diff --git a/domain/modelling/generators/solid_wall_recommendation.py b/domain/modelling/generators/solid_wall_recommendation.py
index dff55b669..32cee3437 100644
--- a/domain/modelling/generators/solid_wall_recommendation.py
+++ b/domain/modelling/generators/solid_wall_recommendation.py
@@ -26,6 +26,7 @@ from domain.building_geometry import gross_heat_loss_wall_area
 from domain.geospatial.planning_restrictions import PlanningRestrictions
 from domain.modelling.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
+from domain.modelling.generators.wall_u_value_gate import wall_insulation_lowers_u
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
 from repositories.product.product_repository import ProductRepository
 
@@ -158,6 +159,15 @@ def recommend_solid_wall(
         measure_type
         for measure_type in measure_types
         if _allowed(measure_type, restrictions, is_flat)
+        # Wall U-Value Gate (ADR-0051): withhold an Option that cannot lower
+        # the wall's derived U — e.g. an at-regs modern age band, where the
+        # as-built and insulated cascades resolve to the same value.
+        and wall_insulation_lowers_u(
+            main,
+            epc.country_code,
+            insulation_type=_INSULATION_TYPE[measure_type],
+            insulation_thickness_mm=_SOLID_WALL_INSULATION_MM,
+        )
     )
     if not allowed:
         return None
diff --git a/domain/modelling/generators/wall_u_value_gate.py b/domain/modelling/generators/wall_u_value_gate.py
new file mode 100644
index 000000000..fbbe3248c
--- /dev/null
+++ b/domain/modelling/generators/wall_u_value_gate.py
@@ -0,0 +1,80 @@
+"""The Wall U-Value Gate (ADR-0051).
+
+A wall insulation Option is offered only when it would actually lower the main
+wall's derived U-value: on a modern construction age band RdSAP's Table 6-10
+cascade assigns an as-built wall its at-regs U-value, and the post-measure
+state resolves to the *same* value — the measure costs thousands for +0.00 SAP
+(property 711795, PRD #1435 WS2: timber frame at age band L, 0.280 → 0.280).
+
+The comparison uses a deliberately thin `u_wall` parameter mapping (the
+`envelope.py` style — construction, age band, insulation type/thickness,
+documentary wall thickness), omitting the calculator's rich extras (surveyor
+description, dry-lining, lodged `wall_u_value`). The omissions bias the gate
+toward OFFERING — both sides are computed identically, so a genuine no-op still
+compares equal — never toward suppressing a real measure.
+"""
+
+from typing import Any, Final, Optional
+
+from datatypes.epc.domain.epc_property_data import SapBuildingPart
+from domain.sap10_ml.rdsap_uvalues import WALL_UNKNOWN, Country, u_wall
+
+# Improvements below this are float noise, not a physical change. NOT a
+# materiality threshold — how much improvement is worth the cost stays with the
+# Optimiser (ADR-0016/0024).
+_NOISE_FLOOR_W_PER_M2K: Final[float] = 0.01
+
+# RdSAP `wall_insulation_type` 4 = as-built / assumed (no added insulation).
+_WALL_INSULATION_NONE: Final[int] = 4
+
+
+def _int_or_none(value: Any) -> Optional[int]:
+    return value if isinstance(value, int) else None
+
+
+def _parse_thickness_mm(value: Any) -> Optional[int]:
+    if value is None or isinstance(value, int):
+        return value
+    if isinstance(value, str) and value.strip().isdigit():
+        return int(value.strip())
+    return None  # "NI" / free text — thickness not known
+
+
+def wall_insulation_lowers_u(
+    main: SapBuildingPart,
+    country_code: Optional[str],
+    *,
+    insulation_type: int,
+    insulation_thickness_mm: Optional[int] = None,
+) -> bool:
+    """Whether insulating the main wall to ``insulation_type`` (+ optional
+    thickness) lowers its derived U-value by more than float noise — the Wall
+    U-Value Gate (ADR-0051)."""
+    country: Country = Country.from_code(country_code)
+    age_band = main.construction_age_band
+    construction: Optional[int] = _int_or_none(main.wall_construction)
+    if construction == WALL_UNKNOWN:
+        construction = None
+    wall_thickness_mm = main.wall_thickness_mm
+    current_type: Optional[int] = _int_or_none(main.wall_insulation_type)
+    current_u: float = u_wall(
+        country,
+        age_band,
+        construction,
+        _parse_thickness_mm(main.wall_insulation_thickness),
+        insulation_present=(
+            current_type is not None and current_type != _WALL_INSULATION_NONE
+        ),
+        wall_insulation_type=current_type,
+        wall_thickness_mm=wall_thickness_mm,
+    )
+    post_u: float = u_wall(
+        country,
+        age_band,
+        construction,
+        insulation_thickness_mm,
+        insulation_present=True,
+        wall_insulation_type=insulation_type,
+        wall_thickness_mm=wall_thickness_mm,
+    )
+    return current_u - post_u > _NOISE_FLOOR_W_PER_M2K