Absent other provisions specified for the particular type of planned development, at least 25% of the gross tract area shall be restricted as common open space. The portion of the gross tract area which is restricted as common open space shall encompass at least 25% of the net tract area. The net tract area is that area of the tract which is not encroached upon by freshwater wetlands, wetlands transition areas, detention basins, the area of the one-hundred-year floodplain, floodways, steep slopes of 15% or greater, existing easements, buildings, parking lots, and private space, such as patios or yards, not available for common use.