OPEN SPACE is that part of a ZONING LOT, including COURTS or YARDS, which:

(a) is open and unobstructed from its lowest level to the sky, except as provided below;
(b) is accessible to and usable by all persons occupying a DWELLING UNIT or a ROOMING UNIT on the ZONING LOT; and
(c) is not part of the roof of that portion of a BUILDING containing DWELLING UNITS or ROOMING UNITS.

OPEN SPACE may include roofed areas, the total area of which is less than 10 percent of the unroofed or uncovered area of a ZONING LOT, provided that such roofed area is not enclosed on more than one side, or on more than 10 percent of the perimeter of the roofed area, whichever is greater.

The roof of any portion of a BUILDING used for ACCESSORY parking or for any permitted non-RESIDENTIAL USE, which is a portion of or attached to a RESIDENTIAL BUILDING, or the roof of a COMMUNITY FACILITY BUILDING, may be considered as OPEN SPACE if such roof area meets the requirements set forth in this definition, and:

(1) is not higher than 23 feet above CURB LEVEL, provided that this restriction does not apply to the roof of a portion of a MIXED BUILDING used for other than RESIDENCES;

(2) is at least two and one-half feet below the sill level of all LEGALLY REQUIRED WINDOWS opening on such roof area;

(3) is directly accessible by a passageway from a BUILDING, or by a ramp (with a grade of less than 10 percent) from a BUILDING, YARD, COURT or STREET, except that in R8 or R9 Districts such roof area need not be accessible to occupants and is therefore exempt from this requirement; and

(4) has no dimension less than 25 feet; except that in R8 or R9 Districts when such roof area adjoins a STREET LINE or a REAR YARD, it may have a minimum depth of nine feet and a minimum length, along such STREET LINE or REAR YARD, equal to at least twice its depth, or the full width of the ZONING LOT, or 50 feet, whichever is the least distance.