Wash. Rev. Code § 85.32.160
Whenever any roll shall have been adopted by the board, the regularity, validity and correctness of the proceedings relating thereto shall be conclusive 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 as provided in RCW 85.18.050 and appealing from the action of the board in confirming such roll in the manner and within the time in this chapter provided. No proceeding of any kind, except proceedings had through 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 a suit in injunction may be brought to prevent collection of charges or assessments or sale of property thereunder upon the following grounds and no other:
RCW 85.32.160