Wash. Rev. Code § 8.12.430
Whenever the assessment for any such improvement shall be payable in installments, the owner of any lot, tract, or parcel of land or other property charged with any such assessment may pay the assessment or any portion thereof, without interest, within thirty days after such notice of the assessment.
The city treasurer shall, as soon as the certified copy of the assessment roll has been placed in his or her hands for collection, publish a notice in the official newspaper of the city for two consecutive daily or two consecutive weekly issues, that the roll is in his or her hands for collection and that any assessment thereon or any portion of any such assessment may be paid at any time within thirty days from the date of the first publication of the notice without penalty, interest, or costs, and the unpaid balance, if any, may be paid in equal annual installments, or any such assessment may be paid at any time after the first thirty days following the date of the first publication of the notice by paying the entire unpaid portion thereof with all penalties and costs attached, together with all interest thereon to the date of delinquency of the first installment thereof next falling due.
The notice shall further state that the first installment of the assessment shall become due and payable during the thirty day period succeeding a date one year after the date of first publication of the notice, and annually thereafter each succeeding installment shall become due and payable in like manner.
If the whole or any portion of any assessment remains unpaid after the first thirty day period herein provided for, interest upon the whole unpaid sum shall be charged at the bond rate, and each year thereafter one of the installments, together with interest due upon the whole of the unpaid balance, shall be collected, except that where the assessment is payable in twenty years, installments of interest only shall be collected for the first ten years, as provided in RCW 8.12.420.
Any installment not paid prior to the expiration of the thirty day period during which the installment is due and payable, shall thereupon become delinquent. All delinquent installments shall be subject to a charge of five percent penalty levied upon both principal and interest due on the installments, and all delinquent installments, except installments of interest when the assessment is payable in twenty years, as provided in RCW 8.12.420, shall, until paid, be subject to a charge for interest at the bond rate.
The bonds herein provided for shall not be issued prior to twenty days after the expiration of the thirty days first above mentioned, but may be issued at any time thereafter. In all cases where any sum is paid as herein provided, the same shall be paid to the city treasurer, or to the officer whose duty it is to collect the assessments, and all sums so paid shall be applied solely to the payment of the awards, interest and costs of the improvements or the redemption of the bonds issued therefor.
RCW 8.12.430