diff --git a/domain/modelling/__init__.py b/domain/modelling/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/domain/modelling/overlay_applicator.py b/domain/modelling/overlay_applicator.py
new file mode 100644
index 00000000..e4df587b
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+++ b/domain/modelling/overlay_applicator.py
@@ -0,0 +1,34 @@
+"""The Overlay Applicator — folds an ordered set of Simulation Overlays onto
+a baseline EpcPropertyData and returns a new one for the calculator.
+
+Sequential fold: overlays are applied in order and a later overlay wins on a
+field it shares with an earlier one. The baseline is never mutated; the
+returned EpcPropertyData is throwaway (handed to the calculator for scoring,
+then discarded). See ADR-0016.
+"""
+
+import copy
+from dataclasses import fields
+from typing import Sequence
+
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.modelling.simulation import EpcSimulation
+
+
+def apply_simulations(
+    baseline: EpcPropertyData, simulations: Sequence[EpcSimulation]
+) -> EpcPropertyData:
+    """Return a copy of ``baseline`` with every Simulation Overlay's non-``None``
+    fields written onto the building part it targets, applied in order."""
+    result: EpcPropertyData = copy.deepcopy(baseline)
+    parts_by_id = {part.identifier: part for part in result.sap_building_parts}
+
+    for simulation in simulations:
+        for identifier, overlay in simulation.building_parts.items():
+            part = parts_by_id[identifier]
+            for overlay_field in fields(overlay):
+                value = getattr(overlay, overlay_field.name)
+                if value is not None:
+                    setattr(part, overlay_field.name, value)
+
+    return result
diff --git a/domain/modelling/simulation.py b/domain/modelling/simulation.py
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index 00000000..a5f36dfa
--- /dev/null
+++ b/domain/modelling/simulation.py
@@ -0,0 +1,38 @@
+"""The Simulation Overlay (`EpcSimulation`) — the change a single Measure
+Option makes to a Property's EpcPropertyData.
+
+An all-optional partial mirror of EpcPropertyData / SapBuildingPart, covering
+the retrofit-relevant surface only (wall fields first). It is *not* an
+EpcPropertyData — composition, not inheritance — and carries no scores.
+Building parts are targeted by `BuildingPartIdentifier` so a measure addresses
+the exact `SapBuildingPart` (the main wall vs an extension). See CONTEXT.md.
+"""
+
+from dataclasses import dataclass, field
+from typing import Mapping, Optional
+
+from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+
+
+@dataclass(frozen=True)
+class BuildingPartOverlay:
+    """All-optional partial of `SapBuildingPart` (wall surface first).
+
+    A `None` field means "leave the baseline value unchanged".
+    """
+
+    wall_insulation_type: Optional[int] = None
+
+
+def _no_building_parts() -> dict[BuildingPartIdentifier, BuildingPartOverlay]:
+    return {}
+
+
+@dataclass(frozen=True)
+class EpcSimulation:
+    """A Simulation Overlay: the per-building-part changes a Measure Option
+    makes, keyed by `BuildingPartIdentifier`."""
+
+    building_parts: Mapping[BuildingPartIdentifier, BuildingPartOverlay] = field(
+        default_factory=_no_building_parts
+    )
diff --git a/tests/domain/modelling/__init__.py b/tests/domain/modelling/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/domain/modelling/test_overlay_applicator.py b/tests/domain/modelling/test_overlay_applicator.py
new file mode 100644
index 00000000..4f208158
--- /dev/null
+++ b/tests/domain/modelling/test_overlay_applicator.py
@@ -0,0 +1,101 @@
+"""Behaviour of the Overlay Applicator: folding Simulation Overlays
+(EpcSimulation) onto a baseline EpcPropertyData to produce a new one for
+the calculator. See ADR-0016 and the Modelling glossary in CONTEXT.md.
+"""
+
+from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
+    EpcPropertyData,
+    SapBuildingPart,
+)
+from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
+from domain.modelling.overlay_applicator import apply_simulations
+from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
+    build_epc,
+)
+
+
+def _part(epc: EpcPropertyData, identifier: BuildingPartIdentifier) -> SapBuildingPart:
+    return next(p for p in epc.sap_building_parts if p.identifier is identifier)
+
+
+def test_apply_writes_targeted_building_part_and_leaves_others_untouched() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()
+    extension_before: int | str = _part(
+        baseline, BuildingPartIdentifier.EXTENSION_1
+    ).wall_insulation_type
+    simulation = EpcSimulation(
+        building_parts={
+            BuildingPartIdentifier.MAIN: BuildingPartOverlay(wall_insulation_type=1)
+        }
+    )
+
+    # Act
+    result: EpcPropertyData = apply_simulations(baseline, [simulation])
+
+    # Assert
+    assert _part(result, BuildingPartIdentifier.MAIN).wall_insulation_type == 1
+    assert (
+        _part(result, BuildingPartIdentifier.EXTENSION_1).wall_insulation_type
+        == extension_before
+    )
+
+
+def test_empty_simulation_is_a_no_op() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()
+
+    # Act
+    result: EpcPropertyData = apply_simulations(baseline, [EpcSimulation()])
+
+    # Assert
+    assert result == baseline
+
+
+def test_later_simulation_wins_on_a_shared_field() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()
+    first = EpcSimulation(
+        building_parts={
+            BuildingPartIdentifier.MAIN: BuildingPartOverlay(wall_insulation_type=1)
+        }
+    )
+    second = EpcSimulation(
+        building_parts={
+            BuildingPartIdentifier.MAIN: BuildingPartOverlay(wall_insulation_type=2)
+        }
+    )
+
+    # Act
+    result: EpcPropertyData = apply_simulations(baseline, [first, second])
+
+    # Assert
+    assert _part(result, BuildingPartIdentifier.MAIN).wall_insulation_type == 2
+
+
+def test_baseline_is_not_mutated() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()
+    original: int | str = _part(
+        baseline, BuildingPartIdentifier.MAIN
+    ).wall_insulation_type
+
+    # Act
+    _: EpcPropertyData = apply_simulations(
+        baseline,
+        [
+            EpcSimulation(
+                building_parts={
+                    BuildingPartIdentifier.MAIN: BuildingPartOverlay(
+                        wall_insulation_type=1
+                    )
+                }
+            )
+        ],
+    )
+
+    # Assert
+    assert (
+        _part(baseline, BuildingPartIdentifier.MAIN).wall_insulation_type == original
+    )