Newly built houses will be allowed to emit tons of CO2 every year and still be called "zero carbon" under new rules being considered by ministers.
In five years time, all new homes built in the UK were expected to be carbon neutral, using technology such as wind turbines, solar panels and ground-source heat pumps. But guidance drawn up for the Housing minister, Grant Shapps, now suggests that some developments will only have to achieve 50 per cent carbon reductions from present rules to qualify.
Developers would then be expected to pay to "offset" the additional carbon emissions – although there is no agreed mechanism to do that yet. The idea of zero-carbon homes was first proposed by Gordon Brown and the then housing minister, Yvette Cooper, in 2006. They announced that all new homes would have to be zero carbon by 2016, but failed to explain how it could be done.
A taskforce was set up to examine how the target could be achieved, and in 2009 it recommended that all new homes would have to reduce their carbon emissions by 70 per cent from 2006 levels in order to be classed as "zero carbon".
But this has been reduced still further in a final report sent this week to Mr Shapps, who is expected to endorse its findings shortly. It will form the basis of new building regulations to be introduced from 2016.
The report's authors conclude that the 70 per cent target "may not be achievable" and suggest that new detached houses should instead reduce emissions by 60 per cent, other houses by 56 per cent and new-build, low-rise apartment blocks by just 44 per cent.
In practical terms it means that a new three-bedroom, semi-detached house will be allowed to emit 2.3 tons of carbon a year compared to seven tons at the moment. Last night critics said while they welcomed the fact that existing regulations would be tightened, it was still a long way from the promise of new homes being zero carbon.
A spokesman for Friends of the Earth said: "Let's stop calling these houses zero carbon when they clearly will not be zero carbon. Zero-carbon housing has been a long standing political pledge and an acid test of the ability of the construction industry to deliver. They have had as long to do this as they have had to prepare for the Olympics, but we are still not there, and they should admit that."
Phil Thornhill, national co-ordinator of the Campaign Against Climate Change, added: "They have just tinkered with the definition of zero carbon. It shows a lack of ambition on behalf of the industry and government and is unfortunately another example of where the politics has not caught up with the science of climate change."
David Adams, director of the Zero Carbon Hub, a partnership of industry and charities tasked with drawing up the new guidelines, chaired the task force. He said the standards were very challenging to the industry.
"It is important to remember that this taskforce was not just an industry body, but had representatives from charities involved in campaigning against climate change on it as well. I think everyone recognised that what we have come up with is a very challenging set of minimum standards, but they are deliverable – which was a very serious concern with the previous target," he said.
"It is important to remember that when the proposal for zero-carbon homes were first introduced in 2006, there were no details at all, and the house-building industry was in much better shape than it is today.
"I would say it is entirely wrong to suggest that this report is watered down. I believe it represents an extremely tough challenge."