Situated in the Sussex town of Lewes on the edge of the South Downs, St Pancras is a small school with 107 children. Still, the existing space was cramped. Teachers had to use the staff room for small group work and the only green area was dingy and so steeply sloping as to be unusable. Head teacher Angela Ferns - with 18 years at St Pancras, 10 of them as head - had always seen the potential for an extension at the back of the school; four years ago, she decided to act. The first step was to ask the children, parents, governors and the community for their ideas. “We began to talk about how we could minimise energy use and maximise the feeling of being in the green space,” she says. “We do teach children about looking after the environment and they were eager for the building to be from renewable resources, and not to use too much energy.”

Alongside these considerations, the idea grew that the new extension should contain a large, flexible space - for science experiments, large-scale art works and performances. “The children wanted an exciting building. None of us wanted a box,” says the head. They decided that the centre would host scientists and artists-in-residence to engage another renewable resource - children’s curiosity and creativity, and are in discussion with the Arts Council, Nesta and others to make this a reality. A number of architects prepared designs. But the best came from local, award-winning architects BBM Sustainable Design Ltd. Their design maximised the possibilities for this unpromising space, with a simple, beautiful building - using locally grown sweet chestnut for beams, sustainably managed cedar shingles from the Duchy of Cornwall estate on the outside, and wool insulation and clay paints for a healthy internal environment. The pound;425,000 costs have been raised from the DfES, East Sussex and the Diocese, with a substantial contribution raised by families at this small, socially mixed school.

St Pancras is using the creation of the building as a learning experience for children. Since being commissioned, the architects have run design workshops with the children, building geodesic structures and talking to them about the principles of sustainability. “They believe the same things as we do,” says Angela Ferns. “By educating the children, they’re going to grow up with the idea that sustainable buildings are possible for the same cost as using materials that are toxic, not renewable and consume a lot of energy in their manufacture.”

Independent film-maker Mick Hawksworth has worked with Year 4, 5 and 6 children to document the whole process, using digital film cameras and editing their work on computers. The builders agreed to allow children on site to film and ask questions at all stages of the work; since then, pupils have interviewed everyone from crane drivers to visitors from the DfES. After their visit earlier this year, the Innovations Unit at the DfES gave a pound;15,000 grant to St Pancras to produce a DVD of the process, to share the lessons with other schools.

By the summer of 2005, the centre was taking shape, with music room, quiet room and an expansive library in place above an airy, accommodating space for art and science. The air is tinged with the smell of lanolin from the untreated wool packed into the walls as insulation material. Some walls already have their vapour control layer - made from recycled paper and plastic. Floors are insulated with timber fibre, rather than polystyrene.

Light “scoops” maximise and spread the limited quantity of natural light.

Three children - Anna Dunne, Harry Dempster and Jake Blake, all aged 11, all hard-hatted - film the laying of a concrete screed for a wet area in the downstairs space. Compacted earth, a Sussex tradition, would have been nice, says architect Duncan Baker-Brown, but the problem is finding craftspeople who can do it. The children step carefully over pipes as they cross the floor; the pipes extend one metre underground and retrieve and store sufficient energy to cool the building in summer and warm it in winter, via a geothermal boiler.

St Pancras school’s new arts and science building will be completed this autumn and fitted out for use by next spring. While the children and staff will be able to enjoy it, head teacher Angela Ferns will not; she took early retirement last term, making way for a new head to oversee the completion of the project. “It means my successor will have a chance to make her input. It is more sustainable this way,” she says.