Any reconstruction, rehabilitation, addition, or other improvement of a structure, the cost of which equals or exceeds 50 percent of the market value of the structure before the "start of construction" of the improvement. This term includes structures which have incurred "substantial damage," regardless of the actual repair work performed. The term does not, however, include either:
- Any project for improvement of a structure to correct existing violations of state or local health, sanitary, or safety code specifications which have been identified by the local code enforcement official and which are the minimum necessary to assure safe living conditions or
- Any alterations of a "historic structure," provided that the alteration will not preclude the structure's continued designation as a "historic structure."
- Floodplain management requirements for new construction apply to substantial improvements.
For more information please check FEMA website