In general, common open space land areas shall be preserved in their natural state and their use shall be limited to appropriate conservation and passive recreation purposes. A portion of such common open space, not to exceed 10% of the gross land area of the average density development, may be reserved and designated for active recreation purposes, provided the size, shape, access and location of such area(s) is approved by the Planning Board. Within a designated active recreation area, there may be located swimming pools, ball fields, facilities for court games, clubhouses, playground equipment, and so forth, provided the use of all such facilities shall be limited to the residents of the average density development and their guests, and further provided that such facilities shall be subject to site plan approval by the Planning Board.