In R3-1 Districts, including Commercial Districts mapped within such Residence Districts, for any large-scale residential development, the City Planning Commission, by special permit, may allow the open space ratio otherwise required for such large-scale residential development as a whole to be reduced by not more than 20 percent, may allow the maximum number of dwelling units to be increased by not more than 10 percent and may allow the maximum residential floor area ratio to be increased by not more than 15 percent, provided that:

(a)        at least one acre or 20 percent of the total open space, whichever is more, is provided in common areas meeting the requirements of Section 78-52 (Common Open Space) and not used for off-street parking;

(b)        the large-scale residential development qualifies for a bonus in accordance with the provisions of Section 78-32; and

(c)        the Commission makes the findings required in Section 78-32 and in paragraph (e) of Section 78-313 (Findings).

Such authorizations shall be instead of, rather than in addition to, those which would be allowable under the provisions of Section 78-32.