Manufacture underground Kiva's in folks backyard. Kivas are awesome. You go down the bellybutton (Sipapu) of the Kiva, which is a ladder descending into a hole. When you are in the place is spacious and womb-like. There is a fireplace in the middle and a simple air vent system which brings in fresh air and helps push the smoke of the fire up. There are crevices all around the walls, like a shelf, in which you can lay or sit.
This could be dug into the backyards of an average home and be practically invisible. It could increase your living space without detracting from your yard or house. There could even be a garden built on the roof of the kiva.
Imagine hanging out inside the kiva and playing guitar loudly without waking the kids. Imagine hanging out with friends and family around the fire, telling stories or playing games. It would be very cozy and warm.
Would need an architect on board!