Any lawful nonconforming use of a portion of a building may be extended throughout the building and any lawful nonconforming use may be extended or enlarged upon the lot occupied by such building that is held in single and separate ownership on the effective date of the ordinance from which this chapter derives, provided that the area of such buildings shall not be increased by more than a total of twenty-five (25) percent of the total area of the first floor of such building existing on the date it first became a lawful nonconforming building or a building on 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 zoning district in which it is located and shall comply with all provisions of this chapter.