The difficulty with doubling up, is it puts the renter at risk of eviction. Their lease can contain language on the specific names of who can liver there. Michael, thank you for writing such an excellent article. There is a movement about building Tiny Homes. It would be a start: https://medium.com/the-engage/americas-finest-city-san-diego-ca68192db16?sk=db3a57679e064c567d66efd9b02a8cc3