diff --git a/docs/adr/0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md b/docs/adr/0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md
index 2028d037d..3efcc1c9f 100644
--- a/docs/adr/0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md
+++ b/docs/adr/0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md
@@ -101,3 +101,27 @@ companions drag from the code). The visible baseline shift it produces is
 correct Rebaselining per [ADR-0039](0039-override-aware-rebaselining.md). It
 **corrects the stated cause of PRD #1361 Class A** (classifier taxonomy gap, not
 fabric/rebaseline).
+
+### Amendment: natural-fuel coherence
+
+A room-heater / direct-acting archetype names a *system*, not its fuel — and fuel
+arrives as its own composable `main_fuel` override. Two rules keep the system
+self-coherent without coupling the two overlays:
+
+- **The heating overlay drags a natural fuel only where the archetype is
+  unambiguous** — electric room heaters / direct-acting / storage are
+  unambiguously electricity (RdSAP `main_fuel` code 29), a gas boiler is mains
+  gas (26). A **solid / oil room heater is fuel-ambiguous** (coal vs wood vs
+  smokeless), so it drags **no** natural fuel and relies on the `main_fuel`
+  override; dragging a guessed coal-vs-wood code would be *more* wrong (very
+  different carbon).
+- **A present `main_fuel` override wins** by the applicator's last-wins
+  composition (`apply_simulations` setattrs non-`None` fields in order), so the
+  `main_fuel` overlay is applied **after** the heating overlay. The agreeing
+  case (electric system + electricity fuel) is order-immaterial; only a
+  *disagreeing* one depends on the ordering.
+- **A landlord fuel that contradicts the archetype's natural fuel is *logged*,
+  not raised** (e.g. "gas" fuel on a solid-fuel room heater) — an
+  override-plausibility signal (cf. the ADR-0039 scanner); what to do with it is
+  deferred. The override is still honoured (the landlord wins); we only record
+  the implausibility.