For a complying or non-complying building existing on December 15, 1961, or in R2X, R3, R4 or R5 Districts on June 30, 1989, and containing residential uses, the Board of Standards and Appeals may permit an enlargement, a change of use or (in the case of a mixed building) an extension, provided that such enlargement, change of use or extension shall not create any new non-compliance or increase the amount or degree of any existing non-compliance except as provided in this Section.

In the districts and for the buildings for which an open space ratio is required, the open space ratio permitted under this Section shall not be less than 90 percent of the open space ratio required under the applicable bulk regulations set forth in Article II or III of this Resolution. In the districts and for the buildings to which a maximum lot coverage applies, the maximum lot coverage permitted under this Section shall not exceed 110 percent of the maximum lot coverage permitted under the applicable bulk regulations set forth in Article II or III of this Resolution. In all districts, the floor area ratio permitted under this Section shall not exceed the floor area ratio permitted under such regulations by more than 10 percent. In R2X, R3 or R4 Districts, the additional floor area permitted pursuant to this Section may be computed using a base floor area ratio including the floor area permitted under a sloping roof with a structural headroom between five and eight feet when such space is provided in the building.