diff --git a/domain/modelling/generators/floor_recommendation.py b/domain/modelling/generators/floor_recommendation.py
index b078a230..f4bd3296 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.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
 from repositories.product.product_repository import ProductRepository
@@ -44,9 +45,9 @@ def _floor_measure_type(construction_type: Optional[str]) -> Optional[str]:
     None when the construction is not a treatable suspended/solid floor."""
     text = (construction_type or "").lower()
     if "suspended" in text:
-        return "suspended_floor_insulation"
+        return MeasureType.SUSPENDED_FLOOR_INSULATION
     if "solid" in text:
-        return "solid_floor_insulation"
+        return MeasureType.SOLID_FLOOR_INSULATION
     return None
 
 
diff --git a/domain/modelling/generators/glazing_recommendation.py b/domain/modelling/generators/glazing_recommendation.py
index ac6c371c..5f0bb006 100644
--- a/domain/modelling/generators/glazing_recommendation.py
+++ b/domain/modelling/generators/glazing_recommendation.py
@@ -23,6 +23,7 @@ from typing import Final, Optional
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
 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.simulation import EpcSimulation, WindowOverlay
 from repositories.product.product_repository import ProductRepository
@@ -52,7 +53,7 @@ class _GlazingTarget:
 # 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",
+    measure_type=MeasureType.DOUBLE_GLAZING,
     description="Replace the single-glazed windows with double glazing",
     glazing_type=5,
     u_value=1.40,
@@ -64,7 +65,7 @@ _DOUBLE: Final[_GlazingTarget] = _GlazingTarget(
 # solar gain). The external units can't be replaced on a protected/over-looked
 # building, so this is the planning-picked Measure.
 _SECONDARY: Final[_GlazingTarget] = _GlazingTarget(
-    measure_type="secondary_glazing",
+    measure_type=MeasureType.SECONDARY_GLAZING,
     description="Fit secondary glazing to the single-glazed windows",
     glazing_type=11,
     u_value=2.90,
diff --git a/domain/modelling/generators/heating_recommendation.py b/domain/modelling/generators/heating_recommendation.py
index c1ec0f3f..1c851541 100644
--- a/domain/modelling/generators/heating_recommendation.py
+++ b/domain/modelling/generators/heating_recommendation.py
@@ -18,14 +18,15 @@ from datatypes.epc.domain.epc_property_data import EpcPropertyData, MainHeatingD
 from datatypes.epc.domain.field_mappings import PROPERTY_TYPE_LOOKUP
 from domain.geospatial.planning_restrictions import PlanningRestrictions
 from domain.modelling.products import AshpCostInputs, AshpExistingSystem, Products
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import EpcSimulation, HeatingOverlay
 from repositories.product.product_repository import ProductRepository
 
 _HEATING_SURFACE = "Heating & Hot Water"
 
-_HHR_STORAGE_MEASURE_TYPE = "high_heat_retention_storage_heaters"
-_ASHP_MEASURE_TYPE = "air_source_heat_pump"
+_HHR_STORAGE_MEASURE_TYPE = MeasureType.HIGH_HEAT_RETENTION_STORAGE_HEATERS
+_ASHP_MEASURE_TYPE = MeasureType.AIR_SOURCE_HEAT_PUMP
 
 # Electricity main-fuel code (Elmhurst → SAP10 Table 12).
 _ELECTRICITY_FUEL = 30
diff --git a/domain/modelling/generators/lighting_recommendation.py b/domain/modelling/generators/lighting_recommendation.py
index e6c7df63..d1628461 100644
--- a/domain/modelling/generators/lighting_recommendation.py
+++ b/domain/modelling/generators/lighting_recommendation.py
@@ -18,11 +18,12 @@ scoring (ADR-0016).
 from typing import Final, Optional
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import EpcSimulation, LightingOverlay
 from repositories.product.product_repository import ProductRepository
 
-_LIGHTING_MEASURE_TYPE: Final[str] = "low_energy_lighting"
+_LIGHTING_MEASURE_TYPE: Final[MeasureType] = MeasureType.LOW_ENERGY_LIGHTING
 
 
 def recommend_lighting(
diff --git a/domain/modelling/generators/roof_recommendation.py b/domain/modelling/generators/roof_recommendation.py
index 0d4f356d..a19921a5 100644
--- a/domain/modelling/generators/roof_recommendation.py
+++ b/domain/modelling/generators/roof_recommendation.py
@@ -17,12 +17,13 @@ from datatypes.epc.domain.epc_property_data import (
     EpcPropertyData,
 )
 from domain.building_geometry import roof_area
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
 from repositories.product.product_repository import ProductRepository
 
