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 edc526a3..e2cea9a9 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -45,6 +45,7 @@ from datatypes.epc.domain.mapper import (
 )
 from domain.sap10_calculator.calculator import calculate_sap_from_inputs
 from domain.sap10_calculator.rdsap.cert_to_inputs import SAP_10_2_SPEC_PRICES, cert_to_inputs
+from domain.sap10_ml.rdsap_uvalues import u_party_wall
 from domain.sap10_calculator.worksheet.tests import (
     _elmhurst_worksheet_000474 as _w000474,
     _elmhurst_worksheet_000477 as _w000477,
@@ -1543,14 +1544,14 @@ def test_summary_000565_bp0_alt1_stone_granite_thin_wall_routes_to_u_value_2p34_
     )
 
 
-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.
+def test_summary_000565_ext1_party_wall_routes_to_cavity_filled_code_11() -> None:
+    # Arrange — RdSAP 10 §5.10 Table 15 row 3 (PDF p.42) "Cavity masonry
+    # filled -> U=0.2 W/m²K". Cert 000565 Ext1 lodges "CF Cavity masonry
+    # filled". The synthetic SAP10 code `WALL_CAVITY_FILLED_PARTY=11`
+    # (introduced S0380.91) distinguishes filled-cavity party walls from
+    # the construction-class-shared code 4 (which `u_party_wall` resolves
+    # to 0.5 per Table 15 row 2). Code 11 is party-wall-only; it never
+    # appears as a main `wall_construction` so `u_wall` is unaffected.
     pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
     site_notes = ElmhurstSiteNotesExtractor(pages).extract()
 
@@ -1558,7 +1559,29 @@ def test_summary_000565_ext1_party_wall_routes_to_cavity_filled_code_4() -> None
     epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
 
     # Assert
-    assert epc.sap_building_parts[1].party_wall_construction == 4
+    assert epc.sap_building_parts[1].party_wall_construction == 11
+
+
+def test_summary_000565_ext1_party_wall_cf_routes_to_u_value_0p2() -> None:
+    # Arrange — cascade integration check for slice S0380.91: route
+    # cert 000565's Summary §8.1 "CF Cavity masonry filled" lodgement
+    # through extractor + mapper + heat_transmission and verify Ext1's
+    # party-wall U-value is 0.2 (Table 15 row 3) rather than the prior
+    # 0.5 (cavity-unfilled approximation). Localises the slice to one
+    # surface area × U product so the cascade aggregate movement (-28
+    # W/K on party_walls, ~-1000 kWh of cert 000565's +1460 SH residual)
+    # is traceable to one BP.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+    construction = epc.sap_building_parts[1].party_wall_construction
+    assert isinstance(construction, int)
+
+    # Act
+    u = u_party_wall(party_wall_construction=construction)
+
+    # Assert
+    assert abs(u - 0.2) <= 1e-4
 
 
 def test_summary_mapper_raises_on_unmapped_wall_type_code() -> None:
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index b9ce5639..7a0d004a 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2193,13 +2193,12 @@ _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).
+    "CF": 11, # Cavity masonry filled (cert 000565 Ext1) — RdSAP 10
+              # §5.10 Table 15 row 3 (PDF p.42) spec U=0.20. Routes via
+              # the synthetic `WALL_CAVITY_FILLED_PARTY=11` code that
+              # `u_party_wall` resolves to 0.2 (slice S0380.91). Code 11
+              # is party-wall-only and never appears as a main wall
+              # `wall_construction` so `u_wall` is unaffected.
     # "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
@@ -2252,10 +2251,10 @@ class UnmappedApiCode(ValueError):
 
