Extension. Any lawful nonconforming use of a portion of a building may be extended throughout the building, and any lawful nonconforming building or any building of which a lawful nonconforming use is made may be extended upon the lot occupied by such building and held in single and separate ownership on the effective date of this chapter, provided that the area of such building shall not be increased by more than a total of 25% of the area of such building existing on the date it first became a lawful nonconforming building or a building of which a lawful nonconforming use is made, and provided further that any structural alteration, extension or addition shall conform with all height, area, width, yard and coverage requirements for the district in which is it located, except in the case of a single-family dwelling or other building converted into a dwelling for two or more families when permitted by special exception issued by the Zoning Hearing Board, subject to Article IV, § 475-29, herein.