Slice 99a: Elmhurst extractor — no attachment line for flats

Cert 9501 (Summary_000784.pdf) is a flat. The Elmhurst Summary's
§1.0 "Property type" section lodges the built-form descriptor
("M Mid-Terrace", "D Detached", ...) only for houses — flats have no
attachment line, and the §2.0 "Number of Storeys" header follows
immediately after the "F Flat" property-type value.

The extractor's prior `_extract_attachment` regex captured the line
right after the property-type value unconditionally, so cert 9501
ended up with `attachment="2.0 Number of Storeys:"` — section-header
noise that the mapper surfaced on `EpcPropertyData.built_form`.
Downstream, this broke the cascade's `_dwelling_exposure` routing
(no prefix match → defaulted to fully-exposed houses) and so the
cert 9501 Summary path was Δ -5.25 SAP vs worksheet 68.5252.

Detect section-header noise via the leading `<digit>.<digit> `
pattern and the "Number of Storeys" substring; return "" in that
case so flats produce empty `built_form`. Houses still pick up their
real attachment (cohort 0330's "M Mid-Terrace" remains correct).

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

backend/documents_parser/elmhurst_extractor.py

@@ -182,8 +182,24 @@ class ElmhurstSiteNotesExtractor:
         )
 
     def _extract_attachment(self) -> str:
+        """Extract the Summary's "attachment" line — the §1.0 built-form
+        descriptor (e.g. "M Mid-Terrace", "D Detached") that sits
+        between the property-type value and the §2.0 section header
+        for HOUSES.
+
+        Flats DON'T lodge an attachment line in the Elmhurst Summary;
+        the §2.0 Number of Storeys header follows immediately after
+        the "F Flat" property-type value. Detect that case and return
+        "" so the mapper's `built_form` doesn't capture section-
+        header noise.
+        """
         m = re.search(r"1\.0 Property type:\n[^\n]+\n([^\n]+)", self._text)
-        return " ".join(m.group(1).strip().split()) if m else ""
+        if not m:
+            return ""
+        candidate = " ".join(m.group(1).strip().split())
+        if re.match(r"^\d+\.\d+\s", candidate) or "Number of Storeys" in candidate:
+            return ""
+        return candidate
 
     def _floors_from_dimensions_body(self, body: str) -> List[FloorDimension]:
         """Parse FloorDimension entries from a single bp's §4 body."""

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -57,6 +57,7 @@ _SUMMARY_000490_PDF = _FIXTURES / "Summary_000490.pdf"
 _SUMMARY_000516_PDF = _FIXTURES / "Summary_000516.pdf"
 _SUMMARY_001479_PDF = _FIXTURES / "Summary_001479.pdf"
 _SUMMARY_000897_PDF = _FIXTURES / "Summary_000897.pdf"
+_SUMMARY_000784_PDF = _FIXTURES / "Summary_000784.pdf"
 
 # GOV.UK EPB API JSON for cert 001479 — the API-path counterpart of the
 # Summary_001479.pdf fixture. Together they drive the API ≡ Summary
@@ -296,6 +297,30 @@ def test_summary_001479_secondary_heating_routes_mains_gas_fuel() -> None:
     assert epc.sap_heating.secondary_fuel_type == 26
 
 
+def test_summary_9501_flat_has_no_built_form_in_summary_pdf() -> None:
+    # Arrange — cert 9501 (Summary_000784.pdf) is a flat. The Elmhurst
+    # Summary's §1.0 "Property type" section lodges the built-form
+    # descriptor (e.g. "M Mid-Terrace", "D Detached") only for houses;
+    # flats have no built-form line — the §2.0 "Number of Storeys"
+    # section follows immediately after the "F Flat" property type.
+    #
+    # The extractor's `_extract_attachment` regex previously captured
+    # the line immediately after the property-type value
+    # unconditionally, so cert 9501 ends up with attachment
+    # "2.0 Number of Storeys:" — pure section-header noise that the
+    # mapper then surfaces on EpcPropertyData.built_form, breaking the
+    # cascade's flat-exposure routing downstream.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000784_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert — built_form is empty for flats. Houses set it to their
+    # attachment descriptor; flats lodge no attachment.
+    assert epc.built_form == ""
+
+
 def test_summary_001479_full_chain_sap_matches_worksheet_pdf_exactly() -> None:
     # Arrange — cert 001479 (Summary_001479.pdf / P960-0001-001479.pdf)
     # is the first cohort cert with a real GOV.UK EPB API counterpart