Architecture: When the best laid plans are turned on their side

The winner of the Ideal Home Concept House could give us the answer to our housing problems.

The Ideal Home exhibition is where you discover the slightly naff things that you think you can't live without. The Butler in the Box that turns off the central heating and puts on the television set and burglar alarm when you leave the house; the pavement sweeper in the slipstream of pets; genetically modified daffodils that bloom in late summer.

This is the place to test British ingenuity in designing knick-knacks and Incredibly Useful Things for the home. But it hasn't been the place to find cutting-edge architecture since the Fifties, when the Smithsons shocked Earls Court with their House of the Future, including kidney-shaped coffee tables and built-in hostess trolleys.

This year we are in for a surprise, with a house of the future that could change the way our cities look. Pierre d'Avoigne's Slim House has won the Ideal Home Concept House for the last show of the century.

The Slim House meets all the Housing Association demands for three-bedroom housing, and it costs just pounds 45,000. He has laid the three-storey terraced house on its side as a single storey and then planted lawns on the flat roof.

Unusually for a cutting-edge modernist, Pierre d'Avoigne likes the British suburbs. His Invisible House, now on site at Ealing in London and the subject of a Channel 4 documentary in April, is just that - invisible. It bunkers into the ground below street level and the approach is through the garage doors into a light-filled one-bedroom house.

Designed specifically for a suburban back garden, it can be eased into the small-scale landscape of fences, hedges, sheds, pergolas, shrubbery, compost and lawns without disruption. All you see from the road is the picket fence it hides behind. Pierre d'Avoigne says he designed it as "an antidote to the over-assertive suburban fabric".

Sometimes this unobtrusiveness counts against him. Slim House hasn't even been built yet for the Ideal Home exhibition, but already it has lost the style trial. Wallpaper* magazine and Elle Decoration highlight the sexy curves and triffid-like pods of the second and third prizewinners of the Concept House, rather than the earnest presentation by d'Avoigne. "We've never gone for glossy interpretation. We didn't have the hard-hitting, seductive glam shots."

As the 12 judges sifted the 150 entries, d'Avoigne's Slim House wasn't an obvious winner. But the more they looked at his simple plans, the more they liked the 25-metre-long house. At the front a double-height room faces the street. Behind it, other rooms lead off the 16-metre-long gallery. All rooms have full-height, glazed sliding doors opening on to paved terrazzo courtyards. A raised threshold from the street to the front door is covered - useful for prams and pushchairs - while mopeds and bikes can be bolted to the metal railings. "The brief asked us to look at transport issues for the city. We deliberately didn't want to include a garage," the architect says.

The roof is an egg-box crate material called Bauder XF307 which drains off rainwater to feed it back into the lawns. One of the problems of flat roofs - drainage - is thus met. The only time it leaks is if gardeners take a pitchfork to the lawn; but then, as d'Avoigne points out, the communal roof gardens aren't allotments. The space behind each pavilion houses pigeon coops, rabbit hutches, containers, window boxes and sheds.

All rooms open out on to courtyards like secret gardens to breathe life - as well as light - into the core of the house. Not since Le Corbusier's tracts in the Twenties, concerning the need for architects to facilitate healthy living and sunshine with their modern buildings, has there been so much attention paid to air circulation and ventilation. Conventional extractors have been replaced with eco-chic models that breathe in condensation and then warm the air and distribute it through underfloor heating ducts. All within an easy, prefab, steel-framed construction kit that should make it a des res for developers.

"The showhouse as a medium to try out new ideas on housing has been limited in this country by property developers blocking architects," d'Avoigne says. "Also, let's face it, architects have been unwilling to be populist and mainstream."

Even on such a modest scale, the realities of building inside Olympia at the Ideal Home exhibition mean that his concept won't be fully realised. Only three terraced houses instead of five, and no access to the roof garden because of the need for wheelchair ramps, plus the space taken up by public viewing-platforms.

However, even when concept houses do get tried and tested it doesn't mean that property developers will risk building them. The Oyster House prototype by Nigel Coates, last year's Ideal Home winner, was a detached suburban house costing pounds 100,000. After the show it was binned - though Bath University wants to install the Oyster House on its campus for students to live in and learn from.

As an experiment for students of architecture and a talking-point, the Oyster House is great, but it isn't exactly Britain's answer to housing in the future. By 2010 we need more than 4 million new homes and this year's Concept House really could be the answer.

The prizewinning designs will be on show at the Royal Institute of British Architects in Portland Street, London, from Wednesday 3 March