The answer is in RCW 59.18.253, which is part of Washington's Residential Landlord Tenant statute. It says:
Deposit to secure occupancy by tenant -- Landlord's duties -- Violation.
(1) It shall be unlawful for a landlord to require a fee from a prospective tenant for the privilege of being placed on a waiting list to be considered as a tenant for a dwelling unit.
(2) A landlord who charges a prospective tenant a fee or deposit to secure that the prospective tenant will move into a dwelling unit, after the dwelling unit has been offered to the prospective tenant, must provide the prospective tenant with a receipt for the fee or deposit, together with a written statement of the conditions, if any, under which the fee or deposit is refundable. If the prospective tenant does occupy the dwelling unit, then the landlord must credit the amount of the fee or deposit to the tenant's first month's rent or to the tenant's security deposit. If the prospective tenant does not occupy the dwelling unit, then the landlord may keep up to the full amount of any fee or deposit that was paid by the prospective tenant to secure the tenancy, so long as it is in accordance with the written statement of conditions furnished to the prospective tenant at the time the fee or deposit was charged. A fee charged to secure a tenancy under this subsection does not include any cost charged by a landlord to use a tenant screening service or obtain background information on a prospective tenant.
(3) In any action brought for a violation of this section a landlord may be liable for the amount of the fee or deposit charged. In addition, any landlord who violates this section may be liable to the prospective tenant for an amount not to exceed one hundred dollars. The prevailing party may also recover court costs and a reasonable attorneys' fee.
You should have a lawyer review your documents to see how this applies.