Friends of mine are working on a passive studio at the minute that will (hopefully) be completely powered by renewables. A big issue is that they need 13 massive batteries as a power bank for night time/winter electricity, and it's not a house, so there's no cooker or shower or whatever, but it's still an interesting experiment to make these buildings work. Not that you have to focus on just one thing of course, but low energy housing and construction can make a real world impact now.
Retrofitting these buildings is really important though. We can't just have loads of new housing stock overnight. My flat's in an old (1800 and something) building and it's pretty leaky wrt heating and there's no insulation, just thick stone walls. Even houses built in the 80's have no insulation, or minimal 50mm rockwool where now 250mm is recommended and the performance is higher. The way buildings are built wrt to environment and landscape is important too to make them more sustainable.
We have an old stone wall basement. There was no insulation on top of these walls on which the house sat. Right there, around the whole house, there was only wood and siding between the inside and the outside. I stuffed insulation all around, and the basement is toasty now. the house went 100 years without that simple improvement.