The lawful use of a building existing at the time of the adoption of this chapter may be continued, although such use does not conform with the provisions hereof, and such use may be extended throughout the building, provided no structural alterations, except these required by law or ordinance, are made therein. If no structural alterations are made, a nonconforming use of a building may be changed to another nonconforming use of the same or a more restricted classification. The foregoing provision shall also apply to nonconforming uses in districts hereafter changed. Whenever a nonconforming use of a building has been changed to a more restricted use or to a conforming use, such use shall not thereafter be changed to a less restricted use.

A building which has been damaged by fire, explosion, or act of God or public enemy, to the extent of more than 50% of its value, may be restored only if it is in conformity with the regulations of this chapter.

In the event that a nonconforming use of any building or premises is discontinued or its normal operation stopped for a period of one year, it shall be considered abandoned and such use shall not be reestablished except in conformity with the regulations of the district in which it is located.

No existing building or premises devoted to a use not permitted by this chapter in the district in which such building or premises is located, except when required to do so by law or ordinance, shall be enlarged, extended, reconstructed, or structurally altered unless such use is changed to one permitted in the district in which such building or premises is located.