Merge pull request #1447 from Hestia-Homes/fix/electric-underfloor-pathway-and-noop-walls

Electric underfloor gets its heating pathway; wall insulation gated on derived U (WS2 of #1435)

CONTEXT.md

@@ -317,7 +317,11 @@ The two competing **Measure Options** for insulating a *solid* (non-cavity) main
 _Avoid_: solid wall insulation (that names the pair, not one Option), cladding, drylining
 
 **Wall Insulation Eligibility**:
-The rule fixing which wall Option(s) the main-wall **Recommendation** offers, by wall construction then planning status. By construction: **cavity** → cavity fill only (never solid insulation); **solid brick** / **system-built** → IWI + EWI; **timber-frame** → IWI only (EWI is not constructable); **cob** / **granite or whinstone** / **sandstone or limestone** → none (breathable fabric — standard insulation risks trapping moisture). Then, as planning gates: a **conservation area** or **flat** removes EWI (external-appearance / whole-block constraints), and a **listed** or **heritage** building removes **both** EWI and IWI (protected fabric).
+The rule fixing which wall Option(s) the main-wall **Recommendation** offers, by wall construction then planning status. By construction: **cavity** → cavity fill only (never solid insulation); **solid brick** / **system-built** → IWI + EWI; **timber-frame** → IWI only (EWI is not constructable); **cob** / **granite or whinstone** / **sandstone or limestone** → none (breathable fabric — standard insulation risks trapping moisture). Then, as planning gates: a **conservation area** or **flat** removes EWI (external-appearance / whole-block constraints), and a **listed** or **heritage** building removes **both** EWI and IWI (protected fabric). Finally the **Wall U-Value Gate** applies.
+
+**Wall U-Value Gate**:
+The eligibility rule that a wall insulation Option is offered **only when it would lower the wall's derived U-value** (by more than a 0.01 W/m²K float-noise guard) — comparing `u_wall` on the effective wall's current state vs its post-measure state (ADR-0051). Kills the **no-op fabric offer**: on a modern construction age band (e.g. band L, 2012–2022) RdSAP's cascade assigns the as-built wall its at-regs U-value and added insulation resolves to the *same* value, so the measure costs thousands for +0.00 SAP. The gate is physical possibility only — how *much* improvement is worth the cost stays with the Optimiser (ADR-0016/0024).
+_Avoid_: no-op filter (it gates generation, not scored output), U-value threshold (there is no absolute target-U; the comparison is before-vs-after)
 _Avoid_: restricted measures (legacy collapsed conservation/listed/heritage into one boolean — they now gate different Options, so keep them distinct)
 
 **Roof Insulation Eligibility**:

docs/adr/0050-electric-underfloor-override-drags-category-8-and-conservative-group-7-control.md

