Wash. Rev. Code § 35.22.580
Whenever any city of the first class shall levy and collect moneys by sale of bonds or otherwise for any local improvement by special assessment therefor, the same shall be carried in a special fund to be used for said purpose, and no part thereof shall be transferred or diverted to any other fund or use: PROVIDED, That any funds remaining after the payment of the whole cost and expense of such improvement, in excess of the total sum required to defray all the expenditures by the city on account thereof, shall be refunded on demand to the amount of such overpayment: PROVIDED FURTHER, That this section shall not be deemed to require the refunding of any balance in any local improvement fund after the payment of all outstanding obligations issued against such fund, where such balance accrues from any saving in interest or from penalties collected upon delinquent assessments, but any such balance may be turned into the general fund or otherwise disposed of, as the legislative authority of such city may direct by ordinance. The provisions of this section relating to the refund of excess local improvement district funds shall not apply to any district whose obligations are guaranteed by the local improvement guaranty fund.
RCW 35.22.580