FEMA's 50% Rule a.k.a. Substantial Damage Rule is a compilation of specific damage assessment guidlelines in relation to structures built before December 31, 1974 or prior to the start date of which a community begins to participate in the National Flood Insurance Program (NFIP). These guidelines apply to waterfront homes and those in nearby areas who want to stay above water figuratively and financially. But they are also structured for homes damaged by wind, fire, earthquake or man. It is highly advisable that you find out what your home's
flood zone is before undergoing any repairs or remodels. In regards to homeowners, FEMA prefers that houses are rebuilt above the flood level to reduce water damage during a major storm. However, these are guidelines. Each county, city or town adopts these rules as they decide. FEMA provides financial incentives in the form of lower insurance rates the more these guidelines are accepted into local laws.
When adopted locally, these regulations restrict a homeowner from spending more than 50% of the value of their home. This does not include the value of the land. If damage to a home which is located in the flood zone would exceed 50%, the home is considered lost and must either be raised above the flood zone or torn down.
If you plan to do any repair, remodel or addition project to your home, be sure to let your planner know that you are located in a flood zone.
Read more about it
here.