Inner sanctuary.

In a hilly suburb of Basel, a family house presents an inscrutable face to the outside world, but its grey skin encloses a transparent light-filled interior open to the garden and the skies.

Herzog & de Meuron's design of a house for the Kochlin family at Riehen, in the leafy suburban outskirts of Basel, is yet another demonstration that the language of Modernism, when applied by imaginative hands to the design of the private dwelling, is as supple a vehicle for individual expression as it ever was.

The first impression of the Kochlin house is daunting. The site slopes sharply downhill to the street; the hill descends by degrees to the valley of a small tributary of the Rhine. The main visible mass of the building, inset into the hillside, is elevated above and set back behind an apparently isolated series of glazed sliding doors denoting the garage, service rooms and subterranean entrance.

From the street, the house seems to be a gloomy cube of slatey concrete and glass. It is impossible from this vantage to appreciate the irregular trapezoidal geometry of the building, and to see how the side walls splay out and away from the rectangular to follow the boundary lines of the site. The lines are prefigured by the awkward outline of the garage forecourt, which seems to concur with no formal design but rather, perhaps, with an impromptu one of the inhabitants. Tricks of illusion are characteristic of this practice, and the deadpan shell encloses a civilised airy elaboration of the villa that turns inwards around a central atrium, secluded from the neighbouring houses that surround it, but admitting the bosky garden at the rear and distant views of the city.

In some respects, this building represents a departure for the practice, away from the architecture of skins for which they are famous. The architects have acknowledged a debt to Joseph Beuys with whom they once collaborated, and lurking somewhere in their treatment of skin and surface there is the memory of felt and fat, the substances that so exercised Beuys' sculptural imagination.

Ambiguity and the message that everyday normality (of the office, the factory, the house) has equivocal depths may be another legacy of that collaboration - though Herzog professes admiration for the films of Alfred Hitchcock - and are present in previous schemes: the Riccola Factory at Mulhouse, the strangely flat sports centre, also near Basel, at St Louis; and the Schutzenmattstrasse apartment block with its undulating screen of cast iron. In all these buildings, rigour and controlled imagination are manifest (both Herzog and de Meuron were taught by Aldo Rossi). But their architecture is permeated by a morbidity that is both fascinating and repellent. The Riccola building seems, in its pure concrete abstraction, the epitome of 'factory' with all its dramatic - and heartless - connotations. Its external concrete surfaces, screen-printed with a leaf motif, are both poetic and funereal like the decoration of antique tombs; similarly, the apartments' screen is lyrical but also suggests imprisonment.

The architects' approach to designing the Kochlin villa seems on the other hand to have been reassuringly straightforward and almost playful. Given the nature of the site and domestic purpose, they designed the house 'from the inside out', beginning with the idea of the private internal atrium, around which spaces for 'day- and night-time life' could be organised horizontally and vertically. The atrium was envisaged as both an interior and exterior room, and the house as securely rooted deep in the hillside. Design of its exterior was left until quite late in the process: 'after months of planning, we still did not know how [the house] would look from the outside'.

You enter as if through a root canal - a subterranean tunnel of unrelieved concrete, illuminated by shafts of light from the atrium and clerestory glazing. A perspective straight out of Alice in Wonderland derives from the odd geometry, trapezium-shaped plan and ramped section. On left and right are wings containing the garage and laundry room respectively, and at the far end is the glass entrance to stairs and subsequent surprising emergence into the transparent crystalline kernel of the house.

There are two levels. On the lower one, three rooms - kitchen-dining, living and children's - form a U around the trapezium-shaped atrium. Expanding on plan under the upper floor towards the garden, the void can be closed off or opened up by sliding doors and a retractable roof. As in a conventional house, interior volumes are clearly delineated sequentially one from another by opaque partitions. But space flows visually and physically through the courtyard's transparent glass walls and sliding doors and because of the geometry, diagonally across it to the opposing room. Big windows that can be slid open across the outside of the external shell frame long views. Framing of walls and windows is emphatic, as in early Modernist exercises (Johnson's New Canaan house of 1949 comes to mind), but here where technology could have made it almost invisible, the frames must be a deliberate device, presumably to emphasise domestic solidity and roots, Here again is a visual trick, for the building's real steel-framed structure has been largely concealed.

The upper bedroom floor is contained within a U-shaped plan, but in a masterly stroke, it has been rotated by 90 degrees and the open end facing south, occupied by the roof of the dining room, has been filled with a rooftop lily pond leaving an inner open plot of emergent greenery to indicate the atrium beneath. Bedrooms ranged along the front, west, and rear east sides of the building, give onto a continuous corridor lining the inner glazed walls and made lyrical by the dappled light reflected off water.

But ambiguity lingers around this clever inversion of the classic Modernist house; and it is interesting that the practice chose to include Rachel Whiteread's inside-out house (AR January 1994, p13) in perspectives of the winning scheme for the Tate Gallery of Modern Art at Bankside in London. If its blank grey and glass skin conveys the idea of a fortress, immune from invasion, the irregular pigmentation of the grey rendering, the architects suggest, recalls something organic and approachable 'like the skin of an animal', or indeed, Beuys' grey felt.