Architecture: Earth above and heaven below: Nicholas Schoon on a remarkable underground company HQ

Europe's largest roof garden is coming to life again. This is its fifth spring; the explosion of growth and colour atop an almost-hidden corporate headquarters is no longer new for the office staff below.

The flow of architects, gardening club members and schoolchildren touring the building has slowed to a trickle. The architecture and landscaping awards - 10 in all - have dried up. But now is the time to assess whether RMC's earth-covered headquarters at Egham, Surrey, is as useful as it is unusual.

The building acts as an office, a company leisure centre, a luxurious directors' palace and a formal, tiered garden with hedges, lawns and pools. It is also an advertisement for corporate imagination, continuity and the long-term view. Only its name, RMC House, is bland.

The principal claim for the much-praised company headquarters, designed by Edward Cullinan Architects, is that it is environmentally friendly, kind to its users and surroundings. 'Green', however, can also mean gimmicky, and environmentalists will always carp. The fact that the building's owner is a construction industry giant with interests ranging from gravel extraction to waste disposal makes RMC House a tempting target.

Critics might also ask what business this building has being where it is: it stands in the London Green Belt, the anti- development zone created to halt the capital's relentless sprawl.

RMC succeeded in winning planning permission against the odds because it was clear that its new headquarters improved the site. Three historic buildings were retained and restored and a jumble of prefabricated Sixties laboratory blocks demolished. And the new offices, for 110 staff, were designed to be almost entirely cloaked in vegetation.

Initially, however, Runnymede Borough Council refused planning permission. RMC then appealed to the Department of the Environment, which held a public inquiry and eventually granted permission, subject to minor design changes. Runnymede was then gracious enough to grant the finished article an award.

The building does work. It saves on fossil fuels and reduces air pollution by storing warmth in winter, coolness in summer, and minimising the need for electric light. The roof is made of 6in-thick concrete slabs, carrying soil up to 2ft deep. This insulates the offices and stores heat, retaining daytime warmth through the night and summer warmth into the autumn. Air is also circulated beneath the building's raised floors, passing over a concrete layer that maintains an even temperature.

Much of the structure is flanked with floor-to-ceiling windows, overhung by box- like structures projecting from the roof. These are filled with soil and planted with yew hedges. The overhangs prevent too much direct sunshine in summer, yet allow in plenty of indirect light. Windows penetrate the thick roof deep within the offices to allow in natural light.

Some heating is needed in winter, but there is no air-

conditioning and the large windows can all be opened to keep the place cool in summer. At pounds 17m, RMC House cost more than a typical office building of its size, but is considerably cheaper to run.

Stale air exits through some brightly painted ventilators on the roof, shaped to look like giant chess pieces. These are the most cheerful and obvious of Cullinan's architectural effects. His combination of old and new buildings makes for busy architecture, with lookout points, secluded corners and pleasing views through linked arches and windows

to more distant greenery and water.

The new structure is largely single storey and self-effacing, consisting of limbs coming off a large reception hall and surrounding three garden courtyards, each with a pond. Stairs link these to the 1.5 acres of roof gardens above. Here, the Derek Lovejoy Partnership, landscape architects, came up with a formal arrangement of yew hedges, small, spherical holly bushes, lawns and gravel paths with an outlook over 18 informal acres of parkland, trees and a lake.

On entering the reception area the visitor is confronted by a swimming-pool. You cannot smell it, because a curtain of air moving from floor to ceiling vents traps both the warmth and the odour of chlorine. Step through this invisible barrier, however, and the air is humid and nearly 10C warmer. Immediately to the right of the entrance are two indoor squash courts; somewhere else is a gym.

The exterior of the newly built offices is glass, white concrete and white-painted steel, which nevertheless fits in well with the old red-brick buildings already on the site. The contrast between the two is eased through clever use of new brickwork and by the sheer mass of greenery, which makes the new offices appear as a formal garden serving the old buildings.

One of these, Eastley End House, is three-storey and Georgian, Grade II listed. Next to that is a Victorian stable block and a little farther off stands an Arts and Crafts house sporting a lanterned turret with a lead-domed roof. All are in use as offices, staff accommodation and training rooms. The Georgian house has been converted to sumptuous directors' offices and a boardroom filled with period furnishings. The architects would have preferred a little more restraint, but British businessmen and, it would seem, their visitors, like nothing more than a bit of 'traditional style'.

Lower down the pecking order, staff seem happy with the new Cullinan building. Alec Gilles, the deputy project architect, says: 'Whenever I've been around, people have come up to me and said good things - some even say they they feel uplifted by it.' Clive Barton, marketing manager of RMC, says: 'The openness of the building has given a different feel to staff relationships. People in the group are proud of it. They come in early and productivity levels have gone up.'

RMC House's one obvious environmental defect is its complete dependence on the car: the nearest railway stations are a long walk, no buses stop nearby and RMC does not provide staff transport.

The company used to organise free Saturday tours of RMC House for groups who wrote in, asking to look around, but these have ended. There is, however, a strong case for opening the building at weekends to visitors, charging them enough to cover the extra security, cleaning and other services required.

Why not contract the National Trust to do this? This is, after all, the modern equivalent of the best kind of stately home - a tasteful and innovative symbol of permanence, wealth and status. The gardens are a delight and the building successfully demonstrates one way forward for the office of the future. Come on, RMC, let the people see.

(Photograph omitted)