Subsequent to Hurricane Andrew in August 1992 many municipalities have adopted rules regarding the renovations of existing structures.  Those guidelines were devised in response to many homes and businesses that were destroyed whose finished floor level (the elevation of the concrete floor slab for "slab on grade structures") was below that of FEMA guidelines.  FEMA also raised the required flood elevation for "finished floors" for new homes and structures.

The rule is "cumulative" meaning that renovations made under subsequent permits count towards the repalcement costs.   If the renovation/new construction proposed for the structure is just under the threshold due to prior renovations, a future improvement could trigger the 50% rule which would preclude any further renovations without complying with all the current building codes and FEMA requirements. 

Triggering the 50% rule to both the floor elevation and all current building codes. *

*Product control of approved building materials to meet hurricane codes are required along with ADA requirements such as handicapped provisions.  Exterior materials such as roofing, doors and windows, must  pass an impact test for approval and certification.  Electrical, mechanical and structural items must also comply with all current codes.  For commercial properties, setbacks, parking open space and other requirements are enforced.