Okay, first of all let me just say how excited I am about how interested everyone is in these houses! I'm in college studying architecture, and they're really pushing the earth-conscious initiative on us.
A few things that might help you out:
1. a cistern. collecting rain (were you the one who lives in seattle?) can really make a difference! of course, you shouldn't drink it, but we use so much water every day. add it up for a week and be surprised! i'm living in south carolina right now in one of the top 3 worst drought conditions in the country. we definitely are aware of the water we use!
2. as for space issues, theres a place here that sells these mobile unit bedrooms, which are probably horrendously expensive, but they have some excellent ideas for having storage and sleeping, even a small office in a small area. you can definitely get some ideas from here! http://www.tumidei.it/home_eng.htm http://freshome.com/2008/09/19/bedroom-ideas-for-kids-tiramolla-loft-bedrooms-from-tumidei/
3. the tumbleweed homes are adorable, but you will use so much less energy if you have more windows (skylights, etc.) allowing natural light into the home will not only save energy, but it has been proven that natural light makes you feel better on the inside, and sleep better as well.
4. don't forget to garden and eat seasonally! shipping vegetables across the globe is awful for the environment, and you will feel better eating fresh and delicious vegetables that you grew yourself
good luck with everything! i can't wait to see how this turns out!