The so-called consequential improvements proposals were to be introduced this October as part of a raft of measures to improve energy efficiency of homes and update Part L of the Building Regulations in 2013.
The draft regulations would mean householders doing extensions or even replacing windows would have to spend an extra 10% on improving the energy efficiency of their homes.
The idea was challenged by the Modern Masonry Alliance when the draft regulations were put out to consultation in January.
Now a growing revolt by Tory ministers has joined in calling for the whole idea to be scrapped.
Conservatives feared the crackpot scheme would deter families wanting to improve their homes at a time when every penny spent stimulating economic growth is needed.
A source in Whitehall said: “We are absolutely not going to have a conservatory tax. It is an attack on aspiration and we want nothing to do with it. It will be blocked.”
Chancellor George Osborne and housing minister Grant Shapps are reported to be firmly against the plan. Even communities secretary Eric Pickles, whose department drew up the plan, is “not massively enthusiastic”.
Mike Leonard, director of the Modern Masonry Alliance, said: “We raised serious concerns about this when the consultation came out.
“It was clear the only outcome would be that fewer extensions would get built, which nobody wants to see.
“This looks like it could be a victory for commonsense. There was no precedent for taxing people that invest their own money in extending their own homes.”
He added: “If the Government wants to do something positive now to improve energy efficiency of homes they should use the £1 billion Eco fund.
“This could deliver a range of free but very simple insulation measures rather than engaging in complex, very expensive measures such as external insulation.”
