RCW 59.18.110

(b) A reasonable time has passed for the landlord to remedy the defective condition following notice to the landlord in accordance with RCW 59.18.070 or such other time as may be allotted by the court or arbitrator; the court or arbitrator may determine the diminution in rental value of the premises due to the defective condition and shall render judgment against the landlord for the rent paid in excess of such diminished rental value from the time of notice of such defect to the time of decision and any costs of repair done pursuant to RCW 59.18.100 for which no deduction has been previously made. Such decisions may be enforced as other judgments at law and shall be available to the tenant as a set-off against any existing or subsequent claims of the landlord.