Bucking the trend of bigger is better, Jay Shafer started a company specializing in designing and building small yet functional homes:
... since 1997 I have been living in a house smaller than some people's bathrooms. I call the first of my little hand built houses Tumbleweed (top left). My decision to inhabit just 100 square feet arose from some concerns I had about the impact a larger house would have on the environment, and because I do not want to maintain a lot of unused or unusable space. My houses have met all of my domestic needs without demanding much in return. The simple, slower lifestyle my homes have afforded is a luxury for which I am continually grateful.
This one above is called the Vardo, inspired by gypsy caravans and shepherd wagons, that you can build for about $4,000.
Link - Thanks Katia (good luck with the mortgage!)