Wash. Rev. Code § 36.88.110
Whenever any assessment roll for construction or improvements shall have been confirmed by the board, as provided in this chapter, the regularity, validity and correctness of the proceedings relating to such construction or improvement and to the assessment therefor, including the action of the board on such assessment roll and the confirmation thereof, shall be conclusive in all things upon all parties and cannot in any manner be contested or questioned in any proceeding whatsoever by any person not filing written objection to such roll in the manner and within the time provided in this chapter, and not appealing from the action of the board in confirming such assessment roll in the manner and within the time provided in this chapter. No proceedings of any kind shall be commenced or prosecuted for the purpose of defeating or contesting any such assessment or for the sale of any property to pay such assessment or any certificate of delinquency issued therefor or the foreclosure of any lien issued therefor, but this section shall not be construed as prohibiting the bringing of injunction proceedings to prevent the sale of any real estate upon the grounds that the property about to be sold does not appear upon the assessment roll, or that the assessment has been paid.
RCW 36.88.110