Nothing in this chapter shall be deemed to prevent normal maintenance and repair of a building or structure containing a nonconforming use. Structural alterations may be made to a building or structure containing a nonconforming use as follows:
(a) When required by law.
(b) To convert to a conforming use.
(c) A building or structure containing residential nonconforming uses may be so altered as to improve interior livability. However, no structural alterations shall be made which exceed the area or height regulations or which would extend into any yard required in the district in which such building is located.