For all large-scale residential developments in R5, R6, R7, R8 or R9 Districts, or in Commercial Districts in which residences are governed by the bulk regulations of such Residence Districts, the City Planning Commission may modify the requirement for open space as set forth in the definition of open space in Section 12-10 (DEFINITIONS) by allowing parking space on the roofs of parking garages not abutting another building and not more than 23 feet in height above curb level, to count as open space and by not requiring connections from such roofs to ground level by exterior passageways or ramps, provided that the following findings are made:

(a)        that the total area occupied by driveways, private streets, or open accessory off-street parking spaces in all areas claimed as common or private open space throughout the large-scale residential development shall not exceed 40 percent of the total required open space for the large-scale residential development; and

(b)        that such arrangement and use of open space results in better site planning and community planning.