diff --git a/domain/modelling/generators/floor_recommendation.py b/domain/modelling/generators/floor_recommendation.py
index f4bd3296..9e3815ba 100644
--- a/domain/modelling/generators/floor_recommendation.py
+++ b/domain/modelling/generators/floor_recommendation.py
@@ -40,7 +40,7 @@ def _is_uninsulated(thickness: Optional[Union[str, int]]) -> bool:
     return thickness.strip() in ("", "0")
 
 
-def _floor_measure_type(construction_type: Optional[str]) -> Optional[str]:
+def _floor_measure_type(construction_type: Optional[str]) -> Optional[MeasureType]:
     """Map the lodged floor construction to the insulation Measure Type, or
     None when the construction is not a treatable suspended/solid floor."""
     text = (construction_type or "").lower()
diff --git a/domain/modelling/generators/glazing_recommendation.py b/domain/modelling/generators/glazing_recommendation.py
index 5f0bb006..897b011c 100644
--- a/domain/modelling/generators/glazing_recommendation.py
+++ b/domain/modelling/generators/glazing_recommendation.py
@@ -43,7 +43,7 @@ class _GlazingTarget:
     `solar_transmittance` — SAP10.2 Table U2 code, then heat-loss U and solar g).
     """
 
-    measure_type: str
+    measure_type: MeasureType
     description: str
     glazing_type: int
     u_value: float
diff --git a/domain/modelling/generators/roof_recommendation.py b/domain/modelling/generators/roof_recommendation.py
index a19921a5..2595e3f9 100644
--- a/domain/modelling/generators/roof_recommendation.py
+++ b/domain/modelling/generators/roof_recommendation.py
@@ -110,7 +110,7 @@ def _roof_recommendation(
     epc: EpcPropertyData,
     products: ProductRepository,
     *,
-    measure_type: str,
+    measure_type: MeasureType,
     description: str,
     thickness_mm: int,
 ) -> Recommendation:
diff --git a/domain/modelling/generators/solid_wall_recommendation.py b/domain/modelling/generators/solid_wall_recommendation.py
index 49addfa5..dff55b66 100644
--- a/domain/modelling/generators/solid_wall_recommendation.py
+++ b/domain/modelling/generators/solid_wall_recommendation.py
@@ -57,7 +57,7 @@ _SOLID_WALL_INSULATION_MM: Final[int] = 100
 # Which solid-wall Options each construction can take (ADR-0019). Solid brick
 # and system-built take both; timber-frame takes IWI only (EWI not
 # constructable). The breathable cob/stone exclusions take neither (never keyed).
-_CONSTRUCTABLE_OPTIONS: Final[dict[int, tuple[str, ...]]] = {
+_CONSTRUCTABLE_OPTIONS: Final[dict[int, tuple[MeasureType, ...]]] = {
     _WALL_SOLID_BRICK: (_EXTERNAL_MEASURE_TYPE, _INTERNAL_MEASURE_TYPE),
     _WALL_SYSTEM_BUILT: (_EXTERNAL_MEASURE_TYPE, _INTERNAL_MEASURE_TYPE),
     _WALL_TIMBER_FRAME: (_INTERNAL_MEASURE_TYPE,),
@@ -75,7 +75,7 @@ _DESCRIPTION: Final[dict[str, str]] = {
 
 
 def _solid_wall_option(
-    epc: EpcPropertyData, products: ProductRepository, measure_type: str
+    epc: EpcPropertyData, products: ProductRepository, measure_type: MeasureType
 ) -> MeasureOption:
     """Build one solid-wall Measure Option: its insulation overlay (100 mm at the
     External/Internal `wall_insulation_type`) priced at the heat-loss wall area."""
diff --git a/domain/modelling/optimisation/measure_dependency.py b/domain/modelling/optimisation/measure_dependency.py
index 9df7d468..bd7e9d1f 100644
--- a/domain/modelling/optimisation/measure_dependency.py
+++ b/domain/modelling/optimisation/measure_dependency.py
@@ -24,17 +24,18 @@ from datatypes.epc.domain.epc_property_data import EpcPropertyData
 from domain.modelling.generators.ventilation_recommendation import (
     recommend_ventilation,
 )
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.optimisation.optimiser import MeasureDependency, ScoredOption
 from domain.modelling.recommendation import MeasureOption, Recommendation
 from repositories.product.product_repository import ProductRepository
 
