Residential buildings and appurtenant buildings such as garages, sheds, barns, etc., presently existing in Business A Districts may be added thereto, altered or rebuilt. No new building or other structure shall be erected for residential purposes, except for a one-family dwelling for one housekeeping unit only or, unless the land so located in the Business A District is part of a subdivision plan duly recorded prior to the enactment of this section or unless individual building lots have been similarly recorded.