For any building developed or enlarged after February 3, 1977, a minimum of 20 percent of the lot area of the zoning lot shall be provided as usable, landscaped open recreation space, accessible to the occupants of such development or enlargement or to the public. Such open recreation space shall be located either at the ground level and/or roof level, and shall be landscaped and properly maintained in accordance with the provisions of Section 109-142 (Landscaping regulations).