Wash. Rev. Code § 85.05.470
Any protestant who filed a protest prior to the final order of the board, may appeal from such final order, but to do so must within ten days from the date said order was entered, bring direct action in the superior court in the county wherein such district or portion thereof is situated, against such board of commissioners in their official capacity, which action shall be prosecuted under the procedure of civil actions, with appellate review as provided in civil actions. In any such action so brought, the order of the board shall be conclusive of the regularity and propriety of the proceedings, and all other matters, except it shall be open to attack upon the ground of fraud, unfair dealing, arbitrary or unreasonable action of the board.
RCW 85.05.470
Severability-1988 c 202: See note following RCW 2.24.050.