diff --git a/backend/documents_parser/tests/fixtures/Summary_000910.pdf b/backend/documents_parser/tests/fixtures/Summary_000910.pdf
new file mode 100644
index 00000000..a9c8b2f3
Binary files /dev/null and b/backend/documents_parser/tests/fixtures/Summary_000910.pdf differ
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 69f09ccb..60f28581 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -72,6 +72,7 @@ _SUMMARY_000898_PDF = _FIXTURES / "Summary_000898.pdf"  # cert 2636
 _SUMMARY_000902_PDF = _FIXTURES / "Summary_000902.pdf"  # cert 9418
 _SUMMARY_000889_PDF = _FIXTURES / "Summary_000889.pdf"  # cert 2536 (Normal cylinder)
 _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)
 
 # GOV.UK EPB API JSON for cert 001479 — the API-path counterpart of the
 # Summary_001479.pdf fixture. Together they drive the API ≡ Summary
@@ -944,6 +945,32 @@ def test_summary_mapper_raises_on_unmapped_glazing_type_label() -> None:
     assert excinfo.value.value == "Quintuple glazed with helium"
 
 
+def test_summary_0036_flat_unknown_party_wall_routes_to_u_zero() -> None:
+    # Arrange — cert 0036-6325-1100-0063-1226 is a "Flat, Mid-Terrace"
+    # whose Summary lodges party_wall_type='U Unable to determine'.
+    # RdSAP 10 Table 15 footnote *: flats/maisonettes with unknown
+    # party-wall construction default to U=0.0, NOT the U=0.25 house
+    # default. Before Slice S0380.18 the cascade routed the lodging's
+    # "unknown" sentinel to the house default → +6.03 W/K HLC excess
+    # → SAP under-prediction of -0.37 vs worksheet 62.7471.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000910_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Act — chain the EPC through cert_to_inputs + the calculator so
+    # the assertion exercises the full cascade `u_party_wall` path,
+    # not just the helper in isolation.
+    result = calculate_sap_from_inputs(
+        cert_to_inputs(epc, prices=SAP_10_2_SPEC_PRICES)
+    )
+
+    # Assert — party walls contribute zero to HLC for this flat with
+    # unknown party-wall construction (matches worksheet line (32) =
+    # 24.13 m² × 0.00 = 0.0000 W/K).
+    assert epc.property_type == "Flat"
+    assert abs(result.intermediate["party_walls_w_per_k"] - 0.0) <= 1e-4
+
+
 def test_summary_2536_normal_cylinder_routes_to_code_2() -> None:
     # Arrange — cert 2536-2525-0600-0788-2292's Summary §15.1 lodges
     # "Cylinder Size: Normal". The dr87 worksheet lodges "Cylinder
diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index 0baf31a4..a658e668 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -119,6 +119,20 @@ _CANTILEVER_MAX_RATIO: Final[float] = 0.25
 # uniformly regardless of source-mapper encoding.
 _PROPERTY_TYPES_HOUSE: Final[frozenset[str]] = frozenset({"0", "House"})
 
+# RdSAP 10 Table 15 footnote * — flats and maisonettes with unknown
+# party-wall construction default to U=0.0 (both sides heated). The
+# Elmhurst Summary mapper produces "Flat" / "Maisonette" descriptive
+# forms; the API SAP/RdSAP schema enum-as-string is "2" (Flat) and "3"
+# (Maisonette) per `datatypes/epc/domain/epc_codes.csv property_type`
+# rows. Bungalows ("1" / "Bungalow") are *houses* for party-wall
+# purposes (treat party walls per the house default 0.25) even though
+# `_is_house` excludes them from cantilever detection — keep these
+# checks distinct so the API encoding doesn't bleed bungalows into
+# the flat path.
+_PROPERTY_TYPES_FLAT_OR_MAISONETTE: Final[frozenset[str]] = frozenset(
+    {"2", "3", "Flat", "Maisonette"}
+)
+
 
 def _is_house(property_type: Optional[str]) -> bool:
     """True when `epc.property_type` encodes a house (not flat / maisonette
@@ -128,6 +142,13 @@ def _is_house(property_type: Optional[str]) -> bool:
     return property_type in _PROPERTY_TYPES_HOUSE
 
