Cal. Government Code § 66479

There may be imposed by local ordinance a requirement that areas of real property within the subdivision be reserved for parks, recreational facilities, fire stations, libraries, or other public uses, subject to the following conditions: (a) The requirement is based upon an adopted specific plan or an adopted general plan containing policies and standards for those uses, and the required reservations are in accordance with those policies and standards. (b) The ordinance has been in effect for a period of at least 30 days prior to the filing of the tentative map. (c) The reserved area is of such size and shape as to permit the balance of the property within which the reservation is located to develop in an orderly and efficient manner. (d) The amount of land reserved will not make development of the remaining land held by the subdivider economically unfeasible. The reserved area shall conform to the adopted specific or general plan and shall be in such multiples of streets and parcels as to permit an efficient division of the reserved area in the event that it is not acquired within the prescribed period; in such event, the subdivider shall make those changes as are necessary to permit the reserved area to be developed for the intended purpose consistent with good subdividing practices.