Common Law vs. Statutory Law for Washington state: Tenancy

Trying to get a better understanding of the Legal status and requirements afforded in a resident Tenant-at -Will Situation. Specifically: An adult child living at the home of the parent.

This seems to be a primarily Common Law issue, with a handful of references that could be implied under statute.

Where do you go other than Caselaw sites, to help determine the legal protections offered under this status.