Wash. Rev. Code § 85.08.450
Whenever any schedule of apportionment of any drainage or diking improvement district shall have been confirmed, and the assessment therefor shall have been levied, by the board of county commissioners, as provided by RCW 85.08.400 through 85.08.430, the regularity, validity and correctness of the proceedings relating to such improvement, and to the assessment therefor, including the action of the board of county commissioners upon 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 objections to such roll in the manner and within the time provided in RCW 85.08.400 through 85.08.430, and not appealing from the action of the board of county commissioners in confirming such assessment roll in the manner and within the time in this chapter provided. No proceeding of any kind shall be commenced or prosecuted for the purpose of defeating or contesting any such assessment, or the sale of any property to pay such assessment, or any certificate of delinquency issued therefor, or the foreclosure of any lien issued therefor: PROVIDED, That this section shall not be construed as prohibiting the bringing of injunction proceedings to prevent the sale of any real estate upon the grounds:
RCW 85.08.450