Where a lawful structure, other than a sign or billboard, exists at the effective date of adoption of the ordinance codified in this chapter that could not be built under terms of this chapter by reason of restrictions on area, lot coverage, height, yards or other characteristics of the structure or its location on the lot, such structure may be continued, so long as it remains otherwise lawful, subject to the following provisions:
A. No such structure may be enlarged or altered in a way which increases its nonconformity.
B. Should such structure be destroyed by any means to an extent of more than 50% of its replacement cost at the time of destruction, then it shall not be reconstructed except in conformity with the provisions of this chapter. A building designed for a nonconforming use, however, may be constructed or altered beyond the limitations provided in this chapter by action of the Board of Appeals, after public hearing, as required by law in the case of variances.