Substantial improvement
Definition: Under FEMA’s National Flood Insurance Program (44 CFR 59.1), any reconstruction, rehabilitation, addition, or improvement whose cost equals or exceeds 50% of the structure’s pre-project market value. Once this threshold is crossed, the entire structure — not just the improved portion — must be brought into full compliance with current NFIP new-construction standards, including elevation to Base Flood Elevation. Costs are tracked cumulatively over a rolling period (often 1–10 years depending on jurisdiction). This is one of the most consequential regulatory triggers in renovation, as it can transform a moderate project into a full code-compliance upgrade.