feat(modelling): flat gate drops EWI on solid-wall insulation

Slice 2d. A flat can take IWI (its own unit) but not EWI (whole-block
coordination) — ADR-0019. _is_flat handles both ingestion representations:
the Elmhurst name form ('Flat') and the API stringified RdSAP code ('2' = Flat
per PROPERTY_TYPE_LOOKUP). Completes slice 2's eligibility surface.

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

domain/modelling/generators/solid_wall_recommendation.py

@@ -22,6 +22,7 @@ from datatypes.epc.domain.epc_property_data import (
     BuildingPartIdentifier,
     EpcPropertyData,
 )
+from datatypes.epc.domain.field_mappings import PROPERTY_TYPE_LOOKUP
 from domain.building_geometry import gross_heat_loss_wall_area
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
@@ -109,11 +110,26 @@ def _solid_wall_option(
     )
 
 
-def _allowed(measure_type: str, restrictions: PlanningRestrictions) -> bool:
+def _is_flat(epc: EpcPropertyData) -> bool:
+    """Whether the dwelling is a flat. The Elmhurst path lodges the name
+    ("Flat"); the API path a stringified RdSAP code (`PROPERTY_TYPE_LOOKUP`,
+    where 2 = Flat) — handle both representations."""
+    raw: str = (epc.property_type or "").strip()
+    if raw.lower() == "flat":
+        return True
+    if raw.isdigit():
+        return PROPERTY_TYPE_LOOKUP.get(int(raw)) == "Flat"
+    return False
+
+
+def _allowed(
+    measure_type: str, restrictions: PlanningRestrictions, is_flat: bool
+) -> bool:
     """Whether a planning-gated Option survives (ADR-0019): EWI is removed by any
-    restriction; IWI only by a listed/heritage protection."""
+    restriction or by the dwelling being a flat; IWI only by a listed/heritage
+    protection."""
     if measure_type == _EXTERNAL_MEASURE_TYPE:
-        return not restrictions.blocks_external
+        return not (restrictions.blocks_external or is_flat)
     return not restrictions.blocks_internal
 
 
@@ -141,10 +157,11 @@ def recommend_solid_wall(
     if not measure_types:
         return None
 
+    is_flat: bool = _is_flat(epc)
     allowed = tuple(
         measure_type
         for measure_type in measure_types
-        if _allowed(measure_type, restrictions)
+        if _allowed(measure_type, restrictions, is_flat)
     )
     if not allowed:
         return None

tests/domain/modelling/test_elmhurst_cascade_pins.py

@@ -14,6 +14,7 @@ is a named generator/overlay/calculator gap to fix, never a tolerance to widen
 
 from __future__ import annotations
 
+from dataclasses import replace
 from typing import Final
 
 from datatypes.epc.domain.epc_property_data import (
@@ -228,6 +229,42 @@ def test_listed_building_blocks_all_solid_wall_insulation() -> None:
     )
 
 
+def test_flat_drops_ewi_but_keeps_iwi() -> None:
+    # Arrange — a flat can take IWI to its own unit, but EWI needs whole-block
+    # coordination (ADR-0019). property_type "Flat" is the Elmhurst name form.
+    before: EpcPropertyData = parse_recommendation_summary(
+        "solid_brick_ewi_001431_before.pdf"
+    )
+    flat: EpcPropertyData = replace(before, property_type="Flat")
+
+    # Act
+    recommendation: Recommendation | None = recommend_solid_wall(flat, _AnyProduct())
+
+    # Assert
+    assert recommendation is not None
+    assert {option.measure_type for option in recommendation.options} == {
+        "internal_wall_insulation"
+    }
+
+
+def test_flat_detected_from_api_property_type_code() -> None:
+    # Arrange — the API path lodges property_type as a stringified code
+    # ("2" = Flat per PROPERTY_TYPE_LOOKUP), not the name.
+    before: EpcPropertyData = parse_recommendation_summary(
+        "solid_brick_ewi_001431_before.pdf"
+    )
+    flat: EpcPropertyData = replace(before, property_type="2")
+
+    # Act
+    recommendation: Recommendation | None = recommend_solid_wall(flat, _AnyProduct())
+
+    # Assert — same gate fires regardless of representation.
+    assert recommendation is not None
+    assert {option.measure_type for option in recommendation.options} == {
+        "internal_wall_insulation"
+    }
+
+
 def test_cavity_wall_gets_no_solid_wall_recommendation() -> None:
     # Arrange — a cavity wall is handled by recommend_cavity_wall, never here.
     before: EpcPropertyData = parse_recommendation_summary(