An area open to the sky at least five feet in depth shall be provided between a required sidewalk and any building, except that any driveway or parking space located within such open area shall have a depth of at least 18 feet measured perpendicular to such sidewalk. Such open area shall be planted except for parking spaces and entryways to buildings. Where an open area at least 18 feet in depth is required, overhangs of buildings above the first story that project not more than three feet into the required open area are permitted, provided the lowest level of the overhang is at least seven feet above grade at the face of the building. Supports for such overhang are permitted, provided that the total area occupied by such supports does not exceed 15 percent of the area underneath the overhang.