if i am a tenant renting in washington state, and i have a guest stay at my house, how long can they stay with me before they legally have to fill out a background check?
There is no legal time limit that defines a guest versus a roomate.
There is no requirement for a background check, though a land owner may require one.
Read your lease. If it prohibits unauthorized occupants, it may define how long a guest may stay before being considered an occupant. If not, it is whatever a court thinks, should it go that far.
If the lease is silent as to addtional occupants, sub-leasing, etc., then the landlord cannot prevent you from having a sub-tenant, roomate, etc.
