diff --git a/backend/documents_parser/elmhurst_extractor.py b/backend/documents_parser/elmhurst_extractor.py
index 1763b8f5..1bb3b3c2 100644
--- a/backend/documents_parser/elmhurst_extractor.py
+++ b/backend/documents_parser/elmhurst_extractor.py
@@ -294,6 +294,13 @@ class ElmhurstSiteNotesExtractor:
                 u_value_known=self._local_bool(
                     lines, f"Alternative Wall {n} U-value Known"
                 ),
+                # RdSAP10 §5.8 + Table 14: dry-lined uninsulated wall adds
+                # R = 0.17 m²K/W to base U. Cohort fixture: cert 7700
+                # Alt 1 "CavityWallPlasterOnDabs" lodges Dry-lining: Yes →
+                # U = 1/(1/1.5 + 0.17) ≈ 1.20.
+                dry_lined=self._local_bool(
+                    lines, f"Alternative Wall {n} Dry-lining"
+                ),
             ))
         return result
 
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 f8a79eef..b98fe999 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -340,6 +340,38 @@ def test_summary_2102_secondary_heating_routes_house_coal_for_open_fire() -> Non
     assert epc.sap_heating.secondary_fuel_type == 11
 
 
+def test_summary_7700_full_chain_sap_matches_worksheet_pdf_exactly() -> None:
+    # Arrange — cohort-2 cert 7700-3362-0922-7022-3563 (Summary_000905.pdf
+    # / dr87-0001-000905.pdf) is the first cohort fixture to exercise
+    # the alt-wall dry-lining adjustment. End-Terrace house age C, main
+    # wall filled cavity (CavityWallDensePlasterDenseBlock, U=0.70),
+    # alt wall 14.44 m² Cavity As-Built, Dry-lining: Yes
+    # (CavityWallPlasterOnDabsDenseBlock, worksheet U=1.20).
+    #
+    # Per RdSAP10 §5.8 + Table 14 page 41: dry-lining adds R = 0.17
+    # m²K/W → U = 1/(1/1.5 + 0.17) = 1.19522... → 2 d.p. half-up = 1.20.
+    # Pre-slice the alt sub-area's `wall_dry_lined="N"` hard-code routed
+    # to the cavity-as-built default (U=1.50), giving fabric (33)
+    # 148.72 W/K vs worksheet 144.38 (Δ +4.33 W/K = ~+0.44 SAP). Worksheet
+    # "SAP value" line lodges unrounded SAP **63.4425**.
+    cert_dir = Path(
+        "sap worksheets/additional with api 2/7700-3362-0922-7022-3563"
+    )
+    summary_pdf = next(cert_dir.glob("Summary_*.pdf"))
+    pages = _summary_pdf_to_textract_style_pages(summary_pdf)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Act
+    result = calculate_sap_from_inputs(
+        cert_to_inputs(epc, prices=SAP_10_2_SPEC_PRICES)
+    )
+
+    # Assert
+    worksheet_unrounded_sap = 63.4425
+    assert abs(result.sap_score_continuous - worksheet_unrounded_sap) < 1e-4
+
+
 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
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 194d6d7d..b1122f09 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2875,7 +2875,7 @@ def _map_elmhurst_alternative_wall(
     case, matching the full-cert-text "TimberWallOneLayer" lodgement)."""
     return SapAlternativeWall(
         wall_area=a.area_m2,
-        wall_dry_lined="N",
+        wall_dry_lined="Y" if a.dry_lined else "N",
         wall_construction=_elmhurst_wall_construction_int(a.wall_type) or 0,
         wall_insulation_type=_elmhurst_wall_insulation_int(a.insulation) or 4,
         wall_thickness_measured="Y" if not a.thickness_unknown else "N",
diff --git a/datatypes/epc/surveys/elmhurst_site_notes.py b/datatypes/epc/surveys/elmhurst_site_notes.py
index b955f5c8..fa87f167 100644
--- a/datatypes/epc/surveys/elmhurst_site_notes.py
+++ b/datatypes/epc/surveys/elmhurst_site_notes.py
@@ -57,7 +57,12 @@ class AlternativeWall:
     gross wall that has a different construction (e.g. a small 1.43 m²
     timber-frame panel on an otherwise cavity-walled extension). Up to
     two alternative walls per bp; Elmhurst lodges them in §7's "1st/2nd
-    Extension" subsection under the "Alternative Wall N <field>" prefix."""
+    Extension" subsection under the "Alternative Wall N <field>" prefix.
+
+    `dry_lined` carries Summary §7 "Alternative Wall N Dry-lining: Yes/No".
+    RdSAP10 §5.8 + Table 14: a dry-lined uninsulated wall adds R = 0.17
+    m²K/W to the base U-value (cavity-as-built age C: U = 1/(1/1.5 + 0.17)
+    ≈ 1.20). Cohort fixture: cert 7700 alt-wall (CavityWallPlasterOnDabs)."""
 
