Not that much, or so it seems from the latest study of California, just published by Arik Levinson in the AER. This seems to be the bottom line:
1. In 1978 California started to enact some of the world’s most ambitious residential building energy codes. These building codes have been updated 13 times since.
2. The promise was for 80% savings for new buildings constructed after 1990. These assumptions assumed everything would go according to plan and there would be no behavioral adjustments.
3. The actual results?: “For electricity, post-1978 houses in California may be using up to 15 percent less than pre-1978 houses, but do not use less per degree-day when the weather gets hot, and do not use relatively less than similar post-1978 houses in other states with less strict building codes.” For natural gas there is a 25% savings, noting that this trend and the electricity trend both predate the 1978 legislation.
4. Levinson conjectures that most of the savings are coming from natural turnover in the housing stock disfavoring the least energy efficient units.
Here are earlier versions of the paper. Of course most regulations never receive a study anything near this thorough.