Slice 56: Elmhurst floor exposed to external air routes through u_exposed_floor

`_is_floor_exposed_to_unheated_space` previously only matched
"U Above unheated space" (semi-exposed floor over a porch / car-park).
Cert 001479 Ext2 §9 lodges "Location: E To external air" — a 1.92 m²
cantilevered exposed timber floor (the upper-storey extension hanging
out over the garden). The worksheet's §3 `Exposed floor Ext2 … 1.92,
1.20, 1.20` pins this surface as U=1.20 via Table 20.

Pre-slice the mapper missed the "external air" lodgement entirely;
`is_exposed_floor=False` routed Ext2's ground SapFloorDimension
through the BS EN ISO 13370 ground-floor cascade (default U≈0.5),
mis-modelling a fully-exposed cantilever as a slab on soil.

Both lodgement strings ("above unheated", "external air") now
trigger the Table 20 path. Function docstring updated; name kept
to minimise the diff (refactor candidate for a future slice).

Cohort 6 certs all still green at 1e-4 (none lodge external-air
floors); cert 001479 cascade SAP 61.90 → 61.93 (+0.03), modest
upward move toward the 69.0094 target.

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

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -286,3 +286,23 @@ def test_summary_001479_main_party_wall_construction_is_cavity_unfilled() -> Non
 
     # Assert
     assert epc.sap_building_parts[0].party_wall_construction == 4
+
+
+def test_summary_001479_ext2_floor_is_exposed_to_external_air() -> None:
+    # Arrange — cert 001479 Ext2 §9 lodges "Location: E To external air"
+    # — a cantilevered exposed timber floor (the upper-storey extension
+    # over the back garden). The worksheet's §3 row `Exposed floor Ext2
+    # … 1.92, 1.20, 1.20` pins this as U=1.20 via Table 20. Pre-slice the
+    # mapper only routed "U Above unheated space" through `is_exposed_
+    # floor=True`; "E To external air" fell through to the BS EN ISO
+    # 13370 ground-floor cascade, dropping the lodged exposure entirely.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_001479_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    ext2 = epc.sap_building_parts[2]
+    assert ext2.floor_type == "To external air"
+    assert ext2.sap_floor_dimensions[0].is_exposed_floor is True

datatypes/epc/domain/mapper.py

@@ -1880,11 +1880,17 @@ _UPPER_FLOOR_HEIGHT_ADD_M: float = 0.25
 
 
 def _is_floor_exposed_to_unheated_space(location: Optional[str]) -> bool:
-    """True when the floor sits above an unheated space (lodged by the
-    Elmhurst surveyor as 'U Above unheated space'). The cascade routes
-    these through `u_exposed_floor` rather than the BS EN ISO 13370
-    ground-floor formula."""
-    return location is not None and "above unheated" in location.lower()
+    """True when the floor sits above an unheated space OR is exposed
+    directly to external air. Both lodgements route through
+    `u_exposed_floor` (Table 20) rather than the BS EN ISO 13370 ground-
+    floor formula. Elmhurst lodges either 'U Above unheated space'
+    (extension over a porch / car-park) or 'E To external air' (cantile-
+    vered floor of an upper-storey extension, first seen on cert 001479
+    Ext2 — a 1.92 m² exposed timber floor at U=1.20)."""
+    if location is None:
+        return False
+    lower = location.lower()
+    return "above unheated" in lower or "external air" in lower
 
 
 def _extract_age_band(age_range: str) -> str: