In R6, R7 or R8 Districts, for any substantial rehabilitation of one or more non-complying multiple dwellings which were in existence prior to December 15, 1961, the City Planning Commission may authorize the existing open space on the zoning lot to be reduced by not more than five percent and the existing building floor area on such zoning lot to be increased by not more than five percent, if the Commission finds that such modification of the applicable bulk regulations as set forth in Section 54-31 will result in an improved apartment design with adequate access of light and air and an improved circulation system. In the Special Clinton District, such authorizations may apply to complying multiple dwellings and may include a five percent increase in lot coverage and floor area. The Commission, in making the findings above may round out the floor area or lot coverage increase to the nearest percent.

No increase in the existing density and apartment floor area shall be permitted for such buildings.