Wash. Rev. Code § 42.24.090
No claim for reimbursement of any expenditures by officers or employees of any municipal corporation or political subdivision of the state for transportation, lodging, meals or any other purpose shall be allowed by any officer, employee or board charged with auditing accounts unless the same shall be presented in a detailed account: PROVIDED, That, unless otherwise authorized by law, the legislative body of any municipal corporation or political subdivision of the state may prescribe by ordinance or resolution the amounts to be paid officers or employees thereof as reimbursement for the use of their personal automobiles or other transportation equipment in connection with officially assigned duties and other travel for approved public purposes, or as reimbursement to such officers or employees in lieu of actual expenses incurred for lodging, meals or other purposes. The rates for such reimbursements may be computed on a mileage, hourly, per diem, monthly, or other basis as the respective legislative bodies shall determine to be proper in each instance: PROVIDED, That in lieu of such reimbursements, payments for the use of personal automobiles for official travel may be established if the legislative body determines that these payments would be less costly to the municipal corporation or political subdivision of the state than providing automobiles for official travel.
All claims authorized under this section shall be duly certified by the officer or employee submitting such claims on forms and in the manner prescribed by the state auditor.
RCW 42.24.090