A portion of a building used for residential uses other than dwelling units in Quality Housing buildings containing affordable independent residents for seniors on zoning lots meeting the criteria set forth in paragraph (a)(4) of Section 23-664 (Modified height and setback regulations for certain Inclusionary Housing buildings or affordable independent residences for seniors) shall be a permitted obstruction within a rear yard or rear yard equivalent on zoning lots in Special Mixed Use Districts with R6 through R10 District designations, provided that the height of such building portion does not exceed one story, or 15 feet above the adjoining grade, whichever is less, and provided that such space shall be accessible to all residents of the building.