     area_m2: float
     wall_type: str  # e.g. "TI Timber Frame"
@@ -65,6 +70,7 @@ class AlternativeWall:
     thickness_unknown: bool
     thickness_mm: Optional[int]
     u_value_known: bool
+    dry_lined: bool = False
 
 
 @dataclass
diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index a397fea1..0950648a 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -921,5 +921,9 @@ def _alt_wall_w_per_k(
         insulation_present=alt_insulation_present,
         description=wall_description,
         wall_insulation_type=alt_wall.wall_insulation_type,
+        # RdSAP10 §5.8 + Table 14: dry-lined alt-wall adds R = 0.17 m²K/W
+        # when no insulation thickness is lodged. Cohort fixture: cert
+        # 7700 Alt 1 (Cavity, As-Built, Dry-lined) → 1.50 → 1.20.
+        dry_lined=alt_wall.wall_dry_lined == "Y",
     )
     return alt_u * net_alt_area
diff --git a/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py b/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py
index dbdeca5d..b26cd227 100644
--- a/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py
@@ -1119,6 +1119,65 @@ def test_basement_alt_wall_uses_table_23_u_value_not_cascade() -> None:
     )
 
 
+def test_dry_lined_alt_wall_cavity_as_built_age_c_applies_rdsap_5_8_r_0_17_adjustment() -> None:
+    """RdSAP10 §5.8 + Table 14 page 41: dry-lined uninsulated alt-wall
+    adds R = 0.17 m²K/W. Cohort fixture: cert 7700-3362-0922-7022-3563
+    Alt 1 lodges 14.44 m² Cavity, As-Built, Dry-lining: Yes, age C —
+    worksheet `CavityWallPlasterOnDabsDenseBlock` row (29a) U=1.20,
+    A×U = 14.44 × 1.20 = 17.3280 W/K. Without dry-lining the cascade
+    would route to the cavity-as-built default (U=1.50, A×U=21.66).
+    Difference: 4.33 W/K → ~+0.44 SAP — the entire cert 7700 residual."""
+    from dataclasses import replace
+    # Arrange — age C single-bp dwelling, main wall filled-cavity (U=0.70,
+    # so the difference we isolate sits entirely on the alt sub-area).
+    main_age_c = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        construction_age_band="C",
+        wall_construction=4,
+        wall_insulation_type=2,  # filled cavity → main wall U=0.70
+        party_wall_construction=1, roof_construction=4,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=80.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=35.0, floor=0,
+            ),
+        ],
+    )
+    with_dry_lined_alt = replace(
+        main_age_c,
+        sap_alternative_wall_1=SapAlternativeWall(
+            wall_area=14.44, wall_dry_lined="Y",
+            wall_construction=4, wall_insulation_type=4,
+            wall_thickness_measured="N",
+        ),
+    )
+    without_dry_lined_alt = replace(
+        main_age_c,
+        sap_alternative_wall_1=SapAlternativeWall(
+            wall_area=14.44, wall_dry_lined="N",
+            wall_construction=4, wall_insulation_type=4,
+            wall_thickness_measured="N",
+        ),
+    )
+
+    # Act
+    epc_dry_lined = make_minimal_sap10_epc(
+        total_floor_area_m2=80.0, country_code="ENG",
+        sap_building_parts=[with_dry_lined_alt],
+    )
+    epc_plain = make_minimal_sap10_epc(
+        total_floor_area_m2=80.0, country_code="ENG",
+        sap_building_parts=[without_dry_lined_alt],
+    )
+    result_dry_lined = heat_transmission_from_cert(epc_dry_lined)
+    result_plain = heat_transmission_from_cert(epc_plain)
+
+    # Assert — the alt-wall A×U delta is exactly 14.44 × (1.50 - 1.20)
+    # = 4.3320 W/K. Closed form: 1/(1/1.5 + 0.17) = 1.19522... → 2 d.p. = 1.20.
+    delta = result_plain.walls_w_per_k - result_dry_lined.walls_w_per_k
+    assert abs(delta - (14.44 * (1.50 - 1.20))) <= 1e-9
+
+
 def test_basement_floor_uses_table_23_u_value_for_whole_floor_when_basement_detected() -> None:
     """User-confirmed convention: when a part has a basement, the WHOLE
     floor=0 is the basement floor. Table 23 F-column overrides the
diff --git a/domain/sap10_ml/rdsap_uvalues.py b/domain/sap10_ml/rdsap_uvalues.py
index c5d630eb..8e6609cf 100644
--- a/domain/sap10_ml/rdsap_uvalues.py
+++ b/domain/sap10_ml/rdsap_uvalues.py
@@ -137,6 +137,12 @@ WALL_INSULATION_CAVITY_PLUS_INTERNAL: Final[int] = 7
 # (cavity + external/internal insulation).
 _WALL_INSULATION_LAMBDA_W_PER_MK: Final[float] = 0.04
 
