the upper storey has a smaller
+    # heat-loss perimeter than the ground (e.g. set-back upper floor or a
+    # wider ground addition). Surfaced by Elmhurst 000474: Main has
+    # ground perim 7.07, first 5.27 — the worksheet sums each storey
+    # separately, but pre-fix code used `ground_perim × avg_height ×
+    # storey_count` which over-counts the upper storey's wall area.
+    main = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=10.0, floor=0,
+            ),
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=6.0, floor=1,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(total_floor_area_m2=100.0, sap_building_parts=[main])
+
+    # Act
+    result = dimensions_from_cert(epc)
+
+    # Assert — Σ (perim × height) = 10×2.5 + 6×2.5 = 40.
+    # Pre-fix would have given 10 × 2.5 × 2 = 50.
+    assert result.gross_wall_area_m2 == pytest.approx(40.0)
+
+
+def test_party_wall_area_sums_per_storey_party_length_times_height_not_ground_party_times_avg() -> None:
+    # Arrange — Same per-storey-differs shape, but applied to the party
+    # wall. Two-storey main, ground party 5 m, upper party 3 m (e.g. the
+    # upper storey is set back from the party line). RdSAP §5.10 party
+    # area is also Σ (party_length_i × height_i), not
+    # ground_party × avg × count.
+    main = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=5.0, heat_loss_perimeter_m=0.0, floor=0,
+            ),
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=3.0, heat_loss_perimeter_m=0.0, floor=1,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(total_floor_area_m2=100.0, sap_building_parts=[main])
+
+    # Act
+    result = dimensions_from_cert(epc)
+
+    # Assert — Σ (party × height) = 5×2.5 + 3×2.5 = 20.
+    # Pre-fix would have given 5 × 2.5 × 2 = 25.
+    assert result.party_wall_area_m2 == pytest.approx(20.0)
+
+
 def test_room_in_roof_on_main_adds_one_to_dwelling_storey_count_only_once() -> None:
     # Arrange — Main 2-storey with RR + same-height 2-storey extension
     # without RR. RR adds one storey to MAIN (giving 3), extension stays

packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py

@@ -518,6 +518,40 @@ def test_thermal_bridging_drops_for_newer_age_band_per_table_21() -> None:
     assert bridging_m == pytest.approx(bridging_g * 0.08 / 0.15, abs=2.0)
 
 
+def test_walls_w_per_k_uses_sum_of_per_storey_perimeter_times_height_not_ground_perim_times_avg() -> None:
+    # Arrange — 2-storey age G cavity, upper storey set back so the
+    # ground perimeter (10 m) exceeds the first-floor perimeter (6 m).
+    # Both storey heights 2.5 m. Σ (perim × height) = 10×2.5 + 6×2.5 = 40
+    # m² gross wall (no windows/doors → 40 m² net). U_wall(G, cavity) = 0.6.
+    # Expected walls_w_per_k = 0.6 × 40 = 24. Pre-fix used
+    # ground_perim × avg × count = 10 × 2.5 × 2 = 50 m² → 30 W/K (overstates).
+    main = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        construction_age_band="G",
+        wall_construction=4, wall_insulation_type=4,
+        party_wall_construction=1, roof_construction=4,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=10.0, floor=0,
+            ),
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=6.0, floor=1,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=100.0, country_code="ENG", sap_building_parts=[main],
+    )
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert — 0.6 × 40 = 24, not 30.
+    assert result.walls_w_per_k == pytest.approx(24.0, abs=0.5)
+
+
 def test_main_plus_extension_sums_per_element_contributions() -> None:
     # Arrange — Main + single-storey age L extension. Each contributes to the
     # element totals. With_extension > main_only on every populated field
@@ -1079,12 +1113,7 @@ def test_section_3_partial_match_against_elmhurst_worksheet(fixture: ModuleType)
 
     Known divergences:
     1. RR walls not computed → smaller (33), smaller (31) for RR fixtures
-    2. Per-storey-different heat-loss perimeters not handled — our code
+    2. Window U-value is per-window in Elmhurst; we pass an area-weighted
-       does `ground_perim × avg_height × storey_count` which over-counts
-       when upper storeys are smaller than the ground (surfaced by
-       worksheet 000474 where Main has ground perim 7.07 / first 5.27).
-       Right formula: Σ (perim_i × height_i). Tracked as follow-up.
-    3. Window U-value is per-window in Elmhurst; we pass an area-weighted
        raw U so our effective transform approximates (27)
     """
     # Arrange — every Elmhurst fixture has known window U=1.4 raw on
@@ -1110,9 +1139,8 @@ def test_section_3_partial_match_against_elmhurst_worksheet(fixture: ModuleType)
     assert result.total_w_per_k == pytest.approx(
         result.fabric_heat_loss_w_per_k + result.thermal_bridging_w_per_k, rel=1e-9
     )
-    # External-area divergence direction depends on the fixture:
+    # External-area: RR fixtures still under-count vs worksheet because RR
-    # - RR fixtures: ours < worksheet (RR walls missing — gap #1)
+    # sub-areas are not modelled (gap #1). Non-RR fixtures should match
-    # - Non-RR with non-constant per-storey perim: ours > worksheet
+    # the worksheet's (31) once the per-storey-perimeter fix lands; for
-    #   (gap #2 — wall-area over-count). Just check non-zero until both
+    # now keep the looser non-zero check until RR closes.
-    #   fixes land.
     assert result.total_external_element_area_m2 > 0