Repairs. Normal maintenance repair or incidental alteration of a nonconforming use or structure is permitted, provided that it does not extend or expand the nonconforming use or structure beyond what existed prior to the alteration or repair. A building or other structure containing residential nonconforming uses or located on a nonconforming lot may be altered in any way to improve interior livability. However, no structural alterations shall be made which would increase the number of dwelling units, increase the floor area, or expand the building footprint.