Structures existing in any identified flood-prone area prior to the enactment of this chapter, but which are not in compliance with these provisions, may continue to remain subject to the following:
A. 
Existing structures located in any designated floodway area shall not be expended or enlarged, unless the effect of the proposed expansion or enlargement on flood heights is fully offset by accompanying improvements.
B. 
Any modification, alteration, reconstruction, or improvement of any kind to an existing structure, to an extent or amount of less than 50% of its market value, shall be elevated and/or floodproofed to the greatest extent possible.
C. 
Any modification, alteration, reconstruction, or improvement of any kind to an existing structure, to an extent or amount of 50% or more of its market value, shall be undertaken only in full compliance with the provisions of this chapter.