(a) Normal maintenance of a building or other structure containing a nonconforming use is permitted, including necessary nonstructural repairs and incidental alterations which do not extend or intensify the nonconforming use.
(b) No structural alteration shall be made in a building or other structure containing a nonconforming use, except in the following situations:
(1) When the alteration is required by law;
(2) When the alteration will result in eliminating the nonconforming use;
(3) When a building containing residential nonconforming uses may be altered in any way to improve livability, provided no structural alteration shall be made which would increase the number of dwelling units or the bulk of the building.