The Board of Zoning Appeals shall have the authority to grant an extension or permit completion of a building devoted to a nonconforming use upon a lot occupied by such building, or on a lot adjoining, provided that such lot was under the same ownership as the lot in question on the date such building became nonconforming, and where such extension is necessary and incidental to the existing use of such building; provided, however, that the floor area of such extension shall not exceed in all one hundred percent (100%) of the floor area of the existing building or buildings devoted to a nonconforming use and provided further that such extension or extensions shall be undertaken within five years of the date when the use of such building became nonconforming.