Wash. Rev. Code § 36.70.060
A county or a city may join with one or more other counties, cities and towns, and/or with one or more school districts, public utility districts, private utilities, housing authorities, port districts, or any other private or public organizations interested in regional planning to form and organize a regional planning commission and provide for the administration of its affairs. Such regional planning commission may carry on a planning program involving the same subjects and procedures provided by this chapter for planning by counties, provided this authority shall not include enacting official controls other than by the individual participating municipal corporations. The authority to initiate a regional planning program, define the boundaries of the regional planning district, specify the number, method of appointment and terms of office of members of the regional planning commission and provide for allocating the cost of financing the work shall be vested individually in the governing bodies of the participating municipal corporations.
Any regional planning commission or municipal corporation participating in any regional planning district is authorized to receive grants-in-aid from, or enter into reasonable agreement with any department or agency of the government of the United States or of the state of Washington to arrange for the receipt of federal funds and state funds for planning in the interests of furthering the planning program.
RCW 36.70.060
Commission as employer for retirement system purposes: RCW 41.40.010.