 # The measure types that force a ventilation dependency (cf. legacy
 # `assumptions.measures_needing_ventilation`).
-MEASURES_NEEDING_VENTILATION: frozenset[str] = frozenset(
+MEASURES_NEEDING_VENTILATION: frozenset[MeasureType] = frozenset(
     {
-        "cavity_wall_insulation",
-        "internal_wall_insulation",
-        "external_wall_insulation",
+        MeasureType.CAVITY_WALL_INSULATION,
+        MeasureType.INTERNAL_WALL_INSULATION,
+        MeasureType.EXTERNAL_WALL_INSULATION,
     }
 )
 
diff --git a/domain/modelling/optimisation/optimiser.py b/domain/modelling/optimisation/optimiser.py
index de5e0225..0e577657 100644
--- a/domain/modelling/optimisation/optimiser.py
+++ b/domain/modelling/optimisation/optimiser.py
@@ -24,6 +24,7 @@ from dataclasses import dataclass
 from typing import Optional, Protocol, Sequence
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.scoring.package_scorer import Score
 from domain.modelling.recommendation import MeasureOption
 from domain.modelling.simulation import EpcSimulation
@@ -48,7 +49,7 @@ class MeasureDependency:
     figure, the repair decision, and the persisted package. Held as data so
     extending the triggers is a data edit, not control flow."""
 
-    triggers: frozenset[str]
+    triggers: frozenset[MeasureType]
     required: ScoredOption
 
 
@@ -300,11 +301,11 @@ def _inject(
     """``chosen`` plus every forced dependency whose triggers intersect the
     chosen measure-types, de-duplicated by required measure-type (a dependency
     several measures trigger is injected once)."""
-    chosen_types: set[str] = {s.option.measure_type for s in chosen}
+    chosen_types: set[MeasureType] = {s.option.measure_type for s in chosen}
     injected: list[ScoredOption] = list(chosen)
-    present: set[str] = set(chosen_types)
+    present: set[MeasureType] = set(chosen_types)
     for dependency in dependencies:
-        required_type: str = dependency.required.option.measure_type
+        required_type: MeasureType = dependency.required.option.measure_type
         if dependency.triggers & chosen_types and required_type not in present:
             injected.append(dependency.required)
             present.add(required_type)
diff --git a/domain/modelling/plan.py b/domain/modelling/plan.py
index 8483359f..9e8349e9 100644
--- a/domain/modelling/plan.py
+++ b/domain/modelling/plan.py
@@ -15,6 +15,7 @@ from typing import Optional
 
 from datatypes.epc.domain.epc import Epc
 from domain.billing.bill import Bill
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.scoring.package_scorer import Score
 from domain.modelling.recommendation import Cost
 from domain.modelling.scoring.scoring import MeasureImpact
@@ -33,7 +34,7 @@ class PlanMeasure:
     billing has run (persisted as NULL — ADR-0014 amendment). They are distinct
     from `impact.energy_savings_kwh_per_yr`, which is *primary* energy."""
 
-    measure_type: str
+    measure_type: MeasureType
     description: str
     cost: Cost
     impact: MeasureImpact
diff --git a/domain/modelling/recommendation.py b/domain/modelling/recommendation.py
index 96331c44..5f9b10a5 100644
--- a/domain/modelling/recommendation.py
+++ b/domain/modelling/recommendation.py
@@ -11,6 +11,7 @@ CONTEXT.md.
 from dataclasses import dataclass
 from typing import Optional
 
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.simulation import EpcSimulation
 
 
@@ -28,7 +29,7 @@ class Cost:
 class MeasureOption:
     """One mutually-exclusive way to treat a Recommendation's surface."""
 
