Wash. Rev. Code § 6.27.210
If the garnishee files an answer, either the plaintiff or the defendant, if not satisfied with the answer of the garnishee, may controvert within twenty days after the filing of the answer, by filing an affidavit in writing signed by the controverting party or attorney or agent, stating that the affiant has good reason to believe and does believe that the answer of the garnishee is incorrect, stating in what particulars the affiant believes the same is incorrect. Copies of the affidavit shall be served on or mailed by first-class mail to the garnishee at the address indicated on the answer or, if no address is indicated, at the address to or at which the writ was mailed or served, and to the other party, at the address shown on the writ if the defendant controverts, or at the address to or at which the copy of the writ of garnishment was mailed or served on the defendant if the plaintiff controverts, unless otherwise directed in writing by the defendant or defendant's attorney.
RCW 6.27.210