Wash. Rev. Code § 4.08.080
Any assignee or assignees of any judgment, bond, specialty, book account, or other chose in action, for the payment of money, by assignment in writing, signed by the person authorized to make the same, may, by virtue of such assignment, sue and maintain an action or actions in his or her name, against the obligor or obligors, debtor or debtors, named in such judgment, bond, specialty, book account, or other chose in action, notwithstanding the assignor may have an interest in the thing assigned: PROVIDED, That any debtor may plead in defense as many defenses, counterclaims and offsets, whether they be such as have heretofore been denominated legal or equitable, or both, if held by him against the original owner, against the debt assigned, save that no counterclaim or offset shall be pleaded against negotiable paper assigned before due, and where the holder thereof has purchased the same in good faith and for value, and is the owner of all interest therein.
RCW 4.08.080