diff --git a/domain/modelling/generators/floor_recommendation.py b/domain/modelling/generators/floor_recommendation.py
index 9e3815ba5..386c2fd76 100644
--- a/domain/modelling/generators/floor_recommendation.py
+++ b/domain/modelling/generators/floor_recommendation.py
@@ -15,6 +15,7 @@ from datatypes.epc.domain.epc_property_data import (
     SapBuildingPart,
 )
 from domain.building_geometry import ground_floor_area
+from domain.modelling.generators.floor_u_value_gate import floor_insulation_lowers_u
 from domain.modelling.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
@@ -69,6 +70,16 @@ def recommend_floor_insulation(
     if measure_type is None:
         return None
 
+    # Floor U-Value Gate (ADR-0051, floor arm): withhold the Option when it
+    # cannot lower the floor's derived U — e.g. an at-regs modern age band,
+    # where the as-built and insulated cascades resolve to the same value.
+    if not floor_insulation_lowers_u(
+        main,
+        epc.country_code,
+        insulation_thickness_mm=_RECOMMENDED_FLOOR_THICKNESS_MM,
+    ):
+        return None
+
     product = products.get(measure_type)
     area: float = ground_floor_area(epc, BuildingPartIdentifier.MAIN)
     cost = Cost(
diff --git a/domain/modelling/generators/floor_u_value_gate.py b/domain/modelling/generators/floor_u_value_gate.py
new file mode 100644
index 000000000..f7d2a5fb2
--- /dev/null
+++ b/domain/modelling/generators/floor_u_value_gate.py
@@ -0,0 +1,76 @@
+"""The Floor U-Value Gate — the ground-floor arm of ADR-0051.
+
+A floor insulation Option is offered only when it would actually lower the
+ground floor's derived U-value: on a modern construction age band RdSAP's
+BS EN ISO 13370 / Table 19 cascade assigns an as-built floor its at-regs
+U-value and the post-measure state resolves to the same value — the measure
+costs thousands for +0.00 SAP (portfolio 796: solid_floor_insulation offered
+at £3-4k on age-band-L dwellings 709782/709790).
+
+Same thin-mapping philosophy as the Wall U-Value Gate (`wall_u_value_gate`):
+the calculator's rich extras (surveyor description, lodged `floor_u_value`)
+are deliberately omitted — both sides are computed identically, so a genuine
+no-op still compares equal, and omissions only ever bias toward OFFERING.
+"""
+
+from typing import Any, Final, Optional
+
+from datatypes.epc.domain.epc_property_data import SapBuildingPart
+from domain.sap10_ml.rdsap_uvalues import Country, u_floor
+
+# Improvements below this are float noise, not a physical change. NOT a
+# materiality threshold — economics stay with the Optimiser (ADR-0016/0024).
+_NOISE_FLOOR_W_PER_M2K: Final[float] = 0.01
+
+
+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 floor_insulation_lowers_u(
+    main: SapBuildingPart,
+    country_code: Optional[str],
+    *,
+    insulation_thickness_mm: int,
+) -> bool:
+    """Whether insulating the main part's ground floor to
+    ``insulation_thickness_mm`` lowers its derived U-value by more than float
+    noise — the Floor U-Value Gate (ADR-0051)."""
+    country: Country = Country.from_code(country_code)
+    age_band = main.construction_age_band
+    ground = next(
+        (fd for fd in main.sap_floor_dimensions if fd.floor == 0),
+        main.sap_floor_dimensions[0] if main.sap_floor_dimensions else None,
+    )
+    construction: Optional[int] = (
+        _int_or_none(ground.floor_construction) if ground is not None else None
+    )
+    area = ground.total_floor_area_m2 if ground is not None else None
+    perimeter = ground.heat_loss_perimeter_m if ground is not None else None
+    current_u: float = u_floor(
+        country,
+        age_band,
+        construction,
+        _parse_thickness_mm(main.floor_insulation_thickness),
+        area,
+        perimeter,
+        main.wall_thickness_mm,
+    )
+    post_u: float = u_floor(
+        country,
+        age_band,
+        construction,
+        insulation_thickness_mm,
+        area,
+        perimeter,
+        main.wall_thickness_mm,
+    )
+    return current_u - post_u > _NOISE_FLOOR_W_PER_M2K