I also live in Renton, and the rules are most likely the same in King county anywhere, but single family home is for up to 4 unrelated adults. 

No, I just think an older adult should own their own house.

Anytime you have to add more instructions/sections to a lease agreement to explain to an ADULT how to use something, you are asking for trouble and proving the point that sometimes we credit our tenants with more responsibility than they may deserve. 

Someone who will be on the lease as an adult signing the lease, who intends to live there.A co-signer is mommy or daddy saying they'll pay the bill if junior doesn't.All I see on the screenshot you provided is two declines.You're right to decline them based on your own interpretation of the situation - late pays and debt to income ratio.  

For example, my city allows no more than 3 unrelated adults.

Fair housing laws are federal, and then some states add additional protected classes.You can say that someone who is younger than 18 may not sign a lease with you, because they aren't an adult yet.  

@Max Tanenbaum excellent point about the puppy vs adult and the landlord reference. 

I suppose for any situation with more than one adult tenant, it would be a good clause to include.

They are grown adults and should abide by the law and lease.if your lease doesn't cover pets, then you should add some pet rules that could help both protect you and your tenants.

Are you on the sexual offender's list?