diff --git a/domain/sap10_ml/rdsap_uvalues.py b/domain/sap10_ml/rdsap_uvalues.py
index fae4a9ff..b75b17a4 100644
--- a/domain/sap10_ml/rdsap_uvalues.py
+++ b/domain/sap10_ml/rdsap_uvalues.py
@@ -226,6 +226,26 @@ def _u_stone_thin_wall_age_a_to_e(
     return None
 
 
+def _table_3_stone_thickness(band: str, country: Country) -> int:
+    """RdSAP 10 §3.5 Table 3 (PDF p.20) — default stone wall thickness (mm)
+    used "only when the wall thickness could not be measured".
+
+    Stone row:  A-D = 500, E = 450, F-H = 420, I+ = 450.
+    Scotland footnote (*): add 200 mm for bands A and B, 100 mm for other
+    bands. Only A-E reach this helper (the §5.6 formula gate), so the F+
+    branches are defensive.
+    """
+    if band in ("A", "B", "C", "D"):
+        base = 500
+    elif band == "E":
+        base = 450
+    else:
+        base = 420
+    if country == Country.SCT:
+        base += 200 if band in ("A", "B") else 100
+    return base
+
+
 def _u_brick_thin_wall_age_a_to_e(wall_thickness_mm: int) -> float:
     """RdSAP 10 §5.7 Table 13 (PDF p.41) — default U-value for an
     uninsulated solid brick wall by lodged thickness, age bands A-E.
@@ -585,12 +605,22 @@ def u_wall(
     # 000565 Ext1 (granite 50 mm, age E, insulation Unknown) → min(6.09, 1.7)
     # = 1.70, matching the U985 worksheet WITHOUT a flat-table detour — the
     # age-E cap, not an insulation gate, is what produces the 1.70.
+    #
+    # When no documentary thickness is lodged, §3.5 Table 3 (PDF p.20) gives
+    # the default thickness to feed the formula (stone A-D = 500 mm, E = 450,
+    # Scotland +200/+100) — NOT a flat 1.7. This matches Elmhurst: an age-B
+    # granite as-built wall with unknown thickness defaults to 500 mm →
+    # 45.315 × 500^(-0.513) = 1.87 (sandstone → 1.68).
     if (
-        wall_thickness_mm is not None
-        and band in _STONE_AGE_A_TO_E
+        band in _STONE_AGE_A_TO_E
         and construction in (WALL_STONE_GRANITE, WALL_STONE_SANDSTONE)
     ):
-        u0 = _u_stone_thin_wall_age_a_to_e(construction, wall_thickness_mm)
+        w = (
+            wall_thickness_mm
+            if wall_thickness_mm is not None
+            else _table_3_stone_thickness(band, ctry)
+        )
+        u0 = _u_stone_thin_wall_age_a_to_e(construction, w)
         if u0 is not None:
             # Footnote (a) cap is age-E only: clamp the as-built U to the
             # Table-6/7 age-E default (1.7, or 1.5 for Scotland sandstone/
diff --git a/domain/sap10_ml/tests/test_rdsap_uvalues.py b/domain/sap10_ml/tests/test_rdsap_uvalues.py
index 14599a33..210b97bf 100644
--- a/domain/sap10_ml/tests/test_rdsap_uvalues.py
+++ b/domain/sap10_ml/tests/test_rdsap_uvalues.py
@@ -826,13 +826,14 @@ def test_u_wall_stone_granite_age_g_with_wall_thickness_ignores_5_6_formula_per_
     assert abs(result - 0.60) <= 1e-3
 
 
-def test_u_wall_stone_granite_age_a_without_wall_thickness_returns_table_6_age_a_default() -> None:
-    # Arrange — §5.6 formula only fires when a wall thickness is
-    # lodged. Without documentary wall-thickness evidence, fall back
-    # to the Table 6 row (which represents typical thickness). For
-    # age A stone granite without thickness, the cascade preserves
-    # its existing "as-built typical" U value rather than the formula
-    # extrapolation.
+def test_u_wall_stone_granite_age_a_without_wall_thickness_uses_table_3_default_500mm() -> None:
+    # Arrange — when no documentary wall thickness is lodged, RdSAP 10 §3.5
+    # Table 3 (PDF p.20) supplies the default stone thickness (A-D = 500 mm),
+    # which feeds the §5.6 formula — NOT a flat 1.7. This matches Elmhurst:
+    # an as-built granite/whinstone wall with unknown thickness defaults to
+    # 500 mm → U = 45.315 × 500^(-0.513) = 1.8693. (The earlier 1.7
+    # expectation was a setup error: Table 6 reads "According to 5.6" for
+    # bands A-D, with no 1.7 entry.)
 
     # Act
     result = u_wall(
@@ -845,8 +846,48 @@ def test_u_wall_stone_granite_age_a_without_wall_thickness_returns_table_6_age_a
         wall_thickness_mm=None,
     )
 
-    # Assert — _TYPICAL_STONE_UNINSULATED at age A = 1.7 (cohort default).
-    assert abs(result - 1.7) <= 1e-3
+    # Assert — §5.6 formula at the Table-3 default 500 mm.
+    assert abs(result - 1.8693) <= 1e-3
+
+
+def test_u_wall_stone_sandstone_age_b_without_wall_thickness_uses_table_3_default_500mm() -> None:
+    # Arrange — sandstone/limestone variant of the Table-3 default: age B,
+    # unknown thickness → 500 mm → 54.876 × 500^(-0.561) = 1.6798. This is
+    # the "500 mm → sandstone 1.68" Elmhurst default.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="B",
+        construction=WALL_STONE_SANDSTONE,
+        insulation_thickness_mm=None,
+        insulation_present=False,
+        wall_insulation_type=4,
+        wall_thickness_mm=None,
+    )
+
+    # Assert
+    assert abs(result - 1.6798) <= 1e-3
+
+
+def test_u_wall_stone_sandstone_scotland_age_a_without_thickness_adds_200mm_per_table_3() -> None:
+    # Arrange — Table 3 Scotland footnote (*): add 200 mm for bands A and B.
+    # Age-A Scotland sandstone unknown thickness → 500 + 200 = 700 mm →
+    # 54.876 × 700^(-0.561) = 1.3909 (< 1.7, no age-E cap at band A).
+
+    # Act
+    result = u_wall(
+        country=Country.SCT,
+        age_band="A",
+        construction=WALL_STONE_SANDSTONE,
+        insulation_thickness_mm=None,
+        insulation_present=False,
+        wall_insulation_type=4,
+        wall_thickness_mm=None,
+    )
+
+    # Assert
+    assert abs(result - 1.3909) <= 1e-3
 
 
 def test_u_wall_stone_granite_age_e_50mm_caps_at_table6_default_1_7() -> None: