diff --git a/datatypes/epc/domain/epc.py b/datatypes/epc/domain/epc.py
index 6c65cf0a9..67b69dbaa 100644
--- a/datatypes/epc/domain/epc.py
+++ b/datatypes/epc/domain/epc.py
@@ -48,4 +48,13 @@ class Epc(Enum):
         return bounds[self]
 
     def sap_lower_bound_continuous(self) -> float:
-        raise NotImplementedError
+        """The band floor on the **continuous** SAP scale — the lowest un-rounded
+        rating whose *published* rating falls in this band (C → 68.5).
+
+        A dwelling is in a band by its published rating, which is the continuous
+        score rounded half up, so the published rating enters the band half a
+        point below the integer floor. Anything judging "has this dwelling
+        reached band X?" against a continuous score must compare with this, not
+        with `sap_lower_bound`: a dwelling at a continuous 68.6 publishes as SAP
+        69 and *is* band C, but reads as 0.4 short of an integer floor of 69."""
+        return self.sap_lower_bound() - 0.5