Common open space totaling not less than 15% of the total PDD shall be provided in perpetuity. Parking areas, roads, buildings, other impervious surfaces, and their improvements shall not be included in the calculation to determine the amount of available open space. However, the entire tract, apart from these exceptions, shall be considered in determining the required amount of open space. The following facilities or improvements may be located on open space land: water systems, stormwater systems, bike paths, walking trails, and other common community facilities that do not involve buildings, such as tennis courts, swimming pools, and playgrounds.