Security deposits in Washington aren’t taxable until they become the landlord’s property. This happens when the security deposit is applied to rent, forfeited, or applied to charges allowed under the lease.
However, when they are used to cover expenses incurred by the landlord due to the tenant’s fault, like the cost of repairs or unpaid utilities, they are not always taxable. The IRS advises to only report them as income if the landlord also reports the costs they covered as expenses. Otherwise, there is no need to report them as income, in which case they will not be taxed.
The information for this answer was found on our Washington Security Deposit Law answers.