Harden Dwelling-Roof Cap on real data: positional segments, ground-floor basis 🟩

Three corrections found by re-running property 742003 end-to-end:
- roofSegmentStats are POSITIONAL — real responses omit the segmentIndex field
  the fixture happened to carry; key the centre/area lookup by array position.
- Base the cap on ground_floor_area (the footprint the roof covers), not the
  greatest per-storey area; roof_area is the fallback.
- Clamp the basis by total_floor_area: predicted EPCs borrow the structural
  template's geometry (742003: a 118.62 m² MAIN ground floor) decoupled from
  the predicted 55 m² (ADR-0029), so without the clamp the cap reads the
  template's larger footprint.

Result: 742003 plan A/92.4 (16 kWp) -> C/74.4 (6.4 kWp). 29 solar tests +
orchestration threading + products green.

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

domain/modelling/generators/solar_recommendation.py

@@ -24,7 +24,7 @@ from datatypes.epc.domain.epc_property_data import (
 )
 from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
 from datatypes.epc.domain.field_mappings import PROPERTY_TYPE_LOOKUP
-from domain.building_geometry import roof_area
+from domain.building_geometry import ground_floor_area, roof_area
 from domain.geospatial.planning_restrictions import PlanningRestrictions
 from domain.modelling.products import Products, SolarCostInputs
 from domain.modelling.measure_type import MeasureType
@@ -338,11 +338,29 @@ def recommend_solar(
 def _dwelling_roof_area_m2(epc: EpcPropertyData) -> Optional[float]:
     """The dwelling's own roof plan area for the Dwelling-Roof Cap (ADR-0038),
     or None when the EPC has no MAIN building part to measure (the cap then
-    falls back to Google's `maxArrayPanelsCount` cap)."""
+    falls back to Google's `maxArrayPanelsCount` cap).
+
+    The roof covers the **ground-floor footprint**, so the basis is
+    `ground_floor_area` (not the greatest per-storey area); `roof_area` is the
+    fallback when no ground (floor 0) dimension is lodged.
+
+    Clamped by total floor area: a footprint can never exceed the whole
+    dwelling's floor area (= footprint × storeys), so the clamp is a no-op on a
+    consistent lodged EPC. It matters for a **predicted** EPC, whose building-part
+    geometry is the structural template's — decoupled from the predicted floor
+    area (ADR-0029) — so without it the cap would read the template neighbour's
+    (larger) footprint rather than the predicted dwelling's size."""
     try:
-        return roof_area(epc, BuildingPartIdentifier.MAIN)
+        footprint: float = ground_floor_area(epc, BuildingPartIdentifier.MAIN)
     except StopIteration:
-        return None
+        try:
+            footprint = roof_area(epc, BuildingPartIdentifier.MAIN)
+        except StopIteration:
+            return None
+    total: Optional[float] = epc.total_floor_area_m2
+    if total is not None and total > 0.0:
+        return min(footprint, total)
+    return footprint
 
 
 def _solar_eligible(

domain/modelling/solar_potential.py

@@ -124,12 +124,16 @@ class SolarPotential:
             or "panelCapacityWatts" not in solar_potential
         ):
             return None
-        # Per-segment centre + area live on the top-level `roofSegmentStats`,
-        # keyed by `segmentIndex`; the per-config `roofSegmentSummaries` carry
-        # only the panel/orientation fields. Build the lookup once.
+        # Per-segment centre + area live on the top-level `roofSegmentStats`;
+        # the per-config `roofSegmentSummaries` carry only the panel/orientation
+        # fields. `roofSegmentSummaries[].segmentIndex` refers to the POSITION in
+        # `roofSegmentStats` (the entries are positional — Google omits an
+        # explicit `segmentIndex` field on them), so key the lookup by position.
         stats_by_index: dict[int, Mapping[str, Any]] = {
-            int(stats["segmentIndex"]): stats
-            for stats in solar_potential.get("roofSegmentStats", [])
+            index: stats
+            for index, stats in enumerate(
+                solar_potential.get("roofSegmentStats", [])
+            )
         }
 
         def _segment(summary: Mapping[str, Any]) -> SolarRoofSegment:

tests/domain/modelling/test_solar_config_selection.py

@@ -11,7 +11,14 @@ import json
 from pathlib import Path
 from typing import Any
 
+from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
+    EpcPropertyData,
+    SapBuildingPart,
+    SapFloorDimension,
+)
 from domain.modelling.generators.solar_recommendation import (
+    _dwelling_roof_area_m2,
     select_conservative_configs,
 )
 from domain.modelling.solar_potential import (
@@ -20,6 +27,37 @@ from domain.modelling.solar_potential import (
     SolarRoofSegment,
 )
 
+
+def _epc_with_roof(per_storey_areas: tuple[float, ...], total_floor_area: float) -> EpcPropertyData:
+    epc: EpcPropertyData = object.__new__(EpcPropertyData)
+    epc.total_floor_area_m2 = total_floor_area
+    part: SapBuildingPart = object.__new__(SapBuildingPart)
+    part.identifier = BuildingPartIdentifier.MAIN
+    dims: list[SapFloorDimension] = []
+    for floor, area in enumerate(per_storey_areas):
+        fd: SapFloorDimension = object.__new__(SapFloorDimension)
+        fd.floor = floor  # 0 = ground
+        fd.total_floor_area_m2 = area
+        dims.append(fd)
+    part.sap_floor_dimensions = dims
+    epc.sap_building_parts = [part]
+    return epc
+
+
+def test_dwelling_roof_basis_is_ground_floor_clamped_by_total_floor_area() -> None:
+    # ADR-0038/0029: the basis is the ground-floor footprint, clamped by total
+    # floor area. A predicted EPC's building-part geometry is the structural
+    # template's (here a 118 m² ground floor), decoupled from the predicted
+    # floor area (55 m²); a footprint can't exceed total floor area, so the cap
+    # basis clamps to 55 — not the borrowed template's 118.
+    predicted = _epc_with_roof(per_storey_areas=(118.0,), total_floor_area=55.0)
+    assert _dwelling_roof_area_m2(predicted) == 55.0
+
+    # A consistent 2-storey house — ground 55, upper 50, total 105 — uses the
+    # GROUND floor (55), not the greatest per-storey area, and the clamp is inert.
+    lodged = _epc_with_roof(per_storey_areas=(55.0, 50.0), total_floor_area=105.0)
+    assert _dwelling_roof_area_m2(lodged) == 55.0
+
 _FIXTURE: Path = (
     Path(__file__).resolve().parent
     / "fixtures"

tests/domain/modelling/test_solar_potential.py

@@ -122,9 +122,9 @@ def test_projection_enriches_segments_with_centre_and_area() -> None:
     # carry its centre + area — sourced from the top-level roofSegmentStats,
     # keyed by segmentIndex (the per-config roofSegmentSummaries omit them).
     insights = _insights()
+    # roofSegmentStats are positional — segmentIndex refers to the array index.
     stats_by_index = {
-        int(s["segmentIndex"]): s
-        for s in insights["solarPotential"]["roofSegmentStats"]
+        i: s for i, s in enumerate(insights["solarPotential"]["roofSegmentStats"])
     }
 
     potential = SolarPotential.from_building_insights(insights)