Google the keyword “sustainability” and you’ll get more results than you know what to do with. Sustainability may bring to mind the protection of natural resources, or issues of economic and human justice, or food production and consumption.

I’ve learned recently that for many, the mark of one’s commitment to sustainability is the size of the place one calls home.

According to the U.S. Census Bureau, the average size of new construction in this country decreased slightly in 2009, the first decline in about 30 years. While there are still plenty of McMansions in this country, the average new construction is about 2,422 square feet. Housing prices range from an average of $156,000 in the Midwest to $244,000 in the Northeast.

There are a variety of reasons given for the decrease in size. Some have less disposable income in this challenging economic environment; young professionals may accept scaled-down urban housing to enjoy the amenities that accompany city living; couples have fewer children; and, increasingly, people view a home purchase as a temporary, not a permanent, investment.

A growing number of homeowners are embracing the idea of more compact, environmentally friendly homes, some of which use recycled building materials and many that feature biomass stoves, solar panels or other measure of energy efficiency. Proponents of smaller homes say they are dedicated to living in a way that is both financially and ecologically sustainable. That makes good sense.

But there is a movement gaining momentum, not just in America, but in locations around the world, that brings a whole new meaning to the word “downsize.” Extreme houses – some as small as 70 square feet and the vast majority less than 1,000 square feet – are making an extreme statement about what smaller and simpler might mean.

The Small House Society movement is not new, but is enjoying greater publicity, thanks to coverage by CNN, National Public Radio and other media sources. The Public Broadcasting Service “Need to Know” hour recently aired a segment on tiny houses called “Living Large.” Dee Williams, featured in the story, lives in an 84-square-foot home in a friend’s back yard in Olympia, Wash. I admire her commitment to downsizing, but cannot imagine myself living in just 84 square feet.

Ms. Williams is proud of the micro-house on wheels that she built with the help of carpenter friends for about $10,000. She installed solar heat panels and cooks with propane at a cost of about $8 a month. If she gets tired of her location, she can hitch the house to a trailer and move to a new spot.

The inside of her home reminds me of a doll house: a cook space with one burner, a closet that holds about four outfits, a composting toilet in one corner and a loft bedroom that looks too low to stand in. Of course, on the positive side, for those of us who aren’t in love with housecleaning, I’m sure that we could spruce up the place for company in less than an hour.

Speaking of company, how does one entertain in a tiny house? Jay Shafer, owner of Tumbleweed Tiny Houses, lives in a micro-house in Sebastopol, Calif. His and his wife are separated by just a few feet when they face each other in the narrow space, so entertaining seems out of the question. A friend described Shafer’s 8-by-12-foot residence as resembling a Swiss army knife – creatively planned seating, storage and dining areas that fold up and disappear into the walls when not in use.

The number of architects and builders featuring tiny houses is growing, too. Ms. Williams has just opened Portland Alternative Dwellings to show novices how to built tiny houses. Tumbleweed Tiny Houses will sell buyers a pre-fabricated home for about $20,000, or allow you to purchase a do-it-yourself kit for a few hundred dollars. Three months, a few good friends to lift the walls onto the trailer and you’re ready to move in.

Alchemy Architects’ wee-house models look like boxcars; Little Green Buildings will move you into a 16-by-20-foot Swiss Chalet. There are domed houses, carriage houses and salt boxes – something for everyone, it seems.

I listened to a young man named Matt discussing the possibility of moving his wife and baby into a tiny house for their primary residence. He’s ready to make the leap, but said his wife is a bit apprehensive.