Wash. Rev. Code § 54.48.030
In aid of the foregoing declaration of policy, any public utility and any cooperative is hereby authorized to enter into agreements with any one or more other public utility or one or more other cooperative for the designation of the boundaries of adjoining service areas which each such public utility or each such cooperative shall observe, for the establishment of procedures for orderly extension of service in adjoining areas not currently served by any such public utility or any such cooperative and for the acquisition or disposal by purchase or sale by any such public utility or any such cooperative of duplicating utility facilities, which agreements shall be for a reasonable period of time not in excess of twenty-five years: PROVIDED, That the participation in such agreement of any public utility which is an electrical company under RCW 80.04.010, excepting cities and towns, shall be approved by the Washington utilities and transportation commission.
RCW 54.48.030