In Young v. Riley the Washington Supreme Court cited a broken line of decisions, called them "unbroken," and held that set-off's and counterclaims cannot be adjudicated in unlawful detainer actions.
Evan L. Schwab,
Washington Case Law,
Landlord and Tenant—Unlawful Detainer—Set-off's & Counterclaims,
Wash. L. & Rev.
Available at: https://digitalcommons.law.uw.edu/wlr/vol37/iss2/10