Whenever under the provisions of the 1916 Zoning Resolution as amended, either the Board of Standards and Appeals or the City Planning Commission with the approval of the Board of Estimate or the City Council, has authorized any use to locate in a district in which it is not permitted as-of-right by issuing a variance, exception or permit, such existing use established pursuant to such grant may be continued, changed, extended, enlarged or structurally altered only as provided in this Section or in Article VII, Chapter 3 or 4, provided that the lot area of the zoning lot occupied by such use is not increased.