For developments containing residences on zoning lots partly within designated open space, the City Planning Commission, on application, may modify the requirements of Section 107-45 (Required Open Space for Residences) to permit more than 50 percent of the required open space, not within the designated open space, to be occupied by driveways, private streets, open accessory off-street parking spaces or open accessory off-street loading berths, if the Commission finds that such facilities are so arranged and landscaped as to afford an acceptable standard of amenity for the development and its surroundings.