It's not just the energy it takes to run a house. It's also the energy and resources it takes to build all these houses, and the land it consumes that could be used to grow food and provide habitat for other creatures. Also I think a home-per-nuclear-family is a major contributor to the disease of isolation and loneliness that infects our society, especially after the home becomes an empty nest. And there is the growing problem of the homeless population, driven out of housing by the high prices us privileged people are willing and able to pay. Our housing system is a mess, but I believe our housing model is a poor one as well.