Structures existing in any identified floodplain area prior to the enactment of this chapter may continue, subject to the following provisions:
A. 
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 constitute a substantial improvement and shall be undertaken only in full compliance with the provisions of this chapter.
B. 
Any modification, alteration, reconstruction, or improvement of any kind to an existing structure or use within the AE Area/District, regardless of cost, shall comply with the requirements of § 300-99F(4)(e)[5] and § 154-401C of this chapter.