Wash. Rev. Code § 36.94.280
Whenever any assessment roll for local improvements has been confirmed by the county legislative authority, the regularity, validity and correctness of the proceedings relating to the improvement and to the assessment therefor, including the action of the county legislative authority upon the 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 by any person not filing written objections to the assessment roll in the manner and within the time provided in this chapter, and not appealing from the action of the county legislative authority in confirming the assessment roll in the manner and within the time in this chapter provided. No proceedings of any kind shall be commenced or prosecuted for the purpose of defeating or contesting any assessment, or the sale of any property to pay an 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:
If federal, local, or state funds become available for a local improvement after the assessment roll has been confirmed by the county legislative authority, the funds may be used to lower the assessments on a uniform basis. Any adjustments to the assessments because of the availability of federal or state funds may be made on the next annual payment.
RCW 36.94.280