Wash. Rev. Code § 35.51.040
For the purpose of securing the payment of the principal of and interest on an issue of local improvement bonds, notes, warrants, or other short-term obligations, the legislative authority of a municipality may create a reserve fund in an amount not exceeding fifteen percent of the principal amount of the bonds, notes, or warrants issued. The cost of a reserve fund may be included in the cost and expense of any local improvement for assessment against the property in the local improvement district to pay the cost, or any part thereof. The reserve fund may be provided for from the proceeds of the bonds, notes, warrants, or other short-term obligations, from special assessment payments, or from any other money legally available therefor. The legislative authority of a municipality shall provide that after payment of administrative costs a sum in proportion to the ratio between the part of the original assessment against a given lot, tract, or parcel of land in a local improvement district assessed to create a reserve fund, if any, and the total original amount of such assessment, plus a proportionate share of any interest accrued in the reserve fund, shall be credited and applied, respectively, to any nondelinquent portion of the principal of that assessment and any nondelinquent installment interest on that assessment paid by a property owner, but in no event may the principal amount of bonds outstanding exceed the principal amount of assessments outstanding. Whether the payment is made during the thirty-day prepayment period referred to in RCW 35.49.010 and 35.49.020 or thereafter and whenever all or part of a remaining nondelinquent assessment or any nondelinquent installment payment of principal and interest is paid, the reserve fund balance shall be reduced accordingly as each such sum is thus credited and applied to a nondelinquent principal payment and a nondelinquent interest payment. Each payment of a nondelinquent assessment or any nondelinquent installment payment of principal and interest shall be reduced by the amount of the credit. The balance of a reserve fund remaining after payment in full and retirement of all local improvement bonds, notes, warrants, or other short-term obligations secured by such fund shall be transferred to the municipality's guaranty fund.
Where, before July 26, 1987, a municipality established a reserve fund under this section that did not provide for a credit or reimbursement of the money remaining in the reserve fund to the owners of the lots, tracts, or parcels of property subject to the assessments, the balance in the reserve fund shall be distributed, after payment in full and retirement of all local improvement district bonds and other obligations secured by the reserve fund, to those owners of the lots, tracts, or parcels of property subject to the assessments at the time the final installment or assessment payment on the lot, tract, or parcel was made. No owner is eligible to receive reimbursement for a lot, tract, or parcel if a lien on an unpaid assessment, or an installment thereon, that was imposed on such property remains in effect at the time the reimbursement is made or was foreclosed on the property. The amount to be distributed to the owners of each lot, tract, or parcel that is eligible for reimbursement shall be equal to the balance in the reserve fund, multiplied by the assessment imposed on the lot, tract, or parcel, divided by the total of all the assessments on the lots, tracts, or parcels eligible for reimbursement.
RCW 35.51.040