A house built over 67 years ago on a waterfall in Pennsylvania is still an architectural landmark. Why is Fallingwater special? Emma Cullinan, just back from a visit to it, reports

IRISH gardener Paul Martin, who was awarded a Silver Gilt Flora at Hampton Court last week, uses a day bed suspended over water in his Falling Waters garden that was inspired by architect Frank Lloyd Wright's Fallingwater, a house that has inspired many. Built between 1935 and 1939, it still attracts tens of thousands of visitors every year, which is astonishing for such a small work of architecture hidden in the depths of the US countryside.

Standing on a diving board or the deck of a ship looking down onto water makes you feel light, forces you to take a deep breath. It's about the height and also the water; knowing what it would feel like to plunge into it. That's part of the excitement of architect Frank Lloyd Wright's Fallingwater house in Pennsylvania, with its vast balconies cantilevering over a river and the top of a water fall.

It is approached down small country lanes two hours to the south east of Pittsburgh, home of Heinz. When I visited with my family, we were the only ones on the road, having reached the spot through deft map reading and unintentionally intimate tours of a couple of isolated towns. We turned onto a dirt track beneath tall trees and were suddenly in a queue of cars waiting to be checked in. Fallingwater is a top visitor attraction!

That promised tourism hell, traipsing around the house with hoards of others on a guided tour. Yet for all its waterfall drama, Fallingwater is actually a house that is hunkered down quietly in nature. You have to work a bit to get there, walking down a pathway deep into the woods. You round a corner and there it is, sitting right down on the water's edge, built using the same sandstone, in the same strata patterns, as the surrounding rock faces.

Trees, rhododendrons, boulders, natural stone and water easily take the edge off the quantities of visitors, and the groups are kept well away from each other so that you can see whole rooms and lean over balconies in the company of a few, equally enthralled, people.

Fallingwater was the weekend retreat of the Kaufmann family - Edgar, Liliane, and their son Edgar Junior - owners of a department store in Pittsburgh.

Edgar Junior had been educating himself in the arts around Europe and began studying at Frank Lloyd Wright's Taliesan Fellowship in Spring Green, Wisconsin, in 1934.

When his parents went to visit him there they were inspired by Wright's home and work and asked the architect to visit the land they owned at Bear Run, with a view to designing them a weekend retreat. Wright chose a constricted site, between a river and a rock face, with a huge boulder near its centre. This vast rock anchors the house: it erupts through the livingroom floor as a hearth and has other parts of the structure clutching onto it.

Wright was known for his organic architecture that respected the landscape. He had designed ground-hugging prairie houses that almost became a part of the landscape in the early 1900s, inspired by the mid-Western plains.

Fallingwater was started when Wright was nearly 70 and had been short of work for a while. It kicked off the 25 most productive years of his career, which included the remarkable spiralling Guggenheim Museum in New York.

As with many self-employed architects, age was no deterrent to hard work in pursuit of great design. Indeed, the thought of lifetime running out and the constant need to design the 'best' building is a strong driving force.

Fallingwater shows that: it contains wit, inventiveness, a respect for nature and it pushed structural boundaries.

It's complex too. Each floor has a different support system. The ground floor is on four stub walls running through the boulder to the edge of the river. The second floor is cantilevered from a central square of reinforced-concrete beams. A two-storey bathroom block at the rear of the house helps to counterbalance the cantilevering to the front.

It was enough to make any client nervous, even the adventurous Edgar Kaufmann, who hired his own engineers to look at the structure. They worried. In one case the engineers insisted on putting a support in under one of the floors. Wright asked the builders to take it out - secretly - and only told the clients much later; pointing out that the structure hadn't fallen down.

There were some structural problems: the balconies began to tilt downwards after the wooden scaffolding was removed. In retrospect, the floors should have been cast sloping upwards slightly to allow for the settling of the reinforced concrete.

There were problems with leaks too but that's what's extraordinary about this building: most people wouldn't have dared to build on this site. Here were an architect and client who chose to take the risk; while the cost of the house went colossally over budget and there have been repair bills over the years, it was worth it.

Financially, the Western Pennsylvanian Conservancy, who were given the building by Edgar Junior after his parents died (his dad's demise happening just a few hours after Wright paid him a social visit in 1955), must be making a good part of the cost of upkeep from ticket sales. Aesthetically it was worth it because Fallingwater is one of the best houses in the world (and a great example of one-off housing in the countryside!).

Wright wanted this building to be part of its surroundings rather than sitting haughtily above the river. Bringing the house close to the river limited its footprint to a narrow strip of land between the water and a small cliff with a track running beside it.

THIS isn't a conventional house with a garden: the garden is the surrounding woods, The design reinforces this link with nature. Inside the stone floors - waxed to look wet like the stones in the river - continue without thresholds onto balconies and out into the landscape. The sandstone for the walls - and tower - was quarried in the area (one guest asked Liliane Kaufmann how she hoped to get wallpaper to stick).

Metal frames for the glazing were made specially as they were required to do unconventional things. In the tower, corner windows open away from each other completely opening up the edge of the structure, leaving it without any apparent support. The window frames receive flooring slabs, whose edges were cut into the shape of the frame, allowing the three-storey span of glass to rise uninterrupted. The glazing plunges straight into grooves in the wall, rather than being interrupted by vertical metal framing.

As well as opening out to greet nature, the structure itself was manipulated to accommodate it: beams across the entrance path were bent around trees on the site and they run into the cliff announcing to visitors that this is a house embedded in the landscape.

While being very much a horizontal building - in line with rock strata and in contrast to the vertical trees - which is rectangular in plan, Wright played on diagonals, as he had in previous buildings. From the entrance you can see across to the southern balcony and out over the waterfall: from earth to air. On the opposite diagonal, the room runs from the fireplace to a glass hatch through which you walk down steps into the river: from fire to water. This hatch works as a clever passive air conditioning system, opening up on hot days to let water-cooled air into the house.

Wright at one stage wanted to coat the balconies in gold leaf - but his clients opted for sand-coloured (painted) concrete to be more in tune with nature and, anyhow, the building is dazzling enough without it.

There are magical moments in life: watching water from above, sitting in woods, walking on natural stone, being at eye level with tree tops, paddling in streams, listening to nature unobserved, sitting by vast open fires, basking on balconies. Fallingwater has it all.