@@ -0,0 +1,84 @@
+# An electric-underfloor override drags category 8 and the conservative Group 7 control
+
+## Status
+
+accepted
+
+## Context
+
+[ADR-0048](0048-heating-override-forces-category-and-control-defers-fuel-and-meter.md)
+established that a system-replacing heating override must emit a **complete,
+coherent companion set** — category and control are archetype-forced, never
+inherited — and explicitly deferred the category expansion for the still-`None`
+archetypes ("electric boiler/CPSU→2, underfloor→8 … flows into the
+category-keyed Table 12a resolver and needs a corpus check") to its own change.
+
+The portfolio-796 audit (PRD #1435, WS2) found the underfloor half of that gap
+end-to-end on property **711795** (uprn 10090343115): a lodged **community
+heating** cert (SAP 301, category 6, control 2311) overridden to *"Electric
+underfloor, in screed above insulation"* (SAP 424,
+[ADR-0046](0046-electric-underfloor-scored-by-its-own-codes-meter-deferred-when-tariff-ambiguous.md)).
+`_category_for(424)` and `_control_for(424)` both returned `None`, so the
+effective cert kept the replaced heat network's **category 6** and community
+control. Two independent mis-ratings followed:
+
+1. **No heating pathway.** `is_heat_network_main` keys on category 6, so the
+   heating Recommendation Generator excluded both HHRSH and ASHP by the
+   heat-network topology rule — `recommend_heating` returned `None` and the
+   flat's whole candidate menu was a no-op wall measure plus lighting. The
+   dwelling was unliftable from band E (the `plan-stuck-in-low-band` audit
+   group counts ~7 electric-underfloor dwellings).
+2. **Baseline mis-rated.** The calculator scored the electric underfloor as
+   community heating: baseline SAP **51 (E)** vs **67 (D)** with coherent
+   companions. With the pathway restored, HHRSH scores **80 (C)** for ~£1k —
+   the dwelling reaches the scenario goal.
+
+## Decision
+
+The three electric-underfloor archetypes (SAP Table 4a **421 / 422 / 424**,
+ADR-0046) join the forced-companion branches of `main_heating_system_overlay`:
+
+1. **Category 8** ("Electric underfloor heating") in `_category_for` —
+   deterministic from SAP Table 4a, per ADR-0048's category rule.
+2. **Control 2701** (SAP Table 4e **Group 7**, "no time or thermostatic control
+   of room temperature", **+0.3 °C**) in `_control_for` — the conservative
+   default within the group the category fixes: the largest mean-internal-
+   temperature adjustment in Group 7, so an unobserved control is never
+   over-credited. Mirrors the storage manual-charge 2401
+   ([ADR-0035](0035-coherent-heating-system-synthesis.md)) and room-heater 2601
+   (ADR-0048) precedents.
+
+Fuel (electricity 29) and meter (Dual for 421/422; **deferred to the cert** for
+tariff-ambiguous 424) are already correct per ADR-0046 and are unchanged.
+
+**Scope**: underfloor only. Electric boiler / CPSU (Group 1) remain
+log-and-continue gaps tracked by #1444 — their category (2) feeds the wet-boiler
+fallback classification in the generators and needs its own check.
+
+## Consequences
+
+- An underfloor override on any lodged system now yields a coherent cert: the
+  Table 12a resolver and the heat-network gates read the dwelling as electric
+  underfloor, the baseline re-rates correctly, and the HHRSH bundle is offered
+  where eligible (a flat's only heating pathway — ASHP stays house/bungalow-only
+  per ADR-0024).
+- The "incomplete companion set" error log no longer fires for 421/422/424.
+- **Known deferred gap (pre-existing, unchanged by this ADR):** the Table 12a
+  space-heating resolver's underfloor rows are wired in the fraction table but
+  `_table_12a_system_for_main` never returns `UNDERFLOOR_HEATING` ("Underfloor
+  heating (421-422) — TODO"), so an off-peak-metered underfloor dwelling falls
+  to the all-low-rate fallback. That affects genuinely-lodged underfloor certs
+  identically and needs spec sight (SAP 10.2 Table 12a Grid 1, PDF p.191) to
+  split slab (storage-like) from screed (direct-acting-like); tracked as a
+  follow-up, not ridden on this override fix.
+
+### Alternatives rejected
+
+- **A zero-adjustment Group 7 control (2703/2704).** Rejected: credits a room
+  thermostat the landlord never reported; the conservative default is the
+  ADR-0035/0048 pattern.
+- **Completing electric boiler / CPSU (category 2) in the same change.**
+  Rejected: no evidence property in this workstream, and category 2 flows into
+  the generators' wet-boiler fallback (`_WET_BOILER_CATEGORIES`), which would
+  make an electric-boiler override newly eligible for tune-up options — a
+  behaviour change needing its own corpus check (#1444).

docs/adr/0051-wall-insulation-offered-only-when-it-lowers-the-derived-wall-u-value.md

