diff --git a/domain/modelling/measure_dependency.py b/domain/modelling/measure_dependency.py
new file mode 100644
index 00000000..758ae789
--- /dev/null
+++ b/domain/modelling/measure_dependency.py
@@ -0,0 +1,82 @@
+"""The ventilation Measure Dependency — a data-declared "fabric insulation
+requires adequate ventilation" edge (CONTEXT.md: Measure Dependency; ADR-0016).
+
+Wall insulation tightens the envelope, so SAP10.2 (and good practice) require
+adequate ventilation alongside it. The optimiser must never *choose* ventilation
+(it only ever costs SAP), so it is excluded from the candidate pool and instead
+injected into the Optimised Package before the whole-package re-score, where its
+real — negative — SAP contribution lands in the truthful figure and the repair
+decision. The trigger set is held as data (mirroring the legacy
+`assumptions.measures_needing_ventilation`), so extending it (e.g. to roof
+insulation) is a data edit, not control flow.
+
+The intervention is decentralised mechanical extract ventilation (MEV), the
+legacy "mechanical, extract only" recommendation; it is only forced when the
+dwelling is not already mechanically ventilated (legacy `has_ventilation`).
+"""
+
+from typing import Optional
+
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.modelling.optimiser import MeasureDependency, ScoredOption
+from domain.modelling.recommendation import Cost, MeasureOption
+from domain.modelling.simulation import EpcSimulation, VentilationOverlay
+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(
+    {
+        "cavity_wall_insulation",
+        "internal_wall_insulation",
+        "external_wall_insulation",
+    }
+)
+
+_VENTILATION_MEASURE_TYPE = "mechanical_ventilation"
+
+# The SAP10.2 §2 mechanical-ventilation kind installed: decentralised MEV
+# ("mechanical extract, decentralised (MEV dc)" → MechanicalVentilationKind
+# name), the legacy "mechanical, extract only" intervention.
+_MEV_KIND = "EXTRACT_OR_PIV_OUTSIDE"
+
+# Best practice installs one MEV unit per wet zone; the legacy recommendation
+# fits two units per dwelling.
+_VENTILATION_UNIT_COUNT = 2
+
+
+def ventilation_dependency(
+    epc: EpcPropertyData, products: ProductRepository
+) -> Optional[MeasureDependency]:
+    """The ventilation Measure Dependency for a dwelling, or None when it is
+    already mechanically ventilated (so MEV must not be forced on). The required
+    Option installs MEV and is priced at two fully-loaded units."""
+    if _already_mechanically_ventilated(epc):
+        return None
+
+    product = products.get(_VENTILATION_MEASURE_TYPE)
+    cost = Cost(
+        total=product.unit_cost_per_m2 * _VENTILATION_UNIT_COUNT,
+        contingency_rate=product.contingency_rate,
+    )
+    option = MeasureOption(
+        measure_type=_VENTILATION_MEASURE_TYPE,
+        description=f"Install {_VENTILATION_UNIT_COUNT} mechanical extract ventilation units",
+        overlay=EpcSimulation(
+            ventilation=VentilationOverlay(mechanical_ventilation_kind=_MEV_KIND)
+        ),
+        cost=cost,
+    )
+    return MeasureDependency(
+        triggers=MEASURES_NEEDING_VENTILATION,
+        required=ScoredOption(option=option, sap_gain=0.0),
+    )
+
+
+def _already_mechanically_ventilated(epc: EpcPropertyData) -> bool:
+    """True when the dwelling already lodges a mechanical ventilation kind
+    (MEV/MVHR) — the legacy `has_ventilation` guard."""
+    return (
+        epc.sap_ventilation is not None
+        and epc.sap_ventilation.mechanical_ventilation_kind is not None
+    )
diff --git a/tests/domain/modelling/test_measure_dependency.py b/tests/domain/modelling/test_measure_dependency.py
new file mode 100644
index 00000000..3c6cdd4c
--- /dev/null
+++ b/tests/domain/modelling/test_measure_dependency.py
@@ -0,0 +1,109 @@
+"""Behaviour of the ventilation Measure Dependency builder: the data-declared
+"fabric insulation requires adequate ventilation" edge, guarded by the
+dwelling's existing ventilation. See CONTEXT.md (Measure Dependency) / ADR-0016.
