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

In the event that there is a cessation of operation of any nonconforming use for a period of 12 consecutive calendar months, the same shall be presumed to be an abandonment of such nonconforming use. Such use shall not thereafter be reinstated, and the structures shall not be reoccupied except in conformance with this chapter.

Nothing herein contained shall require any change in plans, construction or designated use of a building for which a building permit has been heretofore issued and the construction of which shall have been diligently prosecuted within three months of the date of such permit and the ground story framework of which, including the second tier of beams, shall have been completed within six months of the date of the permit, and which entire building shall be completed according to plans as filed within one year from the date of this chapter.

No nonconforming use which has been changed to a conforming use may be changed back to a nonconforming use.

A nonconforming building may be altered, but not enlarged or extended, to an extent not exceeding in aggregate 50% of the recorded true value of the building as appraised in the records of the Tax Assessor, unless said building is changed to a building conforming to the requirements of this chapter.

If any nonconforming building shall be destroyed by reason of windstorm, fire, explosion or other act of God or the public enemy to an extent of more than 50% of the recorded true value as appraised in the records of the Tax Assessor, then such destruction shall be deemed complete destruction, and the structure may not be rebuilt, restored or repaired except in conformity with this chapter. Nothing in this chapter shall prevent the straightening or restoring to a safe condition of any wall, floor or roof which has been declared unsafe.

Whenever the boundaries of a district shall be changed so as to transfer an area from one district to another district of a different classification, the foregoing provisions shall also apply to any nonconforming uses existing therein or created thereby.