Any lawful nonconforming use which existed September 5, 1957, may be continued. Any existing structure designed, arranged, intended or devoted to a nonconforming use may be structurally altered, subject to the following regulations:
A. 
A nonconforming structure shall not be enlarged unless the structure is changed to a conforming structure, provided that where a building meets the use requirements of this chapter and is nonconforming because of height, area and/or yard regulations, the structure may be enlarged, provided that the height, area or yard regulations are not further violated.
The cessation of use or operation in a nonconforming manner over a period of 12 consecutive calendar months, for any reason other than national emergencies or act of God, shall constitute a presumption that the nonconforming use has been abandoned. Upon expiration of 24 consecutive calendar months from the date such use was discontinued, such presumption shall become irrefutable, and thereafter any subsequent exercise of such abandoned nonconforming use shall be deemed a violation of this chapter.
Nothing in this chapter shall require any change in plans, construction or designated use of a structure for which a building permit has been heretofore issued when construction has been diligently prosecuted within six months of the date of the permit.
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 75% of the assessed value, as recorded in the records of the Tax Assessor, and such destruction shall be deemed complete destruction, the structure may not be rebuilt, restored or repaired except in conformity with the regulations of this chapter. Nothing in this chapter shall prevent the strengthening or restoring to a safe condition of any wall, floor or roof which has been declared unsafe by the Zoning Enforcement Officer. If a nonconforming use is partially destroyed, as mentioned above, a building permit to restore the nonconforming use must be applied for within 12 months from the time of destruction.
No nonconforming use shall, if once changed to a conforming use, be changed back to a nonconforming use.