Wash. Rev. Code § 35.68.040
When all or any portion of the cost is to be assessed against the abutting property owner, the city council may create a "sidewalk construction fund No. . . . ." to be numbered differently for each improvement; and with warrants drawn on this fund the cost of the respective improvements may be paid. The city may advance as a loan to the sidewalk construction fund from any available funds the amounts necessary to pay any costs of the improvement. When any assessments are made for the improvement, payments therefor shall be paid into the particular sidewalk improvement fund; and whenever any funds are available over the amounts necessary to pay outstanding warrants any advances or loans made to the fund shall be repaid. Whenever warrants are drawn on any such fund which are not paid for want of sufficient funds, they shall be so stamped and shall bear interest until called and paid at a rate established by the city council by resolution.
RCW 35.68.040