Wash. Rev. Code § 36.61.200
If the special assessments are to be payable at one time, all or any portion of any special assessment may be paid without interest, penalty, or costs during this thirty-day period and placed into a special fund to defray the costs of the lake or beach improvement or maintenance activities. The remainder shall be paid in installments as provided in a resolution adopted by the county legislative authority, but the last installment shall be due at least two years before the maximum term of the bonds issued to pay for the improvements or maintenance. The installments shall include amounts sufficient to redeem the bonds issued to pay for the lake or beach improvement and maintenance activities. A twenty-day period shall be allowed after the due date of any installment within which no interest, penalty, or costs on the installment may be imposed.
The county shall establish by ordinance an amount of interest that will be imposed on late special assessments imposed annually or at once, and on installments of a special assessment. The ordinance shall also specify the penalty, in addition to the interest, that will be imposed on a late annual special assessment, special assessment, or installment which shall not be less than five percent of the delinquent special assessment or installment.
The owner of any lot, tract, parcel of land, or other property charged with a special assessment may redeem it from all liability for the unpaid amount of the installments by paying, to the county treasurer, the remaining portion of the installments that is attributable to principal on the lake or beach management district bonds.
RCW 36.61.200