Except as otherwise provided in this article, the lawful use of land or structures existing at the date of the adoption of this chapter may be continued although such use or structure does not conform to the regulations specified by this chapter for the zone in which such land or structure is located; provided, however, that:

A nonconforming use shall be deemed abandoned when there occurs such overt act as evidences an intent by the owner or the tenant to cease such prior use or activity. In any event, where an owner or tenant ceases such prior use or activity and fails to reinstate the same within a period of one year from the date of cessation or discontinuance, the same shall be conclusive evidence that there has in fact been an abandonment. In either of the aforesaid events, such use shall not thereafter be reinstated, and the structure shall not be reoccupied except in conformance with this chapter.

If any nonconforming structure is destroyed by reason of windstorm, fire, explosion or other act of God or the public enemy to an extent of more than 50%, said structure shall not be rebuilt, restored or repaired or restored except in conformity with this chapter.

No nonconforming use or structure shall, if once changed into a conforming use or structure, be changed back again into a nonconforming use or structure.

A nonconforming building may be altered or improved, provided that the alteration or improvement does not amount to a substantial increase of a nonconforming use, but if such alterations or improvements involve any increase in lands so used or in the cubic content of the building so used, the permit for the alteration or improvement shall be issued only upon variance granted in accordance with law; provided, however, that accessory uses and additions may be constructed on nonconforming lots in conjunction with an existing residential use if the proposed accessory use or addition complies with the setback and yard requirements set forth in § 335-93.