I would love to read a book on this: How far can you get relying purely on design and structure, not technology. "Passive" solutions, as you say. A fascinating topic.
100% is very feasible, even without going the Earthship route. Mostly it has to do with extra insulation but there are a couple other techniques that help. The reason more houses aren't like this is because it adds $10-20k to construction costs so developers don't bother.