@@ -0,0 +1,71 @@
+# Wall insulation is offered only when it lowers the derived wall U-value
+
+## Status
+
+accepted
+
+## Context
+
+The wall Recommendation Generators trigger on the wall's **insulation state**
+alone: `recommend_cavity_wall` offers cavity fill for any cavity wall whose
+`wall_insulation_type` is as-built (4), and `recommend_solid_wall` offers
+EWI/IWI for any as-built suitable construction
+([ADR-0019](0019-solid-wall-insulation-options.md)). Neither asks whether
+adding insulation can actually **change the modelled wall**.
+
+For modern construction age bands it cannot: RdSAP's Table 6-10 cascade
+(`u_wall` in `domain/sap10_ml/rdsap_uvalues.py`) assigns an as-built wall its
+age band's at-regs U-value, and the post-measure state resolves to the **same
+value**. Portfolio-796 evidence (PRD #1435, WS2): property **711795**, timber
+frame at age band **L** (2012–2022) — `u_wall` returns **0.280 W/m²K both
+as-built and with 100 mm IWI**, so the persisted plan offered
+`internal_wall_insulation` at **£4,027 for +0.00 SAP**. Any modern-age-band
+dwelling gets the same no-op fabric offer; on an unliftable dwelling it is the
+*entire* remedial menu, which reads as "we recommend spending £4k for nothing".
+
+Scoring already prices the option at +0.00, but the Optimiser keeps
+zero-uplift candidates in the persisted menu (they are legitimate for some
+measure types, e.g. secondary-heating removal saves bills without SAP), so the
+fix belongs at **generation eligibility**, not a blanket score filter.
+
+## Decision
+
+Both wall generators consult the **derived wall U-value** before offering: the
+measure is offered only when the post-measure state's `u_wall` is **lower than
+the current state's by more than 0.01 W/m²K** (a float-noise guard, not a
+materiality threshold — economics stay with the Optimiser, ADR-0016/0024).
+
+- **Current state**: `u_wall` with the effective main wall's construction, age
+  band, lodged insulation type/thickness — a thin parameter mapping in the
+  `envelope.py` style, deliberately omitting the calculator's rich extras
+  (surveyor description, dry-lining, documentary-thickness routes). Omissions
+  bias the gate toward **offering** (both sides are computed identically, so a
+  no-op still compares equal), never toward suppressing a real measure.
+- **Post-measure state**: the same call with the Option's overlay applied —
+  filled cavity (`wall_insulation_type=2`) for cavity fill; 100 mm
+  internal/external insulation for IWI/EWI.
+
+## Consequences
+
+- Age-band-L (and any at-regs age band) walls stop polluting candidate menus
+  with £4k no-ops; genuinely treatable walls (e.g. age-band-D solid brick,
+  U 2.1 → ~0.3) are offered exactly as before.
+- The generators gain a read on `domain/sap10_ml/rdsap_uvalues.u_wall` — same
+  precedent as the heating generator reading the calculator's PCDB/Table 4b.
+- **Deferred:** (a) a cert that lodges an explicit `wall_u_value` overrides the
+  cascade in the calculator, making fabric measures score +0.00 regardless of
+  this gate — the measure overlay should adjust the lodged U; (b) roof/floor
+  generators have the same anti-pattern in principle (roof age-band eligibility
+  is partly covered by ADR-0047); both tracked as follow-ups.
+
+### Alternatives rejected
+
+- **Filter zero-SAP candidates after scoring.** Rejected: zero-uplift options
+  are legitimate for some measure types (bill-driven), and the pollution would
+  survive in any consumer that reads Generator output before scoring.
+- **Hard-code "age band ≥ K ⇒ no wall offer".** Rejected: duplicates the U
+  tables coarsely; the tables already encode exactly which
+  construction × age-band × insulation combinations move.
+- **Full-fidelity U parity with the calculator (description, dry-lining,
+  lodged U).** Rejected for now: heavier coupling for no observed benefit —
+  the omissions only ever bias toward offering, which is the status quo.

domain/epc/property_overlays/main_heating_system_overlay.py

@@ -134,6 +134,19 @@ _ELECTRIC_BOILER_CODES = frozenset({191, 192})
 # unambiguously electric (ADR-0046).
 _ELECTRIC_UNDERFLOOR_CODES = frozenset({421, 422, 424})
 
+# SAP Table 4a "electric underfloor heating" category. Stamped so an underfloor
+# override never leaves the replaced system's category behind — property 711795
+# (PRD #1435 WS2) kept a replaced heat network's category 6, which the heating
+# generator's `is_heat_network_main` gate read as community heating: no heating
+# upgrade was offered at all and the baseline mis-rated 51 vs 67 (ADR-0050).
+_UNDERFLOOR_CATEGORY = 8
+# SAP Table 4e Group 7 underfloor control: 2701 ("no time or thermostatic
+# control of room temperature", +0.3 C) is the conservative default when the
+# landlord named only the system — the largest mean-internal-temperature
+# adjustment in the group, so an unobserved control is never over-credited
+# (the underfloor mirror of storage-manual 2401 and room-heater 2601, ADR-0050).
+_UNDERFLOOR_CONTROL = 2701
+
 # Heat pumps (SAP Table 4a 211-224 wet, 521-527 warm-air) are category 4 and
 # unambiguously electric (natural fuel 29). Modellable on the default code's SPF
 # without a PCDB index (ADR-0041).
@@ -271,6 +284,8 @@ def _control_for(code: int) -> Optional[int]:
         return _FULL_BOILER_CONTROL
     if code in _CATEGORY_10_ROOM_HEATER_CODES or code in _ELECTRIC_ROOM_HEATER_CODES:
         return _ROOM_HEATER_CONTROL
+    if code in _ELECTRIC_UNDERFLOOR_CODES:
+        return _UNDERFLOOR_CONTROL
     return None
 
