Cameron architect knocks the wind out of his turbine

Alex Michaelis is masterminding the £600,000 revamp of the £1.1 million four-bedroom Edwardian terrace house in North Kensington where Mr Cameron lives with his wife Samantha and their three children.

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The mini-windmill will only make a small contribution towards bills and helping the family of five combat global warming. Critics claim domestic wind turbines are little more than faddish eyesores only capable of powering a hairdryer.

Now Mr Michaelis himself has admitted their failings andMr Cameron, who spent thousands on his model, may have to invest in a new one as early as next year. "It's early days for wind turbines and their efficiency can only get better," said Mr Michaelis, of architects Michaelis Boyd Associates in Ladbroke Grove.

"New types are being developed for the swirling air conditions of the urban environment. They will be able to use air flows from any direction, not just from the direction of a prevailing wind at a given time. Obviously, we got the best turbine available nowbut in a year's time we will do a cost-benefit audit and in three years, when planning permission will need to be renewed, we will look at the issue again in the context of more efficient devices which may then be available."

The makeover also includes the installation of a 660-gallon tank under the garden to collect rainwater. Once cleansed, this will be used for flushing lavatories and to do the washing.

A large basement extension is to be excavated beneath the house, creating a bedroom for the Camerons' disabled son Ivan, who suffers from cerebral palsy, as well as a room for his carer and a playroom. A lift will link the basement to the ground floor, where a spacious kitchen and dining room are being created.

Planners at Kensington and Chelsea council are concerned about disruption to neighbours while the subterranean developmentis carried out, and the possibility-that the works might compromise local water levels.

They have pledged to insist on stringent structural surveys before granting permission for similar projects in future.

Mr Michaelis described Mr Cameron as an "incredibly easy" client to deal with.

He said: "He's been especially interested in the green aspect. We had detailed discussions about what was worth doing."

The Tory leader was equally complimentary about his architect, saying: "I selected him on the basis that I know him, but also because he likes to do developments in as green a way as possible." Builder Sean McWeeney usually carries out commercial developments but Mr Michaelis said: "He impressed us when it came to reusing materials and sticking with our details without cutting corners."

Mr McWeeney said: "Our 34-week programme was at least 10 weeks shorter than the other tenders, which made it 15 per cent cheaper. We saved time by knocking down the existing rear extension rather than propping it up."

Bricks are being reused in the extension, which will also have insulated cavity walls.