-    measure_type: str
+    measure_type: MeasureType
     description: str
     overlay: EpcSimulation
     cost: Optional[Cost] = None
diff --git a/tests/domain/modelling/test_heating_recommendation.py b/tests/domain/modelling/test_heating_recommendation.py
index 50344d14..9a0cf285 100644
--- a/tests/domain/modelling/test_heating_recommendation.py
+++ b/tests/domain/modelling/test_heating_recommendation.py
@@ -55,7 +55,7 @@ def test_electric_storage_dwelling_yields_an_hhr_storage_bundle() -> None:
     # bundle, whose overlay is the absolute HHR end-state.
     assert recommendation is not None
     assert recommendation.surface == "Heating & Hot Water"
-    options = {o.measure_type: o for o in recommendation.options}
+    options = {o.measure_type.value: o for o in recommendation.options}
     assert "high_heat_retention_storage_heaters" in options
     assert options["high_heat_retention_storage_heaters"].overlay.heating == HeatingOverlay(
         main_fuel_type=30,
@@ -150,7 +150,7 @@ def test_gas_boiler_house_yields_an_ashp_bundle() -> None:
 
     # Assert — the ASHP bundle carries the absolute heat-pump end-state.
     assert recommendation is not None
-    options = {o.measure_type: o for o in recommendation.options}
+    options = {o.measure_type.value: o for o in recommendation.options}
     assert "air_source_heat_pump" in options
     assert options["air_source_heat_pump"].overlay.heating == HeatingOverlay(
         main_fuel_type=30,
diff --git a/tests/domain/modelling/test_optimiser.py b/tests/domain/modelling/test_optimiser.py
index 333909d0..80f39d7e 100644
--- a/tests/domain/modelling/test_optimiser.py
+++ b/tests/domain/modelling/test_optimiser.py
@@ -22,6 +22,7 @@ from domain.modelling.optimisation.optimiser import (
     optimise_min_cost,
     optimise_package,
 )
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.scoring.package_scorer import Score
 from domain.modelling.recommendation import Cost, MeasureOption
 from domain.modelling.simulation import (
@@ -37,7 +38,7 @@ from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
 def _scored(measure_type: str, *, gain: float, cost: float) -> ScoredOption:
     return ScoredOption(
         option=MeasureOption(
-            measure_type=measure_type,
+            measure_type=MeasureType(measure_type),
             description=measure_type,
             overlay=EpcSimulation(),
             cost=Cost(total=cost, contingency_rate=0.0),
@@ -70,7 +71,7 @@ def _scored_overlay(
 ) -> ScoredOption:
     return ScoredOption(
         option=MeasureOption(
-            measure_type=measure_type,
+            measure_type=MeasureType(measure_type),
             description=measure_type,
             overlay=overlay,
             cost=Cost(total=cost, contingency_rate=0.0),
@@ -524,10 +525,12 @@ class _VentStubScorer:
 def _ventilation_dependency(*, cost: float) -> MeasureDependency:
     """A forced 'fabric requires ventilation' edge for the tests."""
     return MeasureDependency(
-        triggers=frozenset({"cavity_wall_insulation", "external_wall_insulation"}),
+        triggers=frozenset(
+            {MeasureType.CAVITY_WALL_INSULATION, MeasureType.EXTERNAL_WALL_INSULATION}
+        ),
         required=ScoredOption(
             option=MeasureOption(
-                measure_type="mechanical_ventilation",
+                measure_type=MeasureType.MECHANICAL_VENTILATION,
                 description="mechanical_ventilation",
                 overlay=_VENT_OVERLAY,
                 cost=Cost(total=cost, contingency_rate=0.0),
@@ -549,10 +552,10 @@ def test_min_cost_warm_start_avoids_a_wall_whose_forced_ventilation_dooms_it() -
     ]
     scorer = _VentStubScorer(base=60.0, wall=6.0, roof=8.0, vent=-5.0)
     dependency = MeasureDependency(
-        triggers=frozenset({"cavity_wall_insulation"}),
+        triggers=frozenset({MeasureType.CAVITY_WALL_INSULATION}),
         required=ScoredOption(
             option=MeasureOption(
-                measure_type="mechanical_ventilation",
+                measure_type=MeasureType.MECHANICAL_VENTILATION,
                 description="mechanical_ventilation",
                 overlay=_VENT_OVERLAY,
                 cost=Cost(total=300.0, contingency_rate=0.0),
diff --git a/tests/domain/modelling/test_plan.py b/tests/domain/modelling/test_plan.py
index 678d281f..1b191202 100644
--- a/tests/domain/modelling/test_plan.py
+++ b/tests/domain/modelling/test_plan.py
@@ -9,6 +9,7 @@ from __future__ import annotations
 