 
+def _is_flat_or_maisonette(property_type: Optional[str]) -> bool:
+    """True when `epc.property_type` encodes a flat or maisonette (the
+    RdSAP 10 Table 15 footnote * party-wall-default trigger). Excludes
+    bungalows — they're houses for party-wall purposes per the spec."""
+    return property_type in _PROPERTY_TYPES_FLAT_OR_MAISONETTE
+
+
 @dataclass(frozen=True)
 class HeatTransmission:
     """SAP 10.2 §3 conduction HLC broken down per element type, summed
@@ -630,7 +651,17 @@ def heat_transmission_from_cert(
                 wall_thickness_mm=part.wall_thickness_mm,
                 description=effective_floor_description,
             )
-        upw = u_party_wall(party_wall_construction=party_construction)
+        # RdSAP 10 Table 15 footnote * — flats/maisonettes with unknown
+        # party-wall construction default to U=0.0 (both sides heated),
+        # not the U=0.25 house default. Cert 0036-6325-1100-0063-1226
+        # is the first flat fixture to exercise this branch — without
+        # it, the cascade over-counts party-wall HLC by ~+6 W/K → SAP
+        # under-prediction of -0.37. Bungalows do NOT trigger this
+        # branch (they're houses for party-wall purposes per the spec).
+        upw = u_party_wall(
+            party_wall_construction=party_construction,
+            is_flat=_is_flat_or_maisonette(epc.property_type),
+        )
         # Per-bp `y` for backwards compat: when the bp's own age band
         # differs from the dwelling's primary, the cascade applies the
         # dwelling-wide value (RdSAP10 Table 21 convention).
diff --git a/domain/sap10_ml/rdsap_uvalues.py b/domain/sap10_ml/rdsap_uvalues.py
index f2c8aa9a..c5d630eb 100644
--- a/domain/sap10_ml/rdsap_uvalues.py
+++ b/domain/sap10_ml/rdsap_uvalues.py
@@ -968,14 +968,29 @@ def u_basement_floor(age_band: Optional[str]) -> float:
     return _BASEMENT_FLOOR_BY_BAND.get(age_band.upper(), 0.50)
 
 
-def u_party_wall(party_wall_construction: Optional[int]) -> float:
+def u_party_wall(
+    party_wall_construction: Optional[int],
+    *,
+    is_flat: bool = False,
+) -> float:
     """RdSAP10 party-wall U-value in W/m^2K, never null.
 
     Mapping: solid masonry / timber / system built -> 0.0; cavity unfilled
-    -> 0.5; cavity filled -> 0.2; unknown -> 0.25 (house default).
+    -> 0.5; cavity filled -> 0.2; unknown -> 0.25 (house default) or 0.0
+    when `is_flat=True` per RdSAP 10 Table 15 footnote *: "for flats and
+    maisonettes with unknown party-wall construction, U=0.0" (each side
+    of the party wall is a heated dwelling, so no heat loss is assumed
+    by default; this matches the worksheet Elmhurst produces for flat
+    fixtures such as cert 0036-6325-1100-0063-1226).
+
+    `None` and `0` are both treated as the unknown sentinel — the
+    Elmhurst mapper lodges `0` for the "U Unable to determine" code per
+    the cross-mapper-parity convention in `datatypes/epc/domain/mapper
+    .py:_ELMHURST_PARTY_WALL_CODE_TO_SAP10` (the API mapper translates
+    its own "Not applicable" code to None directly).
     """
-    if party_wall_construction is None:
-        return 0.25
+    if party_wall_construction is None or party_wall_construction == 0:
+        return 0.0 if is_flat else 0.25
     if party_wall_construction in (WALL_SOLID_BRICK, WALL_STONE_GRANITE, WALL_STONE_SANDSTONE, WALL_TIMBER_FRAME, WALL_SYSTEM_BUILT):
         return 0.0
     if party_wall_construction == WALL_CAVITY:
diff --git a/domain/sap10_ml/tests/test_rdsap_uvalues.py b/domain/sap10_ml/tests/test_rdsap_uvalues.py
index 84a31ec0..b5f7df0b 100644
--- a/domain/sap10_ml/tests/test_rdsap_uvalues.py
+++ b/domain/sap10_ml/tests/test_rdsap_uvalues.py
@@ -969,6 +969,44 @@ def test_u_party_wall_unknown_returns_table15_house_default() -> None:
     assert result == pytest.approx(0.25, abs=0.001)
 
 
+def test_u_party_wall_unknown_for_flat_returns_table15_footnote_zero() -> None:
+    # Arrange — RdSAP 10 Table 15 footnote *: "for flats and maisonettes
+    # with unknown party-wall construction, U = 0.0" (both sides of the
+    # party wall are heated dwellings, so no heat loss).
+
+    # Act
+    result = u_party_wall(party_wall_construction=None, is_flat=True)
+
+    # Assert
+    assert abs(result - 0.0) <= 0.001
+
+
+def test_u_party_wall_unknown_sentinel_zero_treated_as_unknown_for_flat() -> None:
+    # Arrange — the Elmhurst mapper lodges `0` as the explicit "unknown"
+    # sentinel (per `datatypes/epc/domain/mapper.py:_ELMHURST_PARTY_WALL_
+    # CODE_TO_SAP10` cross-mapper-parity comment) where the API mapper
+    # would have lodged `None`. The cascade must treat both equivalently
+    # so a flat cert from either source surfaces Table 15 footnote *.
+
+    # Act
+    result = u_party_wall(party_wall_construction=0, is_flat=True)
+
+    # Assert
+    assert abs(result - 0.0) <= 0.001
+
+
+def test_u_party_wall_known_solid_still_returns_zero_when_is_flat_false() -> None:
+    # Arrange — `is_flat` is a fallback for the unknown case only; an
+    # explicit construction code always takes precedence (Solid → 0.0
+    # regardless of property type, matching Table 15 row 1).
+
+    # Act
+    result = u_party_wall(party_wall_construction=3, is_flat=False)
+
+    # Assert
+    assert abs(result - 0.0) <= 0.001
+
+
 # ----- Thermal bridging -----