Wash. Rev. Code § 59.20.145
A tenant in a mobile home park may share his or her mobile home, manufactured home, or park model with any person over eighteen years of age, if that person is providing live-in home health care or live-in hospice care to the tenant under an approved plan of treatment ordered by the tenant's physician. The live-in care provider is not considered a tenant of the park and shall have no rights of tenancy in the park. Any agreement between the tenant and the live-in care provider does not change the terms and conditions of the rental agreement between the landlord and the tenant. The live-in care provider shall comply with the rules of the mobile home park, the rental agreement, and this chapter. The landlord may not charge a guest fee for the live-in care provider.
RCW 59.20.145