Wash. Rev. Code § 36.88.170
Whenever any property shall be bid in by any county or be stricken off to any county under and by virtue of any proceeding for enforcement of the assessment provided in this chapter said property shall be held in trust by said county for the fund of the improvement district for the creation of which fund said assessment was levied and for the collection of which assessment said property was sold: PROVIDED, Such county may at any time after the procuring of a deed pay in to such fund the amount of the delinquent assessment for which said property was sold and all accrued interest and interest to the time of the next call for bonds or warrants issued against such assessment fund at the rate provided thereon, and thereupon shall take and hold said property discharged of such trust: PROVIDED FURTHER, That property deeded to any county and which shall become a part of the trust being exercised by the said county for the benefit of any local improvement district fund of the said county, shall be exempt from taxation for general, state, county and municipal purposes during the period that it is so held.
RCW 36.88.170