Nonconforming structures may be altered, renovated, or enlarged provided that such alteration, renovation or enlargement does not increase the existing gross floor area by more than 25% nor increase or extend the existing nonconformity. The dimension that is nonconforming shall not be altered so as to extend the nonconformity beyond the existing building line. For example, a structure that is nonconforming as to front yard setback may be expanded, so long as the expansion does not further reduce the existing setback. In the case of a nonconforming structure which is occupied by a nonconforming use, such alteration, renovation or enlargement shall also meet the requirements of § 27-1604, Subsection 1., of this Part. In the case of a nonconforming structure which is located on a nonconforming lot, such alteration, renovation or enlargement shall also meet the requirements of § 27-1606, Subsection 4., of this Part.