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

A nonconforming use shall be presumed to have been abandoned when there occurs a cessation of any use or activity by an apparent act or failure to act on the part of the tenant or owners to reinstate such use within a period of 12 months from the date of cessation or discontinuance. In any civil or quasi-criminal action for the enforcement of this chapter, such cessation of use or activity shall be prima facie proof of abandonment.

No nonconforming building or building housing a nonconforming use damaged by fire or other causes to the extent of more than 50% of its assessed valuation shall be repaired or rebuilt except by approval of the Zoning Board of Adjustment. Nothing in this chapter shall prevent the strengthening or restoring to a safe condition of any wall, roof or floor which has been declared unsafe by the Construction Official.

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

A nonconforming building may not be reconstructed or structurally altered during its life unless said building is changed to a conforming use.

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, has been completed according to such plans as filed within one year from the date of this chapter. If such conditions are not met, any building permit shall be held to be in violation of this chapter, subject to the penalties therein.

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