diff --git a/domain/modelling/generators/glazing_recommendation.py b/domain/modelling/generators/glazing_recommendation.py
new file mode 100644
index 00000000..d4b81ab7
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+++ b/domain/modelling/generators/glazing_recommendation.py
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+"""The glazing Recommendation Generator (double / secondary glazing).
+
+Detects a dwelling's single-glazed windows and emits one "Windows"
+Recommendation carrying a single, planning-picked Measure Option (ADR-0022).
+Unlike the wall generator's competing EWI/IWI Options, the Property's
+`PlanningRestrictions` *hard-picks* the Measure: an unrestricted dwelling gets
+`double_glazing`; any conservation/listed/heritage protection (i.e.
+`blocks_external`) forces `secondary_glazing` — an internal second pane that
+leaves the protected external units untouched.
+
+All single-glazed windows are upgraded together in one overlay. The overlay
+writes each window's lodged `u_value` and `solar_transmittance` (not just
+`glazing_type`), because our calculator reads those per-window values directly
+from `WindowTransmissionDetails` rather than deriving them from the glazing
+type (`heat_transmission.py:490`, `solar_gains.py:300`); `glazing_type` is set
+too, for the §5 daylight factor. The target values are pinned from cert 001431's
+before→after re-lodgement. Detection + pricing only; impact is produced later by
+scoring (ADR-0016).
+"""
+
+from dataclasses import dataclass
+from typing import Final, Optional
+
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.geospatial.planning_restrictions import PlanningRestrictions
+from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
+from domain.modelling.simulation import EpcSimulation, WindowOverlay
+from repositories.product.product_repository import ProductRepository
+
+# SAP10.2 Table U2 code 1 = single glazing — the only windows this generator
+# upgrades (the Elmhurst mapper sends "Single"/"Single glazing" → 1 and the API
+# passes int 1, so the trigger is path-consistent).
+_SINGLE_GLAZED: Final[int] = 1
+
+
+@dataclass(frozen=True)
+class _GlazingTarget:
+    """The planning-picked Measure and the per-window values its overlay lodges,
+    pinned from cert 001431's before→after (`glazing_type`, `u_value`,
+    `solar_transmittance` — SAP10.2 Table U2 code, then heat-loss U and solar g).
+    """
+
+    measure_type: str
+    description: str
+    glazing_type: int
+    u_value: float
+    solar_transmittance: float
+
+
+# Unrestricted: replace the units with double glazing (gt=5 "Double post 2022";
+# U 4.80→1.40, g 0.85→0.72).
+_DOUBLE: Final[_GlazingTarget] = _GlazingTarget(
+    measure_type="double_glazing",
+    description="Replace the single-glazed windows with double glazing",
+    glazing_type=5,
+    u_value=1.40,
+    solar_transmittance=0.72,
+)
+
+
+def recommend_glazing(
+    epc: EpcPropertyData,
+    products: ProductRepository,
+    restrictions: PlanningRestrictions = PlanningRestrictions(),
+) -> Optional[Recommendation]:
+    """Return a glazing Recommendation upgrading every single-glazed window with
+    double glazing (its single Option), else None when the dwelling has no
+    single-glazed windows."""
+    single_indices = tuple(
+        index
+        for index, window in enumerate(epc.sap_windows)
+        if window.glazing_type == _SINGLE_GLAZED
+    )
+    if not single_indices:
+        return None
+
+    target: _GlazingTarget = _DOUBLE
+    product = products.get(target.measure_type)
+
+    overlay = EpcSimulation(
+        windows={
+            index: WindowOverlay(
+                glazing_type=target.glazing_type,
+                u_value=target.u_value,
+                solar_transmittance=target.solar_transmittance,
+            )
+            for index in single_indices
+        }
+    )
+    cost = Cost(
+        total=len(single_indices) * product.unit_cost_per_m2,
+        contingency_rate=product.contingency_rate,
+    )
+    option = MeasureOption(
+        measure_type=target.measure_type,
+        description=target.description,
+        overlay=overlay,
+        cost=cost,
+        material_id=product.id,
+    )
+    return Recommendation(surface="Windows", options=(option,))
diff --git a/tests/domain/modelling/test_glazing_recommendation.py b/tests/domain/modelling/test_glazing_recommendation.py
new file mode 100644
index 00000000..58ddb61a
--- /dev/null
+++ b/tests/domain/modelling/test_glazing_recommendation.py
@@ -0,0 +1,103 @@
+"""Behaviour of the glazing Recommendation Generator: detecting single-glazed
+windows and emitting one planning-picked "Windows" Recommendation (ADR-0022).