+# RdSAP10 §5.8 final note + Table 14 page 41: "For drylining including
+# laths and plaster use Rinsulation = 0.17 m²K/W." Applied additively to
+# the base U-value of an otherwise-uninsulated wall when the cert lodges
+# `wall_dry_lined = "Y"` — see `u_wall(dry_lined=True)`.
+_DRY_LINING_RESISTANCE_M2K_PER_W: Final[float] = 0.17
+
 
 _AGE_BANDS: Final[tuple[str, ...]] = tuple("ABCDEFGHIJKLM")
 
@@ -329,6 +335,7 @@ def u_wall(
     insulation_present: bool = False,
     description: Optional[str] = None,
     wall_insulation_type: Optional[int] = None,
+    dry_lined: bool = False,
 ) -> float:
     """RdSAP10 wall U-value in W/m^2K, never null.
 
@@ -344,6 +351,16 @@ def u_wall(
     dedicated "Filled cavity" row is used in preference to the
     thickness-bucketed cascade — the two encode different things (filled-
     cavity is a construction state, not an added-insulation thickness).
+
+    `dry_lined` triggers the RdSAP10 §5.8 + Table 14 adjustment:
+    U_adjusted = 1 / (1/U_base + R_dryline) with R_dryline = 0.17 m²K/W.
+    The adjustment is applied only when the base U comes from the
+    uninsulated bucket (no measured insulation thickness, no filled-cavity
+    branch, no surveyor "described as insulated" override) — for those
+    branches the dry-lining R is already absorbed into the assumed
+    insulation stack. Cohort fixture: cert 7700 Alt 1 cavity-as-built
+    age C with Dry-lining: Yes — base U=1.5 → adjusted U=1.20 (2 d.p.,
+    matching worksheet `CavityWallPlasterOnDabsDenseBlock`).
     """
     measured = _measured_u_from_description(description)
     if measured is not None:
@@ -394,12 +411,21 @@ def u_wall(
     # Country override first.
     overrides = _COUNTRY_KLM_OVERRIDES.get(ctry, {}).get((wall_type, bucket), {})
     if band in overrides:
-        return overrides[band]
-
-    base = _ENG_WALL.get((wall_type, bucket))
-    if base is None:
-        return 1.5
-    return base[age_idx]
+        u_base = overrides[band]
+    else:
+        base = _ENG_WALL.get((wall_type, bucket))
+        u_base = base[age_idx] if base is not None else 1.5
+    # RdSAP10 §5.8 + Table 14 page 41 — dry-lining (including lath and
+    # plaster) adds R = 0.17 m²K/W to an otherwise-uninsulated wall:
+    # U_adjusted = 1 / (1/U_base + 0.17), rounded to 2 d.p. half-up.
+    # Only the as-built uninsulated bucket triggers the adjustment;
+    # insulated buckets already incorporate the dry-lining R via Table 14.
+    if dry_lined and bucket == 0:
+        u_unrounded = 1.0 / (1.0 / u_base + _DRY_LINING_RESISTANCE_M2K_PER_W)
+        return float(
+            Decimal(str(u_unrounded)).quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)
+        )
+    return u_base
 
