The Federal Emergency Management Agency (FEMA) has exempted such structures from the elevation requirements related to Substantial Improvement unless anything is done to the structure that would affect its status as a contributing historic structure. Utilities and machinery are still required to be elevated to proper flood levels. However, your building won’t be exempt from very high flood insurance costs, depending upon the depth of the building below the flood level.
There’s a major difference between "Historic Structures" and "Contributing Historic Structures." Only the latter are exempt from elevation/floodproofing requirements. A "Historic Structure" may be any building older than 50 years. Whereas a "Contributing Historic Structure" is a documented building on the state list of historic structures; this list is maintained by the local Historic Architectural Review Commission (HARC) board.