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 when at least one of the following conditions exist:
(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 requirements or which would extend into any yard required in the district in which such building is located.