That's not yet driven by legislation. There was a consultative document suggesting that all newly built housing should operate as carbon neutral by 2016. That went out for consultation.
On the basis of the consultation, the government then produced a policy statement, called “Building a Greener Future”, that confirmed their intention that all houses should be carbon neutral by 2016, with a progressive tightening of building regulation by 25% in 2010 and by 44% in 2013.
As far as I know, the legislation for that is still to be put through Parliament.