Wash. Rev. Code § 85.15.080
Wherever any roll shall have been adopted by the board of county commissioners, the regularity, validity and correctness of the proceedings relating thereto shall be conclusive upon all parties, and it cannot in any manner be contested or questioned in any proceeding whatsoever by any person not filing written objections to the roll as provided in RCW 85.15.050 and appealing from the action of said board in confirming the roll in the manner and within the time in this chapter provided. No proceeding of any kind, except proceedings had throughout the process of appeal as in this chapter provided, shall be commenced or prosecuted or may be maintained, for the purpose of defeating or contesting any assessment or charge made through levies under this chapter, or the sale of any property to pay such charges: PROVIDED, That suit in injunction may be brought to prevent collection of charges of assessments or sale of property thereunder upon the following grounds and no other:
RCW 85.15.080