For sky exposure plane buildings in R6 through R9 Districts without a letter suffix, the applicable floor area ratio and open space ratio provisions of Section 23-73 (Special Provisions for Sky Exposure Plane Buildings), inclusive, shall be modified by the provisions of this Section.

1. Open space ratio for residential portions of buildings
   
   For zoning lots containing a residential building or for the residential portion of a mixed building, a minimum open space ratio shall be provided in accordance with Section 23-70. For the purposes of applying such regulations:
   1. the floor area counted in determining the open space ratio shall be only that floor area in the residential portion of the building;
   2. the lot coverage shall be deemed to be that portion of the zoning lot which, when viewed directly from above, would be covered by the residential portion of the building at any level; and
   3. the applicable height factor, if the maximum permitted residential floor area ratio is less than the total floor area ratio permitted for such building, shall be the height factor of the residential portion of the building.
       

      A non-residential use occupying a portion of a building that was in existence on December 15, 1961, may be changed to a residential use and the regulations on minimum required open space ratio shall not apply to such change of use.

2. Location of open space
   
   The open space required for a residential building or for the residential portion of a mixed building under the provisions of paragraph (a) of this Section may be provided at ground floor level or upon the roof of a building. Open space on a roof may be located at a level higher than 23 feet above curb level on a community facility building or the community facility portion of a building, provided that the level of any open space may not be higher than 2 feet, 6 inches below the sill level of any legally required window opening on such roof area, in the residential portion of a building used partly for community facility and residential uses. However, open space located on the roof of a community facility building separated by open area from residential buildings or buildings used partly for community facility and residential uses on the same zoning lot may not be at a level higher than 23 feet above curb level.
   
   For the purposes of this Section, abutting buildings on a single zoning lot may be considered to be a single building.