Substantial improvement

Substantial improvement is floodplain regulation's ratchet: when work on a building in a mapped flood zone crosses the rule's cost threshold — measured against the structure's market value, with damage repair counted through the parallel substantial-damage test — the whole building must be brought into compliance with current flood-resistant construction standards, not just the renovated part.

The mechanism is how old flood-zone stock modernizes, and it is a planning trap for the unwary: a renovation budget that drifts across the line converts a kitchen project into an elevation project. For flood-zone buildings, improvement history and headroom under the threshold are genuine diligence items, since they price the next renovation's regulatory exposure.