Circle in the round.

The Japanese architect, practised at dealing with the tough physical constraints of the average urban site, must find enormous relief in a commission for a house in the countryside. The chance to benefit from a natural setting, with the breathing space offered by open country, is rare and refreshing.

When she was asked to design an artist's studio in the forest in Nagano, Kazuyo Sejima chose a circular form, not for any symbolic reason but quite simply, she says, as a celebration of this escape into nature, far from the incessant restrictions on space, orientation and outlook in the city.

The Villa in the Forest is formed from an interlocking pair of drums, not quite centred. The top of the whole outer drum is tipped back slightly against the grain and gradient of the land. Almost extra-terrestrial in its sudden appearance amongst the pines, it makes contact with the site in the form of a series of tentative projections out into the world beyond its shell.

At the core is the white double-height studio, lit from above and with its walls pierced by variously sized perforations, internal windows, balcony and doors giving on to the spaces wrapped around it. In the way that the external shell makes contact with the site with terraces and small extrusions from the main surface, the studio makes constant contact with the encircling space.

The perimeter provides the kitchen and dining areas with bedrooms and the bathroom, built out above a terrace, above. As the outer spiral of rooms is governed by the placing and geometry of the studio, at the narrowest (and lowest) point the space contracts sufficiently to form a double-height passage leading to the main entrance. It widens, and gains height, to become a living room with south-facing terrace.

Wooden walls, white floors and white ceilings make up for the lack of transparency, one of the marks of Sejima's urban projects. Here the solidity of the walls is alleviated by selected views out, while two terraces offer a toehold on the landscape. Although one terrace faces due south, orientation matters little in a thickly forested site such as this.

With its form hardly interrupted by windows or detailing, the Villa in the Forest suggests a kind of tumulus, little more a mound in the landscape -- like many an ancient Japanese site.

Yet, oddly, in view of the freedom offered by its setting in an unsullied natural landscape, in the end this appears to be a house quite as introspective and womb-like as any city house.

Sejima has, in her choice of concentric circular forms, set herself a series of tricky balancing acts which she has resolved in a deft and skilful fashion.

Nature, in the case of the Villa in the Forest, is simply an unlooked-for bonus.