Architecture: A whole house with a view: Ken Powell visits Britain's most radical family home: a space-age greenhouse in a London conservation area

Squeezed into a 20ft gap between an ebullient 1860s pub and a late-Georgian terrace in fashionable Islington is Britain's most striking new house. Designed by Future Systems (Jan Kaplicky and Amanda Levete), this space-age greenhouse is a radical alternative to the conventional three-storey terraced house that has dominated Britain's city streets for the past 300 years.

Even more remarkable is the fact that the house, commissioned by restaurateurs Jeremy King and Debra Hauer (who run the Caprice and the Ivy in central London), has been built in a conservation area. Might it set a precedent: new houses for old areas? Certainly it has cost no more, so client and architect insist, than buying a Victorian house in the area. And it is a full-time family home, not some wilful folly.

The origins of the house, however, date back 20 years, when Jan Kaplicky designed his first London house, an optimistic venture given there was no client and no site. 'House 26' was intended to fill any one of the many gaps in London's Georgian and Victorian streetscape caused by wartime bombing. When Kaplicky arrived in London from Prague in 1968, there were many such gaps plugged with sheds, wrecked cars and willow herb.

Kaplicky's ideal three-storey city house would not be made of London stock brick, but of clip-together steel and glass. The street front would consist of six sheets of glass, one of them opaque and containing a metal door. The interior was to be open plan, with kitchen, bathroom and other services concentrated in freestanding units or 'pods'.

Would the planners have allowed 'House 26' to be built 25 years ago? We shall never know: 'House 26' remained a paper project, as, over the next two decades, did the vast majority of Kaplicky's space-age designs.

These include an inspired entry for the Bibliotheque de France competition held in 1989, won by Dominique Perrault and, the most striking product of Kaplicky's partnership with Amanda Levete, the Acropolis Museum project, which they did not win either. Despite not building, Kaplicky and Levete's inventive two-person practice, Future Systems, enjoys a formidable reputation. They design what might be.

At 40 Douglas Road, what might have been was a bland neo-Victorian confection for which planning permission had been approved. This, however, was not to the liking of Jeremy King and Debra Hauer who wanted a modern house tailored to their tastes and way of life. They had commissioned Future Systems before, but on a rather smaller scale: diners at the Caprice and the Ivy are familiar with the distinctive serving trolleys and champagne buckets designed by Kaplicky and Levete.

Kaplicky's initial sketch for the house shows something very much like the completed building. To the street, there was to be a wall of glass bricks. To the rear, facing south, the house appeared as a ski slope of clear glass, a strategy adopted to make optimum use of the small site and to maximise natural light.

Kaplicky wanted a staircase as the centrepiece of the building, a spectacular structure with views southwards at all levels. This idea had to be dropped, not least because of the need to compartentalise the house to meet fire regulations. The staircase was consequently placed across the front of the house, a solution which has the benefit of leaving the principal living spaces completely unobstructed.

The house is entered across a sinuous metal bridge that curves to avoid a protected tree. Inside, glass bricks predominate: Kaplicky and Levete have no hesitation in conceding the influence of Pierre Chareau's famous Maison de Verre in Paris, with its great facade of glass bricks. 'If anybody wants to call this the Maison de Verre of London, we'd be very flattered,' says Kaplicky.

Beyond the glazed wall which encases the staircase there are rooms on four levels - a kitchen and dining room, plus a tiny flat for an au pair or visitor at garden level, a big living and entertaining space above, with views down into the lower level, a huge, shared bedroom for the couple's two young daughters on the first floor and, on the top floor of this glass ziggurat, the main bedroom.

All the 'core' elements of the house (bathrooms, storage and so on) are treated as free-standing sculptures, brightly coloured in contrast to the luminous all-white walls.

The house is curious for having no roof, or at least not in the conventional sense. Instead, the glazing at the back of the house sweeps over to meet that of the facade. The glass is unframed, giving a sheer and extremely lightweight look; privacy and sun screening are provided by means of specially designed blinds. It will be interesting to see how the house copes with summer heatwaves and sudden downpours.

Kaplicky and Levete are at pains to insist that this is not a 'hi- tech' house. None the less, the glazing and white rubber floors invoke memories of hi-tech monuments of the past, not least Norman Foster's Willis Faber building in Ipswich on which Kaplicky worked. The clients' initial preference had been for wooden floors, but instead they have got white rubber.

The white flooring, carefully separated from the wall surfaces by verges of polished aluminium, will extend out from the kitchen-dining room to cover the garden area, with sliding doors allowing the interior and exterior to merge in fine weather. Some furniture, including a bed and a dining table (on rollers, so that it can be moved between dining room and garden) is being designed by the architects.

Modern houses are few and far between in Britain's cities and most of those which exist - Michael and Patty Hopkins' house in Hampstead, for example - occupy leafy suburban sites. Future Systems have brought an uncompromisingly modern architecture to a typical London street. They have produced a building with a memorable kick in its tail. They have made their mark on London and demonstrated an understanding that a respect for context does not have to result in bland architecture.

In the past Future Systems have veered between a dramatic, if essentially lightweight, monumentalism - epitomised by their rejected designs for the Paris library - and a lingering preoccupation with the impermanent and the portable. (In one sense, their first London building was the movable hospitality tent designed for the South Bank and wrecked by a gale in Croydon last winter.)

Now, finally, they have come down to earth in their own futuristic way, meeting the complex, untidy, sometimes ugly, sometimes sublime, mesh of urban structures and social relationships which mark out the lives of most British people. In Islington they have built an adventurous house at which no one should throw stones.

(Photographs omitted)