+
+Unrestricted dwellings get `double_glazing`; a planning-protected dwelling gets
+`secondary_glazing` instead — the measure is hard-picked by the Property's
+`PlanningRestrictions`, not offered as competing Options. All single-glazed
+windows are upgraded together in one overlay; the overlay writes each window's
+lodged U-value and g-value (not just `glazing_type`) because the calculator
+consumes those per-window values directly. Detection + pricing only, no scores
+(ADR-0016). The before/after cascade pins live in `test_elmhurst_cascade_pins`.
+"""
+
+import copy
+
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.modelling.generators.glazing_recommendation import recommend_glazing
+from domain.modelling.product import Product
+from domain.modelling.recommendation import Recommendation
+from domain.modelling.simulation import WindowOverlay
+from repositories.product.product_repository import ProductRepository
+from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
+    build_epc,
+)
+
+# SAP10.2 Table U2 code 1 = single glazing (the generator's trigger).
+_SINGLE_GLAZED = 1
+
+
+class _StubProducts(ProductRepository):
+    """In-memory ProductRepository returning a fixed per-window glazing price.
+
+    `unit_cost_per_m2` carries the catalogue row's fully-loaded total, reused as
+    a flat average price per window (ADR-0022)."""
+
+    def get(self, measure_type: str) -> Product:
+        return Product(
+            measure_type=measure_type,
+            unit_cost_per_m2=600.0,
+            contingency_rate=0.2,
+            id=42,
+        )
+
+
+def _with_single_glazed(epc: EpcPropertyData, *indices: int) -> EpcPropertyData:
+    """Return a copy of `epc` with the windows at `indices` set single-glazed."""
+    clone: EpcPropertyData = copy.deepcopy(epc)
+    for index in indices:
+        clone.sap_windows[index].glazing_type = _SINGLE_GLAZED
+    return clone
+
+
+def test_single_glazed_dwelling_yields_a_double_glazing_recommendation() -> None:
+    # Arrange — 000490 is all double-glazed; make windows 0 and 2 single-glazed.
+    baseline: EpcPropertyData = _with_single_glazed(build_epc(), 0, 2)
+
+    # Act
+    recommendation: Recommendation | None = recommend_glazing(
+        baseline, _StubProducts()
+    )
+
+    # Assert — one double-glazing Option whose overlay rewrites each single-
+    # glazed window to the pinned double target (gt=5, U=1.40, g=0.72).
+    assert recommendation is not None
+    assert recommendation.surface == "Windows"
+    assert len(recommendation.options) == 1
+    option = recommendation.options[0]
+    assert option.measure_type == "double_glazing"
+    assert dict(option.overlay.windows) == {
+        0: WindowOverlay(glazing_type=5, u_value=1.40, solar_transmittance=0.72),
+        2: WindowOverlay(glazing_type=5, u_value=1.40, solar_transmittance=0.72),
+    }
+
+
+def test_fully_glazed_dwelling_yields_no_recommendation() -> None:
+    # Arrange — 000490 is entirely double-glazed (gt=2); nothing to upgrade.
+    baseline: EpcPropertyData = build_epc()
+
+    # Act
+    recommendation: Recommendation | None = recommend_glazing(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert recommendation is None
+
+
+def test_recommendation_prices_a_flat_average_per_single_glazed_window() -> None:
+    # Arrange — three single-glazed windows at £600 each (a flat per-window
+    # average reused from `unit_cost_per_m2`, ADR-0022).
+    baseline: EpcPropertyData = _with_single_glazed(build_epc(), 0, 2, 4)
+
+    # Act
+    recommendation: Recommendation | None = recommend_glazing(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert recommendation is not None
+    cost = recommendation.options[0].cost
+    assert cost is not None
+    assert cost.total == 3 * 600.0
+    assert cost.contingency_rate == 0.2
+    assert recommendation.options[0].material_id == 42