Wash. Rev. Code § 7.16.170 : SPECIAL PROCEEDINGS AND ACTIONS — CERTIORARI, MANDAMUS, AND PROHIBITION —MANDAMUS — Absence of remedy at law required-Affidavit

The writ must be issued in all cases where there is not a plain, speedy and adequate remedy in the ordinary course of law. It must be issued upon affidavit on the application of the party beneficially interested.