New buildings in the floodplain must meet certain construction requirements to protect them from flood damage. The city's building code requires that all substantial improvements to buildings meet the same requirements. Substantial improvement means any combination of any repair, reconstruction, rehabilitation, addition or other improvement of a building or structure over any given five-year period for which the cumulative cost equals or exceeds 50% of the base market value of the structure before the start of construction of the repair or improvement.
Similar requirements apply to buildings that have sustained substantial damage, which means damage of any origin sustained by a structure whereby the cost of restoring the structure to before the damaged condition would equal or exceed 50% of the market value of the structure before the damage occurred. If the structure has sustained substantial damage, any repairs are considered a substantial improvement regardless of the actual repair work performed.
This does not apply to improvements of a building to correct existing health, sanitary, or safety code violations, or for the alteration of historic buildings provided that the alteration will not preclude the building’s continued designation as historic.
Information from Federal Emergency Management Agency (FEMA)