If the cost of constructing an addition is less than 50% of the main structure’s value, then only the addition needs to be elevated, (Florida Building Code (FBC) 1101.1). If the cost of the addition - along with any renovations to the main building - is more than 50%, then both the addition and the existing building require elevation. An exception is buildings that have legal additional living units, stand on their own merits. Buildings with non-residential uses have an option to floodproof, and the same scenarios apply. View more information to an exert of Florida Building Code concerning such additions (PDF).