In an M1 District, a non-conforming residential use occupying at least 50 percent of the floor area of a building which was designed for residential use and erected prior to December 15, 1961, may be extended on the ground floor level provided that no dwelling unit or rooming unit may be located on or below a story occupied by a commercial or manufacturing use. The total number of dwelling units or rooming units in the building may not be increased by more than one for each 400 square feet of residential floor area created by such extension.