The plans, which could be in force by the end of next year, will insist new basement extensions – and indeed any home extensions of 100sq metres or more - must be zero carbon; that is so environmentally efficient that the extra space does not cause any extra emissions.
The alternative will be a “carbon offset” payment, to make up for the environmental damage the new extension is causing.
If successful the unique scheme will inevitably be taken up in other areas of the capital.
John Alker of UK Green Building Council, broadly welcomed the proposals. “It is effectively a basement tax. I would like to think that the money will go into energy efficiency measures elsewhere in the borough.”
However he queried the wisdom of forcing people to make just one part of their home carbon efficient - and said that even with very thick insulation, low energy lighting, and low energy appliances, making a basement completely neutral would “be a challenge”.
Exactly how much homeowners will have to pay if they cannot meet this challenge is unclear. But, as a rule of thumb, if an extension emits a ton of carbon a year, over 30 years this would result in a payment of just over £7,700.
The rules also suggest control on the size of new basements – no more than one storey, and not extending more than half way beneath the garden of a property.
Councillor Robert Davis, Westminster Council’s deputy leader, said: “Our policy aims to establish what the council considers appropriate for the developments and how we can better regulate them. This means balancing size and environmental impact, without depriving residents of the basic right to extend their own homes.”
The roll-call of the rich and famous who have risked falling out with their neighbours to build basement extensions includes David Cameron and Bernie Ecclestone. Jude Law and Kate Moss have both recently unveiled plans to follow suit.