 from datatypes.epc.domain.epc import Epc
 from domain.billing.bill import Bill, BillSection, BillSectionCost
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.scoring.package_scorer import Score
 from domain.modelling.plan import Plan, PlanMeasure
 from domain.modelling.recommendation import Cost
@@ -28,7 +29,7 @@ def _bill(*, heating_kwh: float, total_gbp: float) -> Bill:
 
 def _measure(measure_type: str, total: float, rate: float) -> PlanMeasure:
     return PlanMeasure(
-        measure_type=measure_type,
+        measure_type=MeasureType(measure_type),
         description=measure_type.replace("_", " "),
         cost=Cost(total=total, contingency_rate=rate),
         impact=MeasureImpact(
diff --git a/tests/domain/modelling/test_scoring.py b/tests/domain/modelling/test_scoring.py
index ab55706a..61cfb454 100644
--- a/tests/domain/modelling/test_scoring.py
+++ b/tests/domain/modelling/test_scoring.py
@@ -10,6 +10,7 @@ from datatypes.epc.domain.epc_property_data import (
     BuildingPartIdentifier,
     EpcPropertyData,
 )
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.scoring.package_scorer import PackageScorer, Score
 from domain.modelling.recommendation import MeasureOption
 from domain.modelling.scoring.scoring import (
@@ -59,7 +60,7 @@ class _CountingScorer(PackageScorer):
 
 def _option(overlay: EpcSimulation) -> MeasureOption:
     return MeasureOption(
-        measure_type="cavity_wall_insulation", description="opt", overlay=overlay
+        measure_type=MeasureType.CAVITY_WALL_INSULATION, description="opt", overlay=overlay
     )
 
 
diff --git a/tests/harness/test_plan_table.py b/tests/harness/test_plan_table.py
index fa7ac6e6..51031151 100644
--- a/tests/harness/test_plan_table.py
+++ b/tests/harness/test_plan_table.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.plan import Plan, PlanMeasure
 from domain.modelling.recommendation import Cost
 from domain.modelling.scoring.package_scorer import Score
@@ -18,7 +19,7 @@ def _plan() -> Plan:
     )
     measures = (
         PlanMeasure(
-            measure_type="cavity_wall_insulation",
+            measure_type=MeasureType.CAVITY_WALL_INSULATION,
             description="Cavity wall insulation",
             cost=Cost(total=500.0, contingency_rate=0.1),
             impact=MeasureImpact(
@@ -30,7 +31,7 @@ def _plan() -> Plan:
             energy_cost_savings=120.0,
         ),
         PlanMeasure(
-            measure_type="mechanical_ventilation",
+            measure_type=MeasureType.MECHANICAL_VENTILATION,
             description="Mechanical extract ventilation",
             cost=Cost(total=900.0, contingency_rate=0.26),
             impact=MeasureImpact(
diff --git a/tests/repositories/plan/test_plan_postgres_repository.py b/tests/repositories/plan/test_plan_postgres_repository.py
index 8ee0f0f8..3e428bd8 100644
--- a/tests/repositories/plan/test_plan_postgres_repository.py
+++ b/tests/repositories/plan/test_plan_postgres_repository.py
@@ -14,6 +14,7 @@ from sqlalchemy import Engine
 from sqlmodel import Session, col, select
 
 from datatypes.epc.domain.epc import Epc
+from domain.modelling.measure_type import MeasureType
 from domain.modelling.scoring.package_scorer import Score
 from domain.modelling.plan import Plan, PlanMeasure
 from domain.modelling.recommendation import Cost
@@ -25,7 +26,7 @@ from repositories.plan.plan_postgres_repository import PlanPostgresRepository
 def _plan() -> Plan:
     measures: tuple[PlanMeasure, ...] = (
         PlanMeasure(
-            measure_type="cavity_wall_insulation",
+            measure_type=MeasureType.CAVITY_WALL_INSULATION,
             description="Cavity wall insulation",
             cost=Cost(total=1000.0, contingency_rate=0.10),
             impact=MeasureImpact(
@@ -112,7 +113,7 @@ def test_save_persists_null_per_measure_savings_when_unbilled(
 ) -> None:
     # Arrange — a Plan Measure whose per-measure bills were never derived.
     measure = PlanMeasure(
-        measure_type="loft_insulation",
+        measure_type=MeasureType.LOFT_INSULATION,
         description="Loft insulation",
         cost=Cost(total=500.0, contingency_rate=0.20),
         impact=MeasureImpact(