 
@@ -287,6 +302,8 @@ def _category_for(code: int) -> Optional[int]:
         return _ROOM_HEATER_CATEGORY
     if code in _STORAGE_HEATER_CODES:
         return _STORAGE_HEATER_CATEGORY
+    if code in _ELECTRIC_UNDERFLOOR_CODES:
+        return _UNDERFLOOR_CATEGORY
     if code in _HEAT_PUMP_CODES:
         return _HEAT_PUMP_CATEGORY
     if code in _HEAT_NETWORK_CODES:

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

domain/modelling/generators/wall_recommendation.py

@@ -12,6 +12,7 @@ from datatypes.epc.domain.epc_property_data import (
     EpcPropertyData,
 )
 from domain.building_geometry import gross_heat_loss_wall_area
+from domain.modelling.generators.wall_u_value_gate import wall_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
@@ -46,6 +47,14 @@ def recommend_cavity_wall(
     ):
         return None
 
+    # Wall U-Value Gate (ADR-0051): withhold the fill when it cannot lower the
+    # wall's derived U — e.g. an at-regs modern age band, where the as-built and
+    # filled-cavity cascades resolve to the same (or a no-better) value.
+    if not wall_insulation_lowers_u(
+        main, epc.country_code, insulation_type=_FILLED_CAVITY
+    ):
+        return None
+
     product = products.get(_CAVITY_MEASURE_TYPE)
     wall_area: float = gross_heat_loss_wall_area(epc, BuildingPartIdentifier.MAIN)
     cost = Cost(

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

scripts/run_modelling_e2e.py

@@ -910,10 +910,12 @@ def main() -> None:
                 print_table=False,
             )
             # The full candidate menu (every Generator Option + its cost), so
-            # measures the Optimiser did not select are still visible. A predicted
-            # Property has no lodged cert, so the synthesised Effective EPC is used.
+            # measures the Optimiser did not select are still visible. Generated
+            # from the Effective EPC — the same picture the plan above modelled;
+            # the lodged cert would print a menu the Optimiser never saw (e.g.
+            # cavity fill on a wall an override moved to timber frame).
             candidates: list[Recommendation] = candidate_recommendations(
-                epc if epc is not None else effective_epc,
+                effective_epc,
                 planning_restrictions=restrictions,
                 solar_insights=solar_insights,
                 considered_measures=considered,

tests/domain/epc/test_main_heating_system_overlay.py

@@ -233,6 +233,80 @@ def test_off_peak_underfloor_asserts_a_dual_meter(main_heating_value: str) -> No
     assert simulation.heating.meter_type == "Dual"
 
 
+@pytest.mark.parametrize(
+    "main_heating_value",
+    [
+        "Electric underfloor, in concrete slab (off-peak)",
+        "Electric underfloor, integrated storage and direct-acting",
+        "Electric underfloor, in screed above insulation",
+    ],
+)
+def test_electric_underfloor_overlays_the_underfloor_category(
+    main_heating_value: str,
+) -> None:
+    # A system-replacing override must stamp the underfloor heating category
+    # (SAP Table 4a Category 8). Leaving it unset lets the replaced system's
+    # category survive onto the effective cert — property 711795's community
+    # heating (category 6) survived an underfloor override, so the heating
+    # generator read the dwelling as a heat network and offered no heating
+    # upgrade at all, and the baseline mis-rated 51 vs 67 (ADR-0050).
+    # Act
+    simulation = main_heating_overlay_for(main_heating_value, 0)
+
+    # Assert — SAP Table 4a Category 8 (Electric underfloor heating).
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.main_heating_category == 8
+
+
+@pytest.mark.parametrize(
+    "main_heating_value",
+    [
+        "Electric underfloor, in concrete slab (off-peak)",
+        "Electric underfloor, integrated storage and direct-acting",
+        "Electric underfloor, in screed above insulation",
+    ],
+)
+def test_electric_underfloor_drags_the_conservative_group_7_control(
+    main_heating_value: str,
+) -> None:
+    # The landlord names the system, not its control, so the overlay assumes the
+    # conservative Table 4e Group 7 default: 2701 ("no time or thermostatic
+    # control", +0.3 C — the largest adjustment in the group, so an unobserved
+    # control is never over-credited). Mirrors storage-manual 2401 / room-heater
+    # 2601; also overwrites a stale control inherited from the replaced system
+    # (711795 kept a community control 2311) — ADR-0050.
+    # Act
+    simulation = main_heating_overlay_for(main_heating_value, 0)
+
+    # Assert — SAP Table 4e Group 7 conservative underfloor control.
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.main_heating_control == 2701
+
+
+@pytest.mark.parametrize(
+    "main_heating_value",
+    [
+        "Electric underfloor, in concrete slab (off-peak)",
+        "Electric underfloor, integrated storage and direct-acting",
+        "Electric underfloor, in screed above insulation",
+    ],
+)
+def test_complete_underfloor_overlay_does_not_log_a_companion_gap(
+    main_heating_value: str, caplog: pytest.LogCaptureFixture
+) -> None:
+    # Electric underfloor now resolves a full companion set (category 8 +
+    # control 2701, ADR-0050), so the overlay must NOT log an
+    # incomplete-companion gap — guarding that the set stays complete.
+    # Act
+    with caplog.at_level(logging.ERROR):
+        main_heating_overlay_for(main_heating_value, 0)
+
+    # Assert
+    assert "incomplete companion set" not in caplog.text
+
+
 def test_screed_underfloor_defers_the_meter_to_the_cert() -> None:
     # "In screed above insulation (standard or off peak)" is tariff-ambiguous (81%
     # of the corpus lodge off-peak), so the archetype cannot know the meter — it

