Repairs or incidental alterations may be made and in appropriate cases the authorizing agency may permit structural alterations, extensions or enlargements limited to the zoning lot that was granted a variance, exception or permit prior to December 15, 1961.  However, the use of any building or other structure shall not be extended, and the building or other structure shall not be enlarged, in excess of 50 percent of the floor area of such building (or size of such structure) occupied or utilized by the use on December 15, 1961, and, except as otherwise provided in Article VII, no structural alterations, extensions or enlargements shall be authorized for a new non-conforming use authorized under the provisions of Section 11-413 (Change of use).