-_LOFT_MEASURE_TYPE = "loft_insulation"
-_SLOPING_CEILING_MEASURE_TYPE = "sloping_ceiling_insulation"
+_LOFT_MEASURE_TYPE = MeasureType.LOFT_INSULATION
+_SLOPING_CEILING_MEASURE_TYPE = MeasureType.SLOPING_CEILING_INSULATION
 # RdSAP 10 Table 16: 0 mm lodged roof insulation is an uninsulated loft. The
 # Elmhurst mapper resolves "As Built" to 0 for pitched/sloping/loft roofs.
 _ROOF_UNINSULATED_MM = 0
@@ -32,7 +33,7 @@ _ROOF_UNINSULATED_MM = 0
 _RECOMMENDED_LOFT_THICKNESS_MM = 300
 # Recommended sloping-ceiling depth (mm); Elmhurst re-lodges 100 mm (ADR-0021).
 _RECOMMENDED_SLOPING_CEILING_THICKNESS_MM = 100
-_FLAT_ROOF_MEASURE_TYPE = "flat_roof_insulation"
+_FLAT_ROOF_MEASURE_TYPE = MeasureType.FLAT_ROOF_INSULATION
 # Recommended flat-roof depth (mm); Elmhurst re-lodges 200 mm (ADR-0021).
 _RECOMMENDED_FLAT_ROOF_THICKNESS_MM = 200
 
diff --git a/domain/modelling/generators/solar_recommendation.py b/domain/modelling/generators/solar_recommendation.py
index 842e5a99..b74c3b63 100644
--- a/domain/modelling/generators/solar_recommendation.py
+++ b/domain/modelling/generators/solar_recommendation.py
@@ -24,6 +24,7 @@ from datatypes.epc.domain.epc_property_data import (
 from datatypes.epc.domain.field_mappings import PROPERTY_TYPE_LOOKUP
 from domain.geospatial.planning_restrictions import PlanningRestrictions
 from domain.modelling.products import Products, SolarCostInputs
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import EpcSimulation, SolarOverlay
 from domain.modelling.solar_potential import (
@@ -37,7 +38,7 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
 from repositories.product.product_repository import ProductRepository
 
 _SOLAR_SURFACE = "Solar PV"
-_SOLAR_MEASURE_TYPE = "solar_pv"
+_SOLAR_MEASURE_TYPE = MeasureType.SOLAR_PV
 
 # The fixed, representative battery capacity for the with-battery variant
 # (ADR-0026) — a flagged estimate (see the rate sheet), 5 kWh.
diff --git a/domain/modelling/generators/solid_wall_recommendation.py b/domain/modelling/generators/solid_wall_recommendation.py
index 3454341e..49addfa5 100644
--- a/domain/modelling/generators/solid_wall_recommendation.py
+++ b/domain/modelling/generators/solid_wall_recommendation.py
@@ -24,12 +24,13 @@ from datatypes.epc.domain.epc_property_data import (
 from datatypes.epc.domain.field_mappings import PROPERTY_TYPE_LOOKUP
 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.simulation import BuildingPartOverlay, EpcSimulation
 from repositories.product.product_repository import ProductRepository
 
-_EXTERNAL_MEASURE_TYPE: Final[str] = "external_wall_insulation"
-_INTERNAL_MEASURE_TYPE: Final[str] = "internal_wall_insulation"
+_EXTERNAL_MEASURE_TYPE: Final[MeasureType] = MeasureType.EXTERNAL_WALL_INSULATION
+_INTERNAL_MEASURE_TYPE: Final[MeasureType] = MeasureType.INTERNAL_WALL_INSULATION
 
 # RdSAP `wall_construction` codes (consistent across paths for 1-5).
 _WALL_SOLID_BRICK: Final[int] = 3
diff --git a/domain/modelling/generators/ventilation_recommendation.py b/domain/modelling/generators/ventilation_recommendation.py
index 0fed5c7b..ff6d598c 100644
--- a/domain/modelling/generators/ventilation_recommendation.py
+++ b/domain/modelling/generators/ventilation_recommendation.py
@@ -16,11 +16,12 @@ mirrors legacy ``Property.has_ventilation``.
 from typing import Optional
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import EpcSimulation, VentilationOverlay
 from repositories.product.product_repository import ProductRepository
 
-_VENTILATION_MEASURE_TYPE = "mechanical_ventilation"
+_VENTILATION_MEASURE_TYPE = MeasureType.MECHANICAL_VENTILATION
 
 # The SAP10.2 §2 mechanical-ventilation kind installed: decentralised MEV
 # ("mechanical extract, decentralised (MEV dc)" → MechanicalVentilationKind
diff --git a/domain/modelling/generators/wall_recommendation.py b/domain/modelling/generators/wall_recommendation.py
index 86c81f41..2c562241 100644
--- a/domain/modelling/generators/wall_recommendation.py
+++ b/domain/modelling/generators/wall_recommendation.py
@@ -12,11 +12,12 @@ from datatypes.epc.domain.epc_property_data import (
     EpcPropertyData,
 )
 from domain.building_geometry import gross_heat_loss_wall_area
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
 from repositories.product.product_repository import ProductRepository
 
