The structural alterations made in such buildings shall in no case exceed 20% of the true value of the buildings, nor shall the building be enlarged, unless the use therein is changed to a conforming use; provided, however, that where a building meets the use requirements of this chapter and is nonconforming because of height and area regulations, structural alterations may be made in such buildings providing the height and area regulations are not further violated.

A nonconforming use changed to a conforming use may not be changed back to a nonconforming use.

In the event that there is a cessation of operation of any nonconforming use, structure, land, or sign for a period of 12 consecutive calendar months, the same shall be presumed to be an abandonment of such nonconforming use. Any subsequent exercise of such abandoned nonconforming use shall be deemed a violation of the terms 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 such permit.

Nothing in this chapter shall be interpreted as authorization for or approval of the continuance of the use of a structure or premises in violation of zoning regulations in effect at the time of the effective date of this chapter.[1]

No nonconforming use shall be reduced in size. An owner of two or more contiguous nonconforming lots at the time of the effective date of this chapter[2] which, when combined, would create a lot of nonconforming size, shall be required to combine such lots to create conforming lots.