Wash. Rev. Code § 8.12.420
In all cases where any city shall issue bonds as provided for in this chapter, the whole or any portion of the separate assessments for any such improvement may be paid during the thirty day period provided for in RCW 8.12.430, and thereafter the sum remaining unpaid may be paid in equal annual installments; the number of which installments shall be less by two than the number of years which the bonds issued to pay for the improvements may run, with interest upon the whole unpaid sum at the bond rate, and each year thereafter one of such installments, together with the interest due thereon and on all installments thereafter to become due, shall be collected in the same manner as shall be provided by law and the resolutions and ordinances of such city for the collection of assessments for such improvements in cases where no bonds are issued: PROVIDED, HOWEVER, That whenever the legislative body of any city of the first class having a population of three hundred thousand inhabitants, or more, shall have, as provided in RCW 8.12.400, by unanimous vote determined that any bonds issued hereunder shall be payable in twenty-two years, such legislative body may by ordinance provide that the principal sum remaining unpaid after the thirty day period specified in RCW 8.12.430 may be paid in ten equal annual installments, beginning with the eleventh year and ending with the twentieth year after said thirty day period, together with interest upon the unpaid installments at the bond rate, and that in each year after the said thirty day period, to and including the tenth year thereafter, one installment of interest on the principal sum of said assessment shall be paid and collected, and that, beginning with the eleventh year after said thirty day period, one installment of the principal, together with the interest due thereon and on all installments thereafter to become due, shall be paid and collected in the same manner as shall be provided by law and the resolutions and ordinances of such city for the collection of assessments for such improvements in cases where no bonds are issued.
In all cases of improvements authorized in this chapter, where, at the time this chapter shall become effective, the notice by the city treasurer of the assessment for such improvement shall not have been published, the city council or other legislative body of such city may by ordinance or resolution provide for the issuance and sale of bonds for such improvement and for the payment of such assessments in installments.
RCW 8.12.420