 
 # ---------------------------------------------------------------------------
diff --git a/domain/sap10_ml/tests/test_rdsap_uvalues.py b/domain/sap10_ml/tests/test_rdsap_uvalues.py
index b5f7df0b..5c5f5942 100644
--- a/domain/sap10_ml/tests/test_rdsap_uvalues.py
+++ b/domain/sap10_ml/tests/test_rdsap_uvalues.py
@@ -278,6 +278,73 @@ def test_u_wall_unfilled_cavity_england_age_band_e_unchanged_at_1_5() -> None:
     assert result == pytest.approx(1.5, abs=0.001)
 
 
+def test_u_wall_dry_lined_cavity_as_built_age_c_applies_rdsap_5_8_r_0_17_adjustment() -> None:
+    # Arrange — RdSAP10 §5.8 final note + Table 14 page 41: "For drylining
+    # including laths and plaster use Rinsulation = 0.17 m²K/W." Applied
+    # additively to the base U-value of an otherwise-uninsulated wall.
+    # Cohort fixture: cert 7700-3362-0922-7022-3563 Alt 1 lodges Cavity,
+    # As-Built, Dry-lining: Yes, age band C → worksheet
+    # `CavityWallPlasterOnDabsDenseBlock` U-value = 1.20 W/m²K.
+    # Closed form: 1 / (1/1.5 + 0.17) = 1.19522... → 2 d.p. half-up = 1.20.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="C",
+        construction=WALL_CAVITY,
+        insulation_thickness_mm=None,
+        insulation_present=False,
+        wall_insulation_type=4,
+        dry_lined=True,
+    )
+
+    # Assert — adjusted U is rounded to 2 d.p. matching the dr87 worksheet's
+    # `UValueFinal` column for this construction.
+    assert abs(result - 1.20) <= 1e-9
+
+
+def test_u_wall_not_dry_lined_cavity_as_built_age_c_returns_unadjusted_1_5() -> None:
+    # Arrange — same age + construction as the dry-lined case above but
+    # without the dry-lining flag. Cascade must return the bare Table 6
+    # "Cavity as built" row value (no R = 0.17 added).
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="C",
+        construction=WALL_CAVITY,
+        insulation_thickness_mm=None,
+        insulation_present=False,
+        wall_insulation_type=4,
+        dry_lined=False,
+    )
+
+    # Assert
+    assert abs(result - 1.50) <= 1e-9
+
+
+def test_u_wall_dry_lined_with_measured_insulation_thickness_no_adjustment() -> None:
+    # Arrange — once a measured insulation thickness is lodged, Table 6's
+    # insulated buckets already incorporate the dry-lining R via Table 14.
+    # Applying R = 0.17 on top would double-count. Cavity + 100 mm
+    # insulation, age band E → Table 6 cavity-100mm row = 0.32 W/m²K
+    # regardless of the dry-lining flag.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="E",
+        construction=WALL_CAVITY,
+        insulation_thickness_mm=100,
+        insulation_present=True,
+        wall_insulation_type=4,
+        dry_lined=True,
+    )
+
+    # Assert
+    assert abs(result - 0.32) <= 1e-9
+
+
 def test_u_wall_cavity_as_built_england_age_band_g_returns_table6_value() -> None:
     # Arrange — Table 6, England, Cavity as built, age band G -> 0.60 W/m^2K.