Speaking of "6' or 8' square bump-out off the rear of the house",
go Monolithic. They're in your neighborhood -- okay, not really, but they are in Texas
The Monolithic Cabin can be used as a rental unit, eco-housing or granny flat, disaster shelter, workshop, office or studio, game room, vacation dome, exercise room, temporary housing for work crews or home builders, guest house or storage. They can be designed as single living units or combined with others. The list goes on and on!