Wash. Rev. Code § 35.58.430
The treasurer of each component county shall create a separate fund into which shall be paid all money collected from taxes levied by the metropolitan municipal corporation on property in such county and such money shall be forwarded quarterly by the treasurer of each such county to the treasurer of the central county as directed by the metropolitan council. The treasurer of the central county shall act as the treasurer of the metropolitan municipal corporation and shall establish and maintain such funds as may be authorized by the metropolitan council. Money shall be disbursed from such funds upon warrants drawn by the auditor of the central county as authorized by the metropolitan council. The central county shall be reimbursed by the metropolitan municipal corporation for services rendered by the treasurer and auditor of the central county in connection with the receipt and disbursement of such funds. The expense of all special elections held pursuant to this chapter shall be paid by the metropolitan municipal corporation.
RCW 35.58.430