Flag works bought for no real SAP gain 🟥

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/audit/anomalies.py

@@ -187,6 +187,11 @@ def _already_meets_goal_with_works(a: PropertyAudit) -> Optional[str]:
     return f"already {a.effective_band} >= goal {a.scenario_goal_band} but cost_of_works £{a.cost_of_works:.0f}"
 
 
+@check("costed-plan-without-real-gain", Severity.HIGH)
+def _costed_plan_without_real_gain(a: PropertyAudit) -> Optional[str]:
+    raise NotImplementedError
+
+
 @check("plan-stops-short-of-goal", Severity.HIGH)
 def _plan_stops_short_of_goal(a: PropertyAudit) -> Optional[str]:
     """The default plan ends BELOW the scenario's goal band on a scenario that is

tests/scripts/test_audit_anomalies.py

@@ -10,6 +10,7 @@ from scripts.audit.anomalies import (
     PropertyAudit,
     Severity,
     _already_meets_goal_with_works,  # pyright: ignore[reportPrivateUsage]
+    _costed_plan_without_real_gain,  # pyright: ignore[reportPrivateUsage]
     _effective_lodged_divergence,  # pyright: ignore[reportPrivateUsage]
     _implausible_lodged_score,  # pyright: ignore[reportPrivateUsage]
     _plan_stops_short_of_goal,  # pyright: ignore[reportPrivateUsage]
@@ -213,3 +214,70 @@ class TestAlreadyMeetsGoalWithWorks:
 
         # Assert
         assert registered["already-meets-goal-with-works"] is Severity.HIGH
+
+
+class TestCostedPlanWithoutRealGain:
+    """Money spent for no meaningful SAP movement (#1652 / #1655).
+
+    The coverage hole the #1652 diagnosis found: `plan-score-below-baseline`
+    only fires on a *drop* beyond 0.5 and `zero-works-post-differs` only on £0
+    plans, so a costed plan landing flat against the effective baseline was
+    caught by neither — including the 112 homes whose modelled post-retrofit SAP
+    sat ~0.2 *below* their effective baseline.
+    """
+
+    def test_fires_when_works_are_bought_for_no_real_gain(self) -> None:
+        # Arrange — £6,240 of works moving the dwelling 0.2 SAP.
+        audit = _make_audit(
+            effective_sap=63.0, post_sap=63.2, cost_of_works=6240.0
+        )
+
+        # Act
+        result = _costed_plan_without_real_gain(audit)
+
+        # Assert
+        assert result is not None
+        assert "6240" in result or "6,240" in result
+
+    def test_fires_when_the_post_score_lands_below_the_baseline(self) -> None:
+        # Arrange — the likely-downgrade case: paid works, post *below* baseline.
+        audit = _make_audit(
+            effective_sap=63.0, post_sap=62.8, cost_of_works=1029.0
+        )
+
+        # Act
+        result = _costed_plan_without_real_gain(audit)
+
+        # Assert
+        assert result is not None
+
+    def test_silent_when_the_works_deliver_a_real_uplift(self) -> None:
+        # Arrange — £6,240 buying 9 SAP points: what a retrofit should look like.
+        audit = _make_audit(
+            effective_sap=63.0, post_sap=72.0, cost_of_works=6240.0
+        )
+
+        # Act
+        result = _costed_plan_without_real_gain(audit)
+
+        # Assert
+        assert result is None
+
+    def test_silent_on_a_zero_cost_plan(self) -> None:
+        # Arrange — no money spent, so there is nothing to justify; a £0 plan
+        # that moves nothing is the correct outcome for an already-compliant
+        # dwelling, and is covered by its own check.
+        audit = _make_audit(effective_sap=63.0, post_sap=63.0, cost_of_works=0.0)
+
+        # Act
+        result = _costed_plan_without_real_gain(audit)
+
+        # Assert
+        assert result is None
+
+    def test_is_registered_at_high_severity(self) -> None:
+        # Act
+        registered = {name: severity for name, severity, _ in _REGISTRY}
+
+        # Assert
+        assert registered["costed-plan-without-real-gain"] is Severity.HIGH