Wash. Rev. Code § 35.56.130
The city council or commission shall have full authority to provide for the issuance of such bonds against the improvement district fund in such denominations as the city council or commission may provide, which shall bear such rate of interest as the city council or commission may fix. Interest shall be paid annually and the bonds shall become due and payable at such time, not exceeding fifteen years from the date thereof, as may be fixed by the said council or commission and shall be payable out of the assessment district funds.
If so ordered by the council or commission, the bonds may be issued in such a way that different numbers of the bonds may become due and payable at different intervals of time, or they may be so issued that all of the bonds against said district mature together. The city may reserve the right to call or mature any bond on any interest paying date when sufficient funds are on hand for its redemption; but bonds shall be called in numerical order.
RCW 35.56.130