Wash. Rev. Code § 35.45.090
Any funds in the treasury of any municipal corporation belonging to the fund of any local improvement district 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 such municipal corporation on account thereof, shall be refunded, on demand, to the payers into such fund. Each such payer shall be entitled to such proportion of such excess as his or her original assessment bears to the entire original assessment levied for such improvement. Such municipal corporation may, after one year from the date on which the last installment becomes due, transfer any balance remaining on hand to the general fund of such municipal corporation, but shall, notwithstanding such transfer remain liable for the refund herein provided for until such refund shall have been made, unless the actual cost involved in making such refund shall exceed the excess in such fund.
Such demand shall be made in writing to the treasurer of such municipal corporation. No action shall be commenced in any court to obtain any such refund, except upon such demand, and until ninety days after making such demand. No excess shall be recovered in any action where the excess in the fund does not average the sum of one dollar in favor of all payers into such fund.
This section shall not be deemed to require the refunding of any balance left 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, whether accruing heretofore or hereafter, may be turned into the general fund or otherwise disposed of, as the legislative authority of the city may direct.
The provisions of this chapter 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 35.45.090