I doubt whether we will build half this number. And most will be additional houses, so no matter how energy efficient they are total energy usage will go up. And of course there is a lot of embodied energy in building materials.
Improving the existing housing stock is a nice idea, but ISTM that gains in some places are being cancelled out by other changes, for example power showers that use twice as much hot water as older ones did, this fashion for having numerous 50W spot bulbs everywhere instead of a single 60W or 100W bulb, and very probably the more widespread adoption of domestic aircon.