Damage of any origin sustained by a building where the cost of restoring the structure to its pre-damaged condition equals or exceeds 50% of the market value before the damage occurred. Substantially damaged buildings must be brought into compliance with current floodplain management regulations when repaired, including meeting elevation requirements. This applies to damage from any cause (flood, fire, earthquake, etc.), not just flooding. The 50% threshold is calculated using the building's market value excluding land value. Communities track cumulative damage over time to prevent property owners from avoiding compliance through multiple smaller repairs. Substantial damage determinations are made by local floodplain administrators.