Wash. Rev. Code § 35.68.070
The assessment roll as affirmed or modified by the city council shall be filed with the city treasurer for collection, and the amount thereof including interest, if any, shall become a lien against the property described therein from the date of such filing. Whenever any payment on any assessment or installment is delinquent and unpaid for a period of thirty days or more the lien may be foreclosed in the same manner and with the same effect as is provided by chapters 35.43 to 35.54 RCW, inclusive; as now or hereafter amended. Whenever the deed is issued after the sale therein provided, the regularity, validity and correctness of the proceedings relating to such improvement and the assessment therefor shall be final and conclusive and no action shall thereafter be brought by or in behalf of any person to set aside said deed.
RCW 35.68.070