When Does a Homeowner Need to Get Certificate of Occupancy

[evident]

My question involves landlord-tenant law in the State of: WA, City of Redmond (part of King County)

I've been living in a house I own for many years by myself. If I want to rent out a room to a friend, do I need to get a certificate of occupancy?
What if I let the friend live for free?

What happens if a CoO is required and I don't have it?

How long is the CoO good for? Do I need to get one to sell the house?
What other use is there for it?

[Mr. Knowitall]

Try asking the City Department of Planning and Community Development about certification of rental housing.

I find it difficult to believe that you have no certificate of occupancy for your home.

[LawResearcherMissy]

You can read about what a CoO is here and here.

A CoO and certification of rental are two entirely different things. As Mr. K points out, you're looking for the latter.