In R1-2, R2 or 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 the large-scale residential development as a whole and for individual zoning lots therein to be reduced by not more than 10 percent, may allow the maximum number of dwelling units to be increased by not more than five percent, and may allow the maximum residential floor area ratio to be increased by not more than 7.5 percent, if the Commission finds that throughout the large-scale residential development the site plan provides a significantly better arrangement of the buildings in relation to one another and to their sites from the standpoints of privacy, access of light, organization of private open spaces and preservation of important natural features than would be possible or practical for a development comprised of similar types built in strict compliance with the applicable district regulations.