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Designed by Sheppard Robson, the 拢25m building looks like a cross between a luxury car showroom and an exclusive Californian country club. In fact, it stands in the grounds of a 1920s country house high on the North Downs near Epsom racecourse in Surrey. Encircled by a ring of stately pine trees, it commands a mesmerising panorama of Greater London, while a small lake and cascading water wall lap right up to its front facade.

Walk through the silvery entrance rotunda on the crest of the hill, and you find yourself in a large, triangular gallery space displaying four of the latest Toyota models, overlooking the lake and trees. Still to be installed is a fifth car, which will dangle acrobatically from the roof of the gallery.

Although never overt, turn-of-the-millennium automobile imagery abounds in the design, from the silvery metallic cladding to the moulded, precast concrete ceiling troughs with inset air-grilles inspired by Toyota car headlights.

Beneath this seductively glamorous imagery, the building is designed to encourage dynamic new ways of working among its 500 staff, offering a variety of work environments from permanent and secluded to transient and gregarious. Four wings of offices spur off a spacious internal street containing the gallery 鈥� an arrangement devised to promote interaction between staff of different departments. This conforms with many state-of-the-art office buildings, including Niels Torp's acclaimed Waterside complex for British Airways near Heathrow, completed in 1998. "The big difference about this building," comments Tim Evans, director of Sheppard Robson, "is that the entire building of 14,000 m2 is one single, open compartment. There are no barriers between departments, as Toyota wanted to promote an open work culture within the company."

As well as encouraging staff interaction, the barrier-free environment creates an astonishing and exhilarating sense of space, with raking views through three or four different spaces and then out through the glazed perimeter walls to the surrounding landscape. Of course, Sheppard Robson's design also creates a functional working environment for staff.

The design has been tailored to Toyota's specific needs, but these had to be reconciled with financial institutions' standard demands for lettable office space 鈥� in case Toyota should relinquish part or all of the building 鈥� and with a tight budget that reflects the balance between bespoke headquarters and speculative office accommodation.

To meet these conflicting requirements, Sheppard Robson divided the building into three main elements. The most basic is the office wing, which is two or three storeys high and 15 m wide, allowing a combination of open-plan and cellular offices, as well as natural daylighting and cross-ventilation. Four of these office wings project like splayed fingers from another, longitudinal, two-storey strip of offices.

The longitudinal office strip overlooks the double-storey-height street-cum-gallery, 80 m in length, which is the second main building element and contains a scattering of black-leather bucket chairs for informal meetings. Running along the interface between the offices and the street is a service strip into which are concentrated nearly all the building's staircases, lifts, toilets, services risers and office equipment.

The third main element is the entrance rotunda, from where visitors are given a wide-angle view of the entire street and the offices behind. The upper level of the rotunda is a conference suite, while an outer ring at the front contains a staff cafe-bar, equipped with counters for plugging in laptops, and a 200-seat restaurant.

These three elements are cleverly arranged in plan and section to make the most of the spectacular, sloping hill-top site.

In plan, the street-cum-gallery and the cafe and restaurant are given pride of place overlooking the lake and facing the London panorama through the pine trees. The office wings are placed at the rear with a more secluded outlook onto landscaped courtyards.

In section, the site rises by one storey height from front to rear. The entrance rotunda is perched dramatically on top of the hill, and it leads through at the same level to the street and to the lower office floor. An undercroft containing computer server rooms, a fitness centre, plant rooms, car parking and loading bays has been neatly tucked out of sight below the building, with its vehicular entrance at the rear. The restaurant is also at this lower level, but at the front of the building where it spills out irresistibly on timber decking over the lake. At the rear and one storey higher, the lower office floors likewise open out directly into the landscaped courtyards they embrace.

The building fabric is suitably high-tech for a car manufacturer, comprising silvery aluminium panels, tubular structural steelwork and plenty of glass. The colour scheme is monochrome, except for the carpet within the office areas, which is a vivid royal blue. In addition, the form and fabric contribute passively to a low-energy, healthy internal environment, which is actively controlled by a mixed-mode system combining a displacement air-conditioning system with natural cross-ventilation through openable windows.

As part of the low-energy strategy, daylight is spread deep into the offices through clear-glazed curtain walls, but solar gain is filtered out by means of perforated metal spandrel panels. The visual effect is that the curtain walls seem nearly transparent from the inside but opaque from the outside.

With the same objective, precast concrete floor slabs have been exposed underneath to absorb fluctuations in internal temperature. These ceiling coffers were precast in a deliciously scalloped form in off-white concrete as smooth and rounded as a bathtub. Circular holes for air-supply grilles or cabling have been cast at angles, echoing the streamlined headlights of Toyota cars, and continuous grooves have also been cast into the curved surfaces to accommodate partitions for cellular offices or meeting rooms. The upper partitions are in smooth, unpainted plasterwork that matches the precast concrete ceiling.

Above the street, a clear-span roof vaults dramatically up and over to meet the curving, outward-sloping window wall overlooking the lake. The vaulted roof is supported on a diamond grid of tubular steel 鈥� "like the spokes of a sports car", comments Evans 鈥� and is partly opaque and partly glazed. The roof of the rotunda is likewise glazed.

Each of the main building elements has a contrasting geometry 鈥� a drum for the entrance, a torus or doughnut-shaped roof over a triangular plan for the street, and rectilinear floors for the office wings. The inevitable weakness of this rich mix of geometry is that it tends to run away with itself at the interfaces. When viewed from the entrance drive, for instance, the outward-sloping smoked-glass facade and torus roof of the street seem to crash like a runaway bull into the more composed, silvery form of the entrance rotunda.

Fortunately, this clash is more than made up for by the rich and exhilarating sense of space within the building. The varied internal landscapes of the three main elements unfold before one's eyes, and to this is added the stunning natural landscape and long-distance views encircling the building. Standing near the rear of the building, for instance, one can look above the desks and below the curved ceiling troughs of the office wing, through the great vaulted hall of the street and right into the curving entrance rotunda. The scene is animated by staff walking through the spaces about their daily work, and by sunlight streaming through the rooflights to cast complex, curvilinear and constantly shifting patterns on the white-plastered walls below.

As Simon Lake, Toyota's project manager for the building, comments: "Normally there are one or two best positions for views through and out of a building. The thing I love about this building is that there are 18 best positions in it."

The organisational objective of the open plan also works brilliantly, according to Lake. "People drift from the offices into the street, and use the cafe-bar for informal meetings. And from the first 20 minutes we moved in at the end of April, travelling sales staff were drawn by the smell of coffee to the cafe-bar, where they plugged in their laptops and got down to work."