Wash. Rev. Code § 36.94.170
The primary authority to construct, operate and maintain a system of sewerage and/or water within the boundaries of a municipal corporation which lies within the area of the county's sewerage and/or water general plan shall remain with such municipal corporation. A county, after it has adopted and received the necessary approvals of its sewer and/or water general plan under the provisions of chapter 36.94 RCW may construct, own, operate and maintain a system of sewerage and/or water within the boundaries of a city or town with the written consent of such city or town and within any other municipal corporation provided such municipal corporation (1) has the legislative authority to operate such a utility; and (2)(a) has given its written consent to the county to operate therein; or (b) after adoption of a comprehensive plan or an amendment thereto for the area involved, the municipal corporation has not within twelve months after receiving notice by the county of its intention to serve that area held a formation hearing for a utility local improvement district.
Prior to exercising any authority granted in this section, the county shall compensate such municipal corporation for its reasonable costs, expenses and obligations actually incurred or contracted which are directly related to and which benefit the area which the county proposes to serve. The county may contract with a municipal corporation to furnish such utility service within any municipal corporation.
Except in the case of annexations provided for in RCW 36.94.180, once a county qualifies under this section to serve within a municipal corporation, no municipal corporation may construct or operate a competing utility in the same territory to be served by the county if the county proceeds within a reasonable period of time with the construction of its proposed facilities including the sale of any bonds to finance the same.
As may be permitted by other statutes, a city or town may provide water or sewer service outside of its corporate limits, but such service may not conflict with the county plan or any county, sewer or water facilities installed or being installed.
A county proposing to exercise any authority granted in this section shall give written notice of such intention to the municipal corporation involved and to the boundary review board, if any, of such county. Within sixty days of the filing of such notice of intention, review by the boundary review board of the proposed action may be requested as provided by the provisions of RCW 36.93.100 through 36.93.180. In the event of such review, the board shall consider the factors set forth in this section in addition to the factors and objectives set forth in RCW 36.93.170 and 36.93.180.
RCW 36.94.170
Construction-Severability-1971 ex.s. c 96: See notes following RCW 36.94.010.