-_CAVITY_MEASURE_TYPE = "cavity_wall_insulation"
+_CAVITY_MEASURE_TYPE = MeasureType.CAVITY_WALL_INSULATION
 
 # RdSAP 10 Table 5 wall_construction: 4 = "Cavity". Table 6
 # wall_insulation_type: 4 = "as-built / assumed" (uninsulated), 2 = "Filled
diff --git a/domain/modelling/measure_type.py b/domain/modelling/measure_type.py
new file mode 100644
index 00000000..d28bff39
--- /dev/null
+++ b/domain/modelling/measure_type.py
@@ -0,0 +1,35 @@
+"""MeasureType — the canonical vocabulary of the measures the Modelling stage
+models.
+
+One member per Recommendation Generator option. A ``StrEnum`` so each member
+*is* its string value: it persists straight into the ``recommendation`` varchar
+column, is the optimiser's group-by key, and compares equal to the raw strings
+the catalogue and EPC carry — so it can replace the per-generator string
+constants as the single source of truth without a persistence or optimiser
+change. It is also the vocabulary the ``considered_measures`` allowlist speaks
+(mirroring the legacy engine's ``inclusions``).
+"""
+
+from __future__ import annotations
+
+from enum import StrEnum
+
+
+class MeasureType(StrEnum):
+    """A measure the Modelling stage can recommend (CONTEXT.md)."""
+
+    CAVITY_WALL_INSULATION = "cavity_wall_insulation"
+    EXTERNAL_WALL_INSULATION = "external_wall_insulation"
+    INTERNAL_WALL_INSULATION = "internal_wall_insulation"
+    LOFT_INSULATION = "loft_insulation"
+    SLOPING_CEILING_INSULATION = "sloping_ceiling_insulation"
+    FLAT_ROOF_INSULATION = "flat_roof_insulation"
+    SUSPENDED_FLOOR_INSULATION = "suspended_floor_insulation"
+    SOLID_FLOOR_INSULATION = "solid_floor_insulation"
+    DOUBLE_GLAZING = "double_glazing"
+    SECONDARY_GLAZING = "secondary_glazing"
+    LOW_ENERGY_LIGHTING = "low_energy_lighting"
+    MECHANICAL_VENTILATION = "mechanical_ventilation"
+    HIGH_HEAT_RETENTION_STORAGE_HEATERS = "high_heat_retention_storage_heaters"
+    AIR_SOURCE_HEAT_PUMP = "air_source_heat_pump"
+    SOLAR_PV = "solar_pv"
diff --git a/tests/domain/modelling/test_measure_type.py b/tests/domain/modelling/test_measure_type.py
new file mode 100644
index 00000000..60c1a816
--- /dev/null
+++ b/tests/domain/modelling/test_measure_type.py
@@ -0,0 +1,52 @@
+"""Slice 1 — `MeasureType`, the canonical enum of the measures the Modelling
+stage actually models.
+
+A `StrEnum` so each member *is* its persisted/optimiser string value (e.g.
+`MeasureType.SOLAR_PV == "solar_pv"`), letting it flow through the `recommendation`
+varchar column and the optimiser's group-by-type key unchanged. Replaces the
+per-generator string constants as the single source of truth, and is the
+vocabulary the `considered_measures` allowlist speaks.
+"""
+
+from domain.modelling.measure_type import MeasureType
+
+# The full set of measures the generators emit today (one per Generator option).
+_EXPECTED_VALUES = {
+    "cavity_wall_insulation",
+    "external_wall_insulation",
+    "internal_wall_insulation",
+    "loft_insulation",
+    "sloping_ceiling_insulation",
+    "flat_roof_insulation",
+    "suspended_floor_insulation",
+    "solid_floor_insulation",
+    "double_glazing",
+    "secondary_glazing",
+    "low_energy_lighting",
+    "mechanical_ventilation",
+    "high_heat_retention_storage_heaters",
+    "air_source_heat_pump",
+    "solar_pv",
+}
+
+
+def test_members_cover_exactly_the_modelled_measures() -> None:
+    # Arrange / Act
+    values = {member.value for member in MeasureType}
+
+    # Assert
+    assert values == _EXPECTED_VALUES
+
+
+def test_member_is_its_string_value() -> None:
+    # Arrange / Act / Assert — a StrEnum member compares and persists as its str.
+    assert MeasureType.SOLAR_PV == "solar_pv"
+    assert MeasureType.HIGH_HEAT_RETENTION_STORAGE_HEATERS == (
+        "high_heat_retention_storage_heaters"
+    )
+    assert isinstance(MeasureType.SOLAR_PV, str)
+
+
+def test_round_trips_through_its_value() -> None:
+    # Arrange / Act / Assert — a raw catalogue/DB string maps back to the member.
+    assert MeasureType("cavity_wall_insulation") is MeasureType.CAVITY_WALL_INSULATION