NFIP Substantial Improvement 50% Rule (44 CFR 59.1)
The rule that decides whether a remodel in a flood zone is a remodel or a rebuild, and almost nobody prices it before starting: cost equal to or over 50% of the MARKET VALUE OF THE STRUCTURE means the whole building must be brought into compliance -- elevated, vented, the works. The denominator is the STRUCTURE, not the property, and the land is not in it, so on an expensive lot with a modest house the threshold is far lower than it looks: a $95,000 job on a $180,000 house is 52.8% and triggers, while against the $400,000 property it reads as a harmless 23.8%. Substantial DAMAGE runs the same 50% from the other direction.
Formula and source
threshold = market value of the structure x the adopted percent (50 federal). Countable cost = improvement cost less any code-correction work identified by the local official. Triggers where the countable cost, or the cumulative cost under a local rolling rule, equals or exceeds the threshold. Substantial damage applies the same percent to the cost of restoring the structure to its before-damaged condition against its market value before the damage.
44 CFR 59.1, the NFIP definitions of substantial improvement and substantial damage. A US federal regulation in the public domain, quoted directly.
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