Gordon Brown talks of getting Africa on its feet, David Cameron puts a jar of fair-trade coffee in his office. Tony Blair talks of English patriotism and the World Cup, Cameron sticks a flag of St George on his bicycle. So, naturally, Cameron is addressing the issue of energy conservation by installing low-flush lavatories in his house. He was offered the ecologically purer option of a waterless urinal, but the man is only human and wants to stay on the right side of his wife.
The next idea you are likely to hear from the Conservative leader is for some kind of surcharge on doing up your house. It may be called the skip tax. I predict this after interviewing Cameron's personal "green" architect, Alex Michaelis.
Michaelis and Cameron are contemporaries. Michaelis studied architecture at Oxford Polytechnic; Cameron studied politics, philosophy and economics at the university down the road. Both men ended up living in Notting Hill or, as they emphatically describe it, "North Kensington" (Michaelis's office is at the Holland Park end of Ladbroke Grove, which is undeniably Notting Hill). Both have three children. Both are evangelical environmentalists.
Michaelis designed the Camerons' previous home in the standard urban, upper-middle-class fashion. He describes it as "contemporary with a soft edge". Like many architects, he refers to "spaces" rather than rooms. He also talks of "light flow" and "muted colours". The next house, down the road from Oxford Gardens, in the same part of London, is going to be more than contemporary: it is going to represent the ecological future. The Camerons and their architect are waiting for Kensington & Chelsea Council to grant permission for a wind turbine on the roof - to the consternation of their neighbours. Barbara Want, wife of the BBC journalist Nick Clarke, has been the most vocal opponent, criticising both the turbine and the basement-level "light wells" planned for the building as "eyesores".
But the project is ready to proceed, starting with the largest construction element: the creation of a basement. The Camerons' son Ivan, who suffers from cerebral palsy, needs 24-hour care; the basement will provide a room for him and for his carer, and also house a playroom. The excavation of basements is a relentless trend in London, where there is a shortage of good-sized family houses. Michaelis urges caution. "There is no regulation yet, but there will be. It is quite a dodgy thing to do in London with these terraced houses. If you get it wrong you can damage not only your house but also the houses on either side." The ground floor will feature that middle-class imperative, a kitchen/conservatory extension. There is a first floor for bedrooms and an attic room that will become a study.
"They are not that big, those houses. There is not a lot of spare space," Michaelis says. This is not an insuperable problem for the architect: when he bought his own plot of land in Oxford Gardens, planning restrictions prevented him from building more than two metres above ground level. This he overcame by building under. The house has a flat green roof planted with sedums. It is also miraculously light, and comes with a swimming pool and slide for the children and the kind of ecological integrity that would make the Prince of Wales dance for joy. Michaelis has a borehole a hundred metres below the house, which gives him access to an unlimited supply of pure, cold water. He has a thermal solar water heater, photovoltaic (PV) panels and an almost musically intricate balance of heat pumps, heat exchangers and humidifiers to keep the inside temperature even.
Michaelis never set out to be fashionable - which may not be altogether true of Cameron - but his environmental building has become so. He is the star of this year's House and Garden Fair, and the commissions are coming thick and fast. Not everyone wants a wind turbine on the roof and almost nobody wants a waterless urinal in the bathroom, but there are some basic house rules that most of us could follow.
He is very strict about televisions being left on standby. "Standbys should be banned. The effect is a fifth of the energy of when the TV is on." I ask if the modern habit of multi-television households is an energy concern. "Well, apart from the energy difference it is slightly sad anyway," he says. "In my own house we have pop-up screens so the children don't think about it."
A more heinous offence is the stage lighting that we have all come to expect. The banks of ceiling lights are a property advantage but an environmental disgrace. We would be better with bedsit-style flexes hanging from the centre. There is also the question of water and heat retention. Cameron will have a tank in his garden for "grey water" spraying of the plants. More complicated is the underground butt that should give him enough rainwater to work his dishwasher, washing machine and low-flush lavatories.
Michaelis also takes a dim view of the fashion for large baths. "The really big ones are naughty: you can be very comfortable in an old-fashioned steel bath. But we can't stop people," he says. I ask about Jacuzzis and he looks momentarily pained. "No, no. Germ pools." To him, the most disturbing phrase in the vocabulary is "air conditioning": "We don't do it. It is unacceptable."
The trouble is that doing up your house is, in itself, an environmental act of vandalism. Michaelis's advice is: "Do not demolish, dismantle. Since the war we have become used to throwing things out and just buying new." Like Cameron, he wants reclamation yards to set up next to builders' merchants, and he plans to build a Turner Prize-like installation from the contents of a skip and donate it to a children's playground. Cameron, meanwhile, may refuse to skip tax, but he is almost certainly going to tax skips.
Alex Michaelis appears at the House and Garden Fair 2006, which runs from 29 June to 2 July at the Olympia Exhibition Centre, London W14 (www.houseandgarden.co.uk)