Wash. Rev. Code § 35.44.190
Whenever any assessment roll for local improvements has been confirmed by the council, the regularity, validity, and correctness of the proceedings relating to the improvement and to the assessment therefor, including the action of the council upon the assessment roll and the confirmation thereof shall be conclusive in all things upon all parties. They cannot in any manner be contested or questioned in any proceeding by any person unless he or she filed written objections to the assessment roll in the manner and within the time required by the provisions of this chapter and unless he or she prosecutes his or her appeal in the manner and within the time required by the provisions of this chapter.
No proceeding 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 therefor, except that injunction proceedings may be brought to prevent the sale of any real estate upon the ground (1) that the property about to be sold does not appear upon the assessment roll or, (2) that the assessment has been paid.
If federal, local, or state funds become available for a local improvement after the assessment roll has been confirmed by the city 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 35.44.190