 # GOV.UK API party_wall_construction enum → SAP10 wall_construction
 # integer (the domain `u_party_wall` consumes). The API uses a different
-# enum from the regular wall_construction field — RdSAP 10 Table 15
-# (p.31 "U-values of party walls") defines 5 categories, mapped to the
-# nearest SAP10 wall_construction code that `u_party_wall` resolves to
-# the spec U-value:
+# enum from the regular wall_construction field — RdSAP 10 §5.10 Table 15
+# (PDF p.42 "U-values of party walls") defines 5 categories, mapped to
+# the SAP10 wall_construction code that `u_party_wall` resolves to the
+# spec U-value:
 #   0 = "Not applicable" / no party wall (detached etc.) → cascade
 #       returns 0.25 by default but party_wall_length is 0 so the
 #       contribution is 0 regardless.
@@ -2263,10 +2262,10 @@ class UnmappedApiCode(ValueError):
 #       (WALL_SOLID_BRICK) → u_party_wall = 0.0 (Table 15 row 1).
 #   2 = "Cavity masonry unfilled" → SAP10 code 4 (WALL_CAVITY) →
 #       u_party_wall = 0.5 (Table 15 row 2).
-#   3 = "Cavity masonry filled" → spec U=0.2 (Table 15 row 3) — not
-#       yet representable; the cascade only emits 0.0 / 0.5 / 0.25 from
-#       the current u_party_wall, so this code rounds up to the
-#       conservative 0.5 (matches the cavity-unfilled W/K).
+#   3 = "Cavity masonry filled" → synthetic SAP10 code 11
+#       (WALL_CAVITY_FILLED_PARTY) → u_party_wall = 0.2 (Table 15 row 3,
+#       slice S0380.91). Distinct from code 4 because Table 15 lists
+#       the filled / unfilled cavity rows as separate party-wall types.
 #   4 = "Unable to determine, house or bungalow" → None (cascade
 #       default 0.25).
 #   5 = "Unable to determine, flat or maisonette" → cascade should
@@ -2278,7 +2277,7 @@ _API_PARTY_WALL_CONSTRUCTION_TO_SAP10: Dict[int, Optional[int]] = {
     0: None,
     1: 3,   # Solid masonry / timber / system → U=0.0
     2: 4,   # Cavity masonry unfilled → U=0.5
-    3: 4,   # Cavity masonry filled (cascade falls through to 0.5 — TODO)
+    3: 11,  # Cavity masonry filled → U=0.2 (WALL_CAVITY_FILLED_PARTY)
     4: None,  # Unable to determine, house — cascade default 0.25
     5: None,  # Unable to determine, flat — TODO: u_party_wall=0.0 path
 }
diff --git a/domain/sap10_ml/rdsap_uvalues.py b/domain/sap10_ml/rdsap_uvalues.py
index 0de18568..38973a0b 100644
--- a/domain/sap10_ml/rdsap_uvalues.py
+++ b/domain/sap10_ml/rdsap_uvalues.py
@@ -115,6 +115,12 @@ WALL_COB: Final[int] = 7
 WALL_PARK_HOME: Final[int] = 8
 WALL_CURTAIN: Final[int] = 9
 WALL_UNKNOWN: Final[int] = 10
+# Party-wall only: distinguishes "Cavity masonry filled" from "Cavity masonry
+# unfilled" per RdSAP 10 §5.10 Table 15 row 3 (PDF p.42) — the spec lists
+# them as separate party-wall types with U=0.2 vs U=0.5. Main wall U-value
+# cascade (`u_wall`) does not consume this code; cavity-wall insulation
+# state on a main wall flows through `wall_insulation_type` + Table 6.
+WALL_CAVITY_FILLED_PARTY: Final[int] = 11
 
 
 # RdSAP schema `wall_insulation_type` codes (empirically confirmed across
@@ -1094,13 +1100,12 @@ def u_party_wall(
 ) -> 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) 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).
+    Mapping per RdSAP 10 §5.10 Table 15 (PDF p.42):
+      - Solid masonry / timber / system built -> 0.0  (row 1)
+      - Cavity masonry unfilled                -> 0.5  (row 2)
+      - Cavity masonry filled                  -> 0.2  (row 3)
+      - Unable to determine, house             -> 0.25 (row 4)
+      - Unable to determine, flat / maisonette -> 0.0  (row 5, footnote *)
 
     `None` and `0` are both treated as the unknown sentinel — the
     Elmhurst mapper lodges `0` for the "U Unable to determine" code per
@@ -1114,6 +1119,8 @@ def u_party_wall(
         return 0.0
     if party_wall_construction == WALL_CAVITY:
         return 0.5
+    if party_wall_construction == WALL_CAVITY_FILLED_PARTY:
+        return 0.2
     return 0.25
 
 
diff --git a/domain/sap10_ml/tests/test_rdsap_uvalues.py b/domain/sap10_ml/tests/test_rdsap_uvalues.py
index 1b4b28c3..1d36ca15 100644
--- a/domain/sap10_ml/tests/test_rdsap_uvalues.py
+++ b/domain/sap10_ml/tests/test_rdsap_uvalues.py
@@ -23,6 +23,7 @@ import pytest
 from domain.sap10_ml.rdsap_uvalues import (
     Country,
     WALL_CAVITY,
+    WALL_CAVITY_FILLED_PARTY,
     WALL_CURTAIN,
     WALL_INSULATION_FILLED_CAVITY,
     WALL_SOLID_BRICK,
@@ -1233,6 +1234,22 @@ def test_u_party_wall_unfilled_cavity_returns_table15_value() -> None:
     assert result == pytest.approx(0.5, abs=0.001)
 
 
+def test_u_party_wall_cavity_masonry_filled_returns_0p2_per_rdsap_10_table_15_row_3() -> None:
+    # Arrange — RdSAP 10 §5.10 Table 15 row 3 (PDF p.42) "Cavity masonry
+    # filled -> 0.2 W/m²K". Before slice S0380.91 the `u_party_wall`
+    # cascade only resolved 0.0 / 0.5 / 0.25 for code 4 so Elmhurst
+    # "CF" lodgements rounded up to the conservative cavity-unfilled
+    # U=0.5 — over-counting party-wall heat loss by (0.5 - 0.2) × area.
+    # New synthetic code `WALL_CAVITY_FILLED_PARTY = 11` distinguishes
+    # filled cavity from the construction-class-shared code 4.
+
+    # Act
+    result = u_party_wall(party_wall_construction=WALL_CAVITY_FILLED_PARTY)
+
+    # Assert
+    assert abs(result - 0.2) <= 1e-4
+
+
 def test_u_party_wall_unknown_returns_table15_house_default() -> None:
     # Arrange — Table 15: unable to determine, house -> 0.25 W/m^2K.