David JC MacKay described his struggle to improve the energy efficiency of his 1940's era semi-detached house, with a history of his gas usage. He did quite well, but concluded that you get better results when you build with energy efficiency in mind, as did energy consultant Alan Foster. MacKay's book on sustainable energy is highly recommended, and available on the web site and as a free PDF.
Wow! Thanks! Guess I got some reading to do. Although, of course the finance of it runs deeper than energy efficiency, as it seems quite a bit cheaper per square foot to buy a preexisting home. But on the other hand, there's probably some non-quantifiable benefit to designing a home from scratch. Also, finding a suitable piece of property that doesn't already have a home isn't so easy, either.