The FEMA 50 percent rule, in plain terms

In a designated floodplain, local ordinance generally prohibits improving or repairing a structure by more than fifty percent of its market value unless the entire structure is brought into current floodplain compliance — which usually means elevation, and usually means a great deal of money.

The determination turns on the market value of the structure alone, excluding the land. This is where most appraisers go wrong. A conventional appraisal reports a combined land-and-improvements value, and using that figure inflates the denominator and produces the wrong answer.

Because the structure-only value sets the ceiling on permitted work, the number has direct financial consequence for the owner. A defensible, correctly framed determination can be the difference between a permitted repair and a required elevation.

Local floodplain administrators review these closely and reject reports that get the framing wrong. The report must state clearly that it is a structure-only market value determination prepared for substantial improvement or substantial damage purposes.