As another house sharing/renting idea, my MIL had an average size 2 floor house. When her husband passed away, she split the house in two, not bottom level/top level, but half/half.
The existing housing plan was that the master bedroom (with en-suite) had a study built on top of it (which they had built years ago), and the rest of the house contained a kitchen, 2 other bedrooms, lounge and bathroom. She simply closed the bedroom door that connected the master bedroom to the rest of the house, and added a small kitchenette to the study room upstairs. Voila - instant private "suite" to be rented out to a bachelor or young couple that doesn't mind a small living space. The bedroom and study already had their own doors outside, and a private patio.
She makes a tidy amount renting that "room" and the converted garage space outside that she also managed to squeeze a private suite into.
She's a very smart woman!
As an aside, subletting over here is *severely* frowned upon, mainly because of the perceived wear on the house. If the landlord expects 2 adults, 1 kid to be in the house, they would not be happy to hear that you've sublet the extra bedroom to another couple. I guess that's different in the US.