+"""
+
+from typing import Optional
+
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.modelling.measure_dependency import (
+    MEASURES_NEEDING_VENTILATION,
+    ventilation_dependency,
+)
+from domain.modelling.optimiser import MeasureDependency
+from domain.modelling.product import Product
+from domain.modelling.simulation import EpcSimulation, VentilationOverlay
+from repositories.product.product_repository import ProductRepository
+from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
+    build_epc,
+)
+
+
+class _StubProducts(ProductRepository):
+    """In-memory ProductRepository returning a fixed per-unit ventilation cost."""
+
+    def get(self, measure_type: str) -> Product:
+        # unit_cost_per_m2 carries the catalogue row's fully-loaded total cost;
+        # for ventilation that total is per installed unit.
+        return Product(
+            measure_type=measure_type, unit_cost_per_m2=450.0, contingency_rate=0.10
+        )
+
+
+def test_triggers_are_the_fabric_wall_measures() -> None:
+    # Arrange / Act / Assert — the data-held trigger set (cf. legacy
+    # assumptions.measures_needing_ventilation).
+    assert MEASURES_NEEDING_VENTILATION == frozenset(
+        {
+            "cavity_wall_insulation",
+            "internal_wall_insulation",
+            "external_wall_insulation",
+        }
+    )
+
+
+def test_builds_a_ventilation_dependency_for_a_naturally_ventilated_dwelling() -> None:
+    # Arrange — 000490 lodges no mechanical ventilation kind.
+    baseline: EpcPropertyData = build_epc()
+
+    # Act
+    dependency: Optional[MeasureDependency] = ventilation_dependency(
+        baseline, _StubProducts()
+    )
+
+    # Assert — a forced edge whose required Option installs MEV.
+    assert dependency is not None
+    assert dependency.triggers == MEASURES_NEEDING_VENTILATION
+    option = dependency.required.option
+    assert option.measure_type == "mechanical_ventilation"
+    assert isinstance(option.overlay, EpcSimulation)
+    assert option.overlay.ventilation == VentilationOverlay(
+        mechanical_ventilation_kind="EXTRACT_OR_PIV_OUTSIDE"
+    )
+
+
+def test_dependency_costs_two_installed_units_with_contingency() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()
+
+    # Act
+    dependency: Optional[MeasureDependency] = ventilation_dependency(
+        baseline, _StubProducts()
+    )
+
+    # Assert — two MEV units at £450 each, carrying the product's contingency.
+    assert dependency is not None
+    cost = dependency.required.option.cost
+    assert cost is not None
+    assert abs(cost.total - 900.0) <= 1e-9
+    assert abs(cost.contingency_rate - 0.10) <= 1e-9
+
+
+def test_no_dependency_when_already_mechanically_ventilated() -> None:
+    # Arrange — the dwelling already has a mechanical ventilation kind, so MEV
+    # must not be forced on (legacy has_ventilation guard).
+    baseline: EpcPropertyData = build_epc()
+    assert baseline.sap_ventilation is not None
+    baseline.sap_ventilation.mechanical_ventilation_kind = "EXTRACT_OR_PIV_OUTSIDE"
+
+    # Act
+    dependency: Optional[MeasureDependency] = ventilation_dependency(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert dependency is None
+
+
+def test_builds_a_dependency_when_the_dwelling_lodged_no_ventilation() -> None:
+    # Arrange — no SapVentilation at all counts as not mechanically ventilated.
+    baseline: EpcPropertyData = build_epc()
+    baseline.sap_ventilation = None
+
+    # Act
+    dependency: Optional[MeasureDependency] = ventilation_dependency(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert dependency is not None