Actually, if they rent the home, then they still don't have that control. The landlord does. S/he is free to put someone you don't like in the upstairs unit. Or even in the 5th bedroom of the house if it's a room letting type situation.
No matter what. You rent? Yeah, sorry. The landlord makes the rules.
The hypothetical foursome would need to purchase their property. At that point, they would be able to control for who could live there.
No matter what. You rent? Yeah, sorry. The landlord makes the rules.
The hypothetical foursome would need to purchase their property. At that point, they would be able to control for who could live there.