Aggregates group RMC has the finest, most magical head office of any company in the construction industry. In fact, hanging gardens make it look more like uninterrupted parkland than a headquarters. So why did its owner want to change it? Martin Spring found out.
In the tradition of cobblers' kids being THE worst shod in the street, the building industry has a record of developing dismal buildings for its own use. The most glorious exception to this is probably the RMC Group of ready-mixed concrete and aggregates companies. The group's international headquarters, which was designed by Edward Cullinan Architects and built in the Surrey green belt in 1990, scooped nine architectural awards and commendations, including the supreme Civic Trust award for the that year, ironically presented by RMC's rival, Steetley.

Even award-winning buildings can impede the activities of their occupants, however, and by 1999 RMC decided it needed to double the number of staff that worked in it. Enter architect and space planner Aukett Europe to remodel the headquarters. Had Cullinan's scheme been so functionally deficient as to require drastic action? Or has Aukett's remodelling destroyed the magic that made the original so special? As Building discovered on its visit last month, the answer is no to both of these questions.

The 6 ha estate lies alongside Thorpe Park adventure playground – also developed by RMC – and includes three historic houses set in landscaped gardens. The local authority, Runnymede council, had been terrified of setting a precedent of developing the green belt. Cullinan, working with landscape architect Derek Lovejoy Partnership, cracked the planning problem on appeal by submerging new single-storey buildings into an interconnected network of luxuriantly planted courtyards and roof gardens. "Our strategy was to dig up the gardens and put them on the roof," says Cullinan director, Robin Nicholson. The effect is that the buildings are all but invisible when viewed from the nearby St Ann's Hill.

As built in 1990, the two larger historic houses are linked by five new-build wings capped in richly landscaped roof gardens, and these coalesce into a magical parkland in the sky made up of bushes, hedges, creepers, lawns, viewing platforms and sculptures. Another layer of parkland lies at ground level and follows through from the courtyards between the wings to the surrounding estate and lakes. The surreal nature of the scheme is symbolised in large ventilation cowls that pop up through the roof gardens, inspired by the Red Queen's chess set in Alice Through the Looking Glass.

Cullinan's interiors matched the parkland exteriors in novelty and delight. Most staff were given individual offices that looked out through floor-to-ceiling glazing to the landscaped courtyards. A spacious reception hall doubled as the social hub of the building, as it incorporated a staff restaurant, swimming pool and two squash courts. Visitors entering the reception hall were also stopped in their tracks by an extended vista that penetrated without interruption past the reception desk, across the pool and through a window wall to the parkland and lakes beyond.

The building came with a pioneering low-energy design by M&E engineer Max Fordham Associates. This exploited exposed concrete as a heat sink and used groundwater drawn from a site borehole as coolant. As a result, the internal environment relies on mechanical ventilation rather than extravagant air-conditioning.

Not surprisingly, Cullinan's highly original, award-winning design attracted wide interest and radically boosted RMC's image. Staff relished their offices and amenities, which dramatically reduced employee turnover. Twelve years on, the building is immaculately maintained by a 13-strong facilities department. Facilities manager Gary Foy says: "I think it's a stunning building. It was very advanced for its time – and it works."

By 1999, however, attitudes to the building were changing.

A new head of facilities, Diana Kilmartin, was appointed from the dynamic corporate world of call centres. "When I came, the building was full and 24 training staff were being housed in temporary site huts," she says. "I looked around and said there was plenty of space to fit more staff in."

Kilmartin brought in Aukett Europe, a firm she had worked with at Doxford Business Park in Sunderland. "We considered asking Cullinan again, but this project was less of an architectural issue and started off as just a minor space-planning exercise," she says. Aukett's commercial director Stuart McLarty puts the perception of the building by Kilmartin and himself in overtly commercial terms. "The building was from another time. It was out of sync with corporate values these days, and was not expressing shareholder value."

By the time Aukett's project was completed last December, much of the building had been radically remodelled, at a cost of £2m, to house the group's 120-strong UK headquarters, which had been relocated from a 1960s office block in Feltham seven miles away. At a stroke, the permanent staff occupying the building doubled in number.

Other organisational changes had assisted the relocation of the UK company. Group training, for which much of the building had been purpose-designed by Cullinan, had been superseded by computer-based training in the group's subsidiary companies. A concrete-testing laboratory within the complex had also become redundant. The upshot was that three areas of the complex were ripe for a change of use – the reception hall, the store rooms, laboratory and kitchen that lay to one side of it, and the Victorian stable wing to the Georgian house beyond the courtyards.

In the reception hall, the two squash courts were hardly used and have now been converted to meeting rooms. Next to these meeting rooms, the pool has been screened off to provide more privacy for swimmers. Finally, McLarty suggested that the restaurant could be relocated to the Victorian stable wing to replace the study bedrooms created for trainees.

The main loss in the reception area is that the partition to the pool now cuts off Cullinan's great tracking view through the entire reception and pool area and out through the rear window wall. Fortunately, this is partly compensated for by a group of luxurious black leather sofas arranged around a vivid scarlet carpet in the area vacated by the staff restaurant. Located to one side of the reception desk, this is an ideal meeting and greeting area.

As for the two converted squash courts, they have made excellent meeting rooms with only minimal alteration. In each court, a ceiling canopy containing acoustic absorbers and downlighters has been slung from