Wash. Rev. Code § 35.68.020
No such improvement shall be undertaken or required except pursuant to a resolution of the council or commission of the city or town, hereinafter referred to as the city council. The resolution shall state whether the cost of the improvement shall be borne by the city or whether all or a specified portion shall be borne by the city or whether all or a specified portion shall be borne by the abutting property owner; or whether the abutting owner is required to construct the improvement at his or her own cost and expense. If the abutting owner is required to construct the improvement the resolution shall specify the time within which the construction shall be commenced and completed; and further that if the improvement or construction is not undertaken and completed within the time specified that the city will perform or complete the improvement and assess the cost against the abutting owner.
RCW 35.68.020