Wash. Rev. Code § 35.21.426
Whenever a county or school district affected by the project sustains such financial loss or is affected by an increased financial burden as above set forth or it appears that such a financial loss or burden will occur beginning not later than within the next three months, such county or school district shall immediately notify the city in writing setting forth the particular losses or increased burden and the city shall immediately enter into negotiations to effect a contract. In the event the city and the county or school district are unable to agree upon terms and conditions for such contract, then in that event, within sixty days after such notification, the matter shall be submitted to a board of three arbitrators, one of whom shall be appointed by the city council of the city concerned; one by the board of county commissioners for the county concerned or by the school board for the school district concerned, and one by the two arbitrators so appointed. In the event such arbitrators are unable to agree on a third arbitrator within ten days after their appointment then the third arbitrator shall be selected by the state auditor. The board of arbitrators shall determine the loss of revenue and/or the cost of the increased financial burden placed upon the county or school district and its findings shall be binding upon such city and county or school district and the parties shall enter into a contract for reimbursement by the city in accordance with such findings, with the payment under such findings to be retroactive to the date when the city was first notified in writing.
RCW 35.21.426