diff --git a/domain/modelling/package_scorer.py b/domain/modelling/package_scorer.py
new file mode 100644
index 00000000..bacf9e18
--- /dev/null
+++ b/domain/modelling/package_scorer.py
@@ -0,0 +1,47 @@
+"""The Package Scorer — the reusable scoring primitive (ADR-0016).
+
+Composes an ordered set of Simulation Overlays onto a baseline EpcPropertyData
+(via the Overlay Applicator) and scores the throwaway result on a deterministic
+SAP calculator, returning the headline metrics. The same primitive powers the
+optimiser's whole-package re-score and any future live re-score of a
+user-assembled plan.
+"""
+
+from dataclasses import dataclass
+from typing import Sequence
+
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.modelling.overlay_applicator import apply_simulations
+from domain.modelling.simulation import EpcSimulation
+from domain.sap10_calculator.calculator import SapCalculator, SapResult
+
+
+@dataclass(frozen=True)
+class Score:
+    """The headline metrics of a scored package. `sap_continuous` is the
+    un-rounded SAP rating (used for deltas); carbon and primary energy are the
+    annual totals."""
+
+    sap_continuous: float
+    co2_kg_per_yr: float
+    primary_energy_kwh_per_yr: float
+
+
+class PackageScorer:
+    """Scores a package of Simulation Overlays against a baseline EpcPropertyData
+    on an injected SAP calculator (depends on the `SapCalculator` abstraction,
+    not a concrete engine)."""
+
+    def __init__(self, calculator: SapCalculator) -> None:
+        self._calculator = calculator
+
+    def score(
+        self, baseline: EpcPropertyData, simulations: Sequence[EpcSimulation]
+    ) -> Score:
+        simulated: EpcPropertyData = apply_simulations(baseline, simulations)
+        result: SapResult = self._calculator.calculate(simulated)
+        return Score(
+            sap_continuous=result.sap_score_continuous,
+            co2_kg_per_yr=result.co2_kg_per_yr,
+            primary_energy_kwh_per_yr=result.primary_energy_kwh_per_yr,
+        )
diff --git a/tests/domain/modelling/test_package_scorer.py b/tests/domain/modelling/test_package_scorer.py
new file mode 100644
index 00000000..ffe50cd5
--- /dev/null
+++ b/tests/domain/modelling/test_package_scorer.py
@@ -0,0 +1,54 @@
+"""Behaviour of the Package Scorer: composing Simulation Overlays onto a
+baseline EpcPropertyData and scoring the result on the deterministic SAP10
+calculator. The reusable compute primitive (ADR-0016). Elmhurst before/after
+cascade pins land with #1154 once the cert parses; here we exercise the real
+calculator on a hand-built EPD.
+"""
+
+from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
+    EpcPropertyData,
+)
+from domain.modelling.package_scorer import PackageScorer, Score
+from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
+from domain.sap10_calculator.calculator import Sap10Calculator, SapResult
+from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
+    build_epc,
+)
+
+_CAVITY_FILL = EpcSimulation(
+    building_parts={
+        BuildingPartIdentifier.MAIN: BuildingPartOverlay(wall_insulation_type=2)
+    }
+)
+
+
+def test_filling_the_main_cavity_improves_sap() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()  # MAIN: uninsulated cavity
+    scorer = PackageScorer(Sap10Calculator())
+
+    # Act
+    base: Score = scorer.score(baseline, [])
+    filled: Score = scorer.score(baseline, [_CAVITY_FILL])
+
+    # Assert
+    assert filled.sap_continuous > base.sap_continuous
+
+
+def test_empty_package_scores_the_unmodified_baseline() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()
+    calculator = Sap10Calculator()
+    direct: SapResult = calculator.calculate(baseline)
+
+    # Act
+    score: Score = PackageScorer(calculator).score(baseline, [])
+
+    # Assert
+    assert abs(score.sap_continuous - direct.sap_score_continuous) <= 1e-9
+    assert abs(score.co2_kg_per_yr - direct.co2_kg_per_yr) <= 1e-9
+    assert (
+        abs(score.primary_energy_kwh_per_yr - direct.primary_energy_kwh_per_yr)
+        <= 1e-9
+    )