Common open space for recreation purposes, either active or passive, shall be an essential and a major element of the plan. A minimum of 25% of the gross tract acreage shall be reserved as open space for the benefit of all residents of the development. Such open space may include areas of land and water, but shall exclude all roads, parking areas, structures, and service lanes. Where possible, such open space shall include the preservation of existing natural features and shall be easily accessible to all units in the development. Portions of the area to be used for recreation shall have suitable physical characteristics, including well-drained soils, gentle topography, and suitable shape and size.