diff --git a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
index 4c56fce0..7042c32f 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -75,6 +75,7 @@ _SUMMARY_000889_PDF = _FIXTURES / "Summary_000889.pdf"  # cert 2536 (Normal cyli
 _SUMMARY_000884_PDF = _FIXTURES / "Summary_000884.pdf"  # cert 9421 (Normal cylinder)
 _SUMMARY_000910_PDF = _FIXTURES / "Summary_000910.pdf"  # cert 0036 (Flat, party wall U=0)
 _SUMMARY_000890_PDF = _FIXTURES / "Summary_000890.pdf"  # cert 7800 (two electric showers)
+_SUMMARY_000565_PDF = _FIXTURES / "Summary_000565.pdf"  # cert 000565 (5-bp Elmhurst-only)
 
 # GOV.UK EPB API JSON for cert 001479 — the API-path counterpart of the
 # Summary_001479.pdf fixture. Together they drive the API ≡ Summary
@@ -1090,6 +1091,93 @@ def test_summary_mapper_raises_on_unmapped_glazing_type_label() -> None:
     assert excinfo.value.value == "Quintuple glazed with helium"
 
 
+def test_summary_000565_ext1_wall_construction_routes_to_stone_granite() -> None:
+    # Arrange — RdSAP 10 §3.3 + Table 4: cert 000565 Ext1 lodges
+    # "SG Stone: granite or whinstone" which routes to SAP10
+    # WALL_STONE_GRANITE=1. Pre-S0380.64 fell through silent-None,
+    # losing the Ext1 wall channel (worksheet line 29a: 91.83 m² ×
+    # U=1.7 = 156.11 W/K) from the cascade fabric subtotal.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.sap_building_parts[1].wall_construction == 1
+
+
+def test_summary_000565_ext3_ext4_wall_constructions_route_to_basement_code_6() -> None:
+    # Arrange — RdSAP 10 §5.17 / Table 23: cert 000565 Ext3 + Ext4
+    # lodge "B Basement wall". The canonical `BASEMENT_WALL_
+    # CONSTRUCTION_CODE=6` triggers the cascade's
+    # `part.main_wall_is_basement` route to `u_basement_wall` at
+    # heat_transmission.py:640. Pre-S0380.64 silent-None bypassed
+    # the basement-wall override entirely.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.sap_building_parts[3].wall_construction == 6
+    assert epc.sap_building_parts[3].main_wall_is_basement is True
+    assert epc.sap_building_parts[4].wall_construction == 6
+    assert epc.sap_building_parts[4].main_wall_is_basement is True
+
+
+def test_summary_000565_ext1_party_wall_routes_to_cavity_filled_code_4() -> None:
+    # Arrange — RdSAP 10 Table 15 row 3 "Cavity masonry filled":
+    # cert 000565 Ext1 lodges "CF Cavity masonry filled". Routes
+    # to SAP10 code 4 (Cavity). TODO(S0380.64+1): Table 15 row 3
+    # spec U=0.20; today's `u_party_wall` only returns 0.0 / 0.5 /
+    # 0.25 for code 4 so the cascade conservatively rounds up to
+    # the cavity-unfilled U=0.5 — matches the pre-existing
+    # `_API_PARTY_WALL_CONSTRUCTION_TO_SAP10[3]` approximation.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.sap_building_parts[1].party_wall_construction == 4
+
+
+def test_summary_mapper_raises_on_unmapped_wall_type_code() -> None:
+    # Arrange — strict-coverage gate per [[reference-unmapped-api-
+    # code]] mirror: an Elmhurst wall_type lodgement that isn't in
+    # `_ELMHURST_WALL_CODE_TO_SAP10` raises `UnmappedElmhurstLabel`
+    # rather than silently routing through wall_construction=None.
+    # The silent-None failure mode is what hid cert 000565 Ext1/3/4
+    # ~300 W/K cascade gap until the S0380.64 fabric-loss audit.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    site_notes.walls.wall_type = "XX Unknown construction"
+
+    # Act / Assert
+    with pytest.raises(UnmappedElmhurstLabel) as excinfo:
+        EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+    assert excinfo.value.field == "walls.wall_type"
+    assert excinfo.value.value == "XX Unknown construction"
+
+
+def test_summary_mapper_raises_on_unmapped_party_wall_type_code() -> None:
+    # Arrange — mirror strict-coverage gate for party-wall-type
+    # lodgements (same silent-None failure mode at the
+    # `_elmhurst_party_wall_construction_int` boundary).
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    site_notes.walls.party_wall_type = "YY Unknown party wall"
+
+    # Act / Assert
+    with pytest.raises(UnmappedElmhurstLabel) as excinfo:
+        EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+    assert excinfo.value.field == "walls.party_wall_type"
+    assert excinfo.value.value == "YY Unknown party wall"
+
+
 # ----------------------------------------------------------------------
 # API mapper strict-raise — mirror the Elmhurst UnmappedElmhurstLabel
 # coverage gate on the GOV.UK API path. The same failure mode (silently
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index d56ffaaa..f6f3abe6 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2077,6 +2077,9 @@ def _leading_code(value: str) -> str:
 _ELMHURST_WALL_CODE_TO_SAP10: Dict[str, int] = {
     "ST": 1,   # Stone (granite/sandstone) — placeholder; sandstone vs granite
                # ambiguity resolved downstream via walls[].description.
+    "SG": 1,   # Stone: granite or whinstone (cert 000565 Ext1) — the
+               # granite-specific Elmhurst variant of "ST"; same SAP10
+               # WALL_STONE_GRANITE=1 cascade entry.
     "SB": 3,   # Solid brick (cohort cert lodgement)
     "SO": 3,   # Solid brick (newer Elmhurst PDF variant — same SAP10
                # mapping; cert 9501 lodges "SO Solid Brick" where the
@@ -2085,6 +2088,15 @@ _ELMHURST_WALL_CODE_TO_SAP10: Dict[str, int] = {
     "TF": 5,   # Timber frame
     "TI": 5,   # Timber frame (Elmhurst's alt-wall code; same SAP10 mapping)
     "SY": 6,   # System build
+    "B":  6,   # Basement wall (cert 000565 Ext3+Ext4) — routes to the
+               # `BASEMENT_WALL_CONSTRUCTION_CODE=6` canonical signal so
+               # the cascade's `part.main_wall_is_basement` triggers the
+               # RdSAP 10 §5.17 / Table 23 `u_basement_wall` override
+               # (heat_transmission.py:640). Collides numerically with
+               # "SY" System build — the cascade's basement check
+               # precedes `u_wall(construction=6)` so SY would be
+               # silently mis-routed to u_basement_wall today; no cohort
+               # fixture exercises SY yet so the conflict is dormant.
     "CO": 7,   # Cob
     "PH": 8,   # Park home
     "CW": 9,   # Curtain wall
@@ -2151,9 +2163,19 @@ def _elmhurst_dwelling_type(
 
 def _elmhurst_wall_construction_int(coded: str) -> Optional[int]:
     """Map an Elmhurst wall_type string ('CA Cavity') to the SAP10
-    integer code (4). Returns None when the leading code isn't a known
-    SAP10 wall type."""
-    return _ELMHURST_WALL_CODE_TO_SAP10.get(_leading_code(coded))
+    integer code (4). Returns None when the lodging is absent (empty
+    string). Raises `UnmappedElmhurstLabel` when a non-empty code
+    isn't in `_ELMHURST_WALL_CODE_TO_SAP10` — that's a mapper-coverage
+    gap that should be made explicit so the next fixture forces a
+    dict entry rather than silently routing to wall_construction=None
+    (the failure mode that hid cert 000565 Ext1/Ext3/Ext4's ~300 W/K
+    cascade fabric-loss gap until the S0380.64 audit)."""
+    code = _leading_code(coded)
+    if not code:
+        return None
+    if code not in _ELMHURST_WALL_CODE_TO_SAP10:
+        raise UnmappedElmhurstLabel("walls.wall_type", coded)
+    return _ELMHURST_WALL_CODE_TO_SAP10[code]
 
 
 # Elmhurst Party Wall Type codes — distinct category-set from the Wall
@@ -2168,6 +2190,13 @@ _ELMHURST_PARTY_WALL_CODE_TO_SAP10: Dict[str, int] = {
     "CU": 4,  # Cavity masonry unfilled — same U=0.5 cascade; Elmhurst
               # encodes party-wall cavity type with the masonry sub-code
               # (CU vs CF filled) — observed first on cert 001479 Main.
+    "CF": 4,  # Cavity masonry filled (cert 000565 Ext1) — RdSAP 10
+              # Table 15 row 3 spec U=0.20. The cascade's `u_party_wall`
+              # only returns 0.0 / 0.5 / 0.25 for code 4 today, so CF
+              # rounds up to the conservative cavity-unfilled U=0.5 —
+              # matches the existing `_API_PARTY_WALL_CONSTRUCTION_TO
+              # _SAP10[3]` approximation until u_party_wall gains the
+              # filled-cavity branch (TODO).
     # "U Unable to determine" — the cohort's modal lodgement. The cohort
     # hand-built convention uses 0 as the explicit "unknown" sentinel
     # (rather than None) so cross-mapper field parity is preserved; the
@@ -2178,10 +2207,19 @@ _ELMHURST_PARTY_WALL_CODE_TO_SAP10: Dict[str, int] = {
 
 
 def _elmhurst_party_wall_construction_int(coded: str) -> Optional[int]:
-    """Map an Elmhurst party-wall-type string to a SAP10 wall_construction
-    integer. Returns None for 'U Unable to determine' (cascade default
-    U=0.25 then applies) and for unrecognised codes."""
-    return _ELMHURST_PARTY_WALL_CODE_TO_SAP10.get(_leading_code(coded))
+    """Map an Elmhurst party-wall-type string to a SAP10
+    wall_construction integer. Returns None when lodging is absent
+    (empty string — cascade default U=0.25 applies). Raises
+    `UnmappedElmhurstLabel` when a non-empty code isn't recognised
+    (same strict-coverage gate as `_elmhurst_wall_construction_int`;
+    silent-None routes to the same cascade default but hides genuinely-
+    new party-wall variants from the next fixture probe)."""
+    code = _leading_code(coded)
+    if not code:
+        return None
+    if code not in _ELMHURST_PARTY_WALL_CODE_TO_SAP10:
+        raise UnmappedElmhurstLabel("walls.party_wall_type", coded)
+    return _ELMHURST_PARTY_WALL_CODE_TO_SAP10[code]
 
 
 class UnmappedApiCode(ValueError):