tests/domain/modelling/test_wall_recommendation.py

@@ -90,6 +90,69 @@ def test_non_cavity_main_wall_yields_no_recommendation() -> None:
     assert recommendation is None
 
 
+def test_at_regs_age_band_cavity_yields_no_cavity_fill_recommendation() -> None:
+    # Arrange — a cavity wall lodged as-built at construction age band L
+    # (2012-2022): the as-built cascade already assigns the at-regs U-value, so
+    # filling the cavity cannot lower it. The Wall U-Value Gate withholds the
+    # Option (ADR-0051); the legacy insulation-state trigger alone would offer
+    # a paid no-op.
+    baseline: EpcPropertyData = build_epc()  # MAIN: cavity (4), uninsulated (4)
+    _part(baseline, BuildingPartIdentifier.MAIN).construction_age_band = "L"
+
+    # Act
+    recommendation: Recommendation | None = recommend_cavity_wall(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert recommendation is None
+
+
+def test_at_regs_age_band_wall_yields_no_solid_wall_recommendation() -> None:
+    # Arrange — a timber-frame wall lodged as-built at construction age band L
+    # (2012-2022). RdSAP's cascade assigns it the at-regs U-value (0.28), and
+    # 100 mm IWI resolves to the SAME value — the measure cannot change the
+    # modelled wall, so offering it sells £4k of insulation for +0.00 SAP
+    # (property 711795, PRD #1435 WS2). The Wall U-Value Gate withholds it
+    # (ADR-0051).
+    baseline: EpcPropertyData = build_epc()
+    main: SapBuildingPart = _part(baseline, BuildingPartIdentifier.MAIN)
+    main.wall_construction = 5  # timber frame — IWI-only construction
+    main.wall_insulation_type = 4  # as-built / uninsulated — the legacy trigger
+    main.construction_age_band = "L"
+
+    # Act
+    recommendation: Recommendation | None = recommend_solid_wall(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert recommendation is None
+
+
+def test_treatable_old_band_solid_wall_keeps_its_insulation_options() -> None:
+    # Arrange — solid brick as-built at age band B: the cascade U (~2.1) far
+    # exceeds what 100 mm EWI/IWI achieves, so the Wall U-Value Gate must let
+    # both Options through unchanged (the gate is physical possibility only —
+    # ADR-0051).
+    baseline: EpcPropertyData = build_epc()  # age band B
+    main: SapBuildingPart = _part(baseline, BuildingPartIdentifier.MAIN)
+    main.wall_construction = 3  # solid brick — EWI + IWI constructable
+    main.wall_insulation_type = 4
+
+    # Act
+    recommendation: Recommendation | None = recommend_solid_wall(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert recommendation is not None
+    assert {option.measure_type for option in recommendation.options} == {
+        "external_wall_insulation",
+        "internal_wall_insulation",
+    }
+
+
 def test_park_home_wall_yields_no_solid_wall_recommendation() -> None:
     # Arrange — a park home (wall_construction code 8) with an uninsulated
     # as-built wall. Code 8 is NOT system-built (ADR-0019); a park home's