The lawful use of any land or building existing at the time of the enactment of this chapter may be continued although such use may not conform with the provisions of this chapter, except as hereinafter provided.

A nonconforming use may be changed to a conforming use, and when so changed to a more restrictive classification, it shall not thereafter resume the less restrictive use.

No building which has been damaged by fire or other natural causes to the extent of more than 50% of its equalized assessed value for county tax purposes shall be repaired or rebuilt except in conformity with the regulations of this chapter.

No nonconforming building shall be altered nor a nonconforming use of building or land extended, except as authorized by the Board of Appeals.

Nothing in this article shall prohibit the completion of any lawful structure for which the excavation has been prepared and the foundation walls constructed at the date this chapter takes effect; provided, however, that construction must be completed within a period of one year from that date.

Whenever a nonconforming use has been abandoned or discontinued for a period of one year, any future use shall be in conformity with the provisions of this chapter.

The foregoing restrictions on nonconforming uses shall not apply to or limit alteration or extension to nonconforming residential or agricultural uses, provided that such alteration or extension does not increase the nonconformity.