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Global warming means a more energetic atmosphere and more extreme weather; buildings are the construction industry's contribution to mitigating its causes.

"Some kind of penny has dropped in the last year or two," says Bennetts, who has found himself cast as spokesman and role model for ecologically correct architects. "The majority of the construction industry is looking for guidance, and most clients ask for energy efficiency from the outset." As Bennetts says, this is a major step forward compared with a few years ago, when most clients considered environmentally sustainable design a costly no-go zone and many architects viewed it as an exercise in feel-good tokenism.

Today, energy-efficient buildings, such as the headquarters designed by Bennetts Associates for PowerGen and Wessex Water, have a green philosophy as deeply embedded as the lettering in a stick of rock. As the man who designed them, the straight-talking, 47-year-old Scot was an obvious choice to chair the DETR taskforce devising key performance indicators for environmental sustainability.

But Bennetts, who set up his practice with his wife Denise 13 years ago, is not a green guru who lives in a grass-roofed, solar-powered bunker with the dogged principles to match. His style is more that of the friendly vicar who knows that a life of unassailable green virtue is beyond him and most of us. "We're not saints or religious about it. We're just normal architects trying to design good buildings," he says. "We still drive, we still design buildings that need air-conditioning and use glass. We don't have to be perfect, but we do all have to try and do something."

The nearest he comes to religious fervour is in condemning the practice of "greenwash" – covering buildings in large expanses of glass and making extravagant energy-saving claims. "I'm always suspicious of buildings that have a high percentage of glass, but no external shading. Famous names do it and people don't like to be critical, but the chances of a building like that performing well are limited," he says.

The DETR working group has been compiling environmental KPIs for exactly a year, the sheer scale of the task and the lack of data slowing its progress. So Bennetts decided to sidestep the thorny questions of social and economic sustainability to concentrate purely on environmental factors. His pragmatic view is that "people want something to be going on with. It is self-defeating to wait for a perfect performance indicator."

We’re not saints or religious about it. We’re just normal architects trying to do good buildings

The taskforce identified six areas where energy can be trimmed or natural resources saved: embodied energy, operational energy, site wastage, transport energy, water and biodiversity. A report commissioned from the BRE concluded that there is enough reliable data to measure the first two categories, and benchmarks and performance targets are due to be published by Easter.

The report also advised that energy saved and expended should be measured not in technical formulae such as kilowatts per hour, but more emotively and directly in terms of CO2 emissions. That simple change could help hardwire the scale of the challenge into the mind of every designer and architect. "The Kyoto deadline is reducing CO2 emissions by 20% by 2010 – now there's a KPI if ever there was one," says Bennetts.

"You can probably reduce a building's CO2 emissions by half if you design it thoughtfully. We renew our stock by 1% per annum, so for all new buildings, over 10 years, we only get a 5% improvement," he says. The other 15% of UK construction's contribution has to come from insulating and reducing energy consumption in existing buildings.

But, again, Bennetts isn't dogmatic about targets and is anxious not to be seen as the type of architect who would oppose his clients' wishes. From his own practice's portfolio, he is proud of the "close on 50% reduction" in CO2 emissions achieved in a new business school for City University in London. But on a hotel project, he concedes that "there is limited opportunity [to reduce CO2], because guests demand air-conditioning". The headquarters building Bennetts Associates designed for a software company also has air-conditioning to offset the extravagant heat gain from PCs.

In struggling at the coal-face of practical change, Bennetts maintains that he was not depressed by the sight of world leaders squabbling at November's Hague convention. "I'm sure it will generate another emergency meeting, and it did manage to get across a sense of real urgency," he says. "Up until then, the USA would not even acknowledge that there was a problem. Now, everyone is so alarmed that the heads of government will not allow the issue to remain unresolved."

Whether looking for answers at the negotiating table or on a PC screen in an architect's office, Bennetts believes that simplicity is the key. At governmental level, he is concerned that "if the formula is too complicated [on issues such as carbon trading], people will look for exemptions". And as a designer, he argues: "There is no need for complicated devices. If your building is energy-efficient and complicated – for instance, with huge chimney stacks for ventilation – you lose more energy in constructing it than you save in using it. Simple is best, and the most adaptable."