Some cities in WA require landlords to have permits and licenses. Some cities do not.
Seattle just passed on ordinance that will require all rentals in Seattle to be inspected. If your rental is in Seattle, you should review the Seattle ordinances on rentals. Seattle has ordinances regarding rentals that are not found anywhere else in WA.
If you do not follow the Seattle ordinances, in addition to the WA Residential Landlord-Tenant Act, you may be finding yourself paying the tenant to live in your property.