Cobtun House might sound like something from the Third World, but it represents the height of ecological design. Overlooking the river Avon near Worcester, it combines modern principles of sustainable development with ancient techniques of mud construction.
Behind a primitive-looking outer wall of piled clay, the house has an airy modern interior with ceiling-high windows. It was built to order in 2001 for its current owner, Nicholas Worsley, a retired barrister. "Sustainable houses are usually very worthy - but very dull," he said. "This was an attempt to introduce aesthetics to the equation."
Mr Worsley's design brief was only 10 words long: "Humour, mystery, fantasy, ecological, sustainable, independent, contextual, agricultural, invisible." From that vague outline, the architect John Christophers, of Associated Architects in Birmingham, devised a plan to use a mixture of mud and straw, known as cob. The mud was taken from a nearby building site, and piled into shape by hand.
"We mixed the cob with a digger, then pitchforked it on by hand. It's just like making mud pies, but on a massive scale," said Mr Worsley.
To prevent rain or rising damp from washing the mud away, the metre-thick exterior wall is mounted on a stone slab and capped with a corrugated iron roof.
Every element of the house is designed to reduce its impact on the environment: shelves are built from recycled plastic, the shower is heated with solar panels and the washing machine and lavatories use rainwater collected from the roof.
According to Mr Worsley, the house is cheaper to heat than a conventional house, thanks to the metre-thick earth wall and heat-retaining blinds on every window.
Cobtun House includes an open-plan living space, a modern kitchen and an organic vegetable garden. "It is a daily joy to live here," said Mr Worsley, who has put the house up for sale after the death of his wife.
The house won the Royal Institute of British Architects' Sustainable Building of the Year award last year. The judges said it "emerges seamlessly from the surrounding landscape. The technology is primitive, but the architecture is refreshing and modern".
The mud outer wall cost only £20,000 to build - a fraction of the total £300,000 construction bill, said Mr Christophers. "It was wonderful to find a client whose ideals were exactly the same as ours. Design and sustainability are equally important," he said.
THE ECO HOUSE
The walls of Cobtun House are built from piled mud, bound together with clay and straw. Taken from a nearby building site to reduce transport costs, the mud would otherwise have been dumped in landfill.
To prevent rain from washing the mud away, the exterior wall is capped with a corrugated iron roof and stands on a stone base.
The wall was built up by hand and left to dry over three months. At a metre thick, the wall gives Cobtun excellent insulation, improving energy efficiency and keeping the house warm in winter and cool in summer. Heat-retaining blinds are on the windows.
Inside, the wall is insulated with recycled newspapers. Elsewhere, the designers used materials with low environmental impact, including railway sleepers, manhole covers and recycled bricks. Shelves and cabinets are built from boards made from recycled plastic cartons.
A rainwater tank collects water from the roof to supply the toilets, washing machine and garden hose. The shower is heated with solar panels.