A structure which was lawful before the adoption of this regulation but which is not in conformity with the provisions hereof, shall be subject to the following conditions (to be enforced by the appropriate authority as to Class I and Class II projects, respectively, under §§ 415.40 through 415.43 of this part):

(a) A non-conforming structure in the floodway may not be expanded, except that it may be modified, altered or repaired to incorporate flood proofing measures provided such measures do not raise the level of the 100-year flood.

(b) A non-conforming structure in the floodway which is destroyed or damaged by any means, including a flood, to the extent of 50 percent or more of its market value at that time may not be restored, repaired, reconstructed or improved except in conformity with the provisions of these regulations.