Wash. Rev. Code § 35A.63.040
A code city with one or more municipalities within a region, otherwise authorized by law to plan, including municipalities of adjoining states, when empowered by ordinances of their respective legislative bodies, may cooperate to form, organize, and administer a regional planning commission to prepare a comprehensive plan and perform other planning functions for the region defined by agreement of the respective municipalities. The various agencies may cooperate in all phases of planning, and professional staff may be engaged to assist in such planning. All costs shall be shared on a pro rata basis as agreed among the various entities. A code city may also cooperate with any department or agency of a state government having planning functions.
RCW 35A.63.040
Effective date-1969 ex.s. c 81: See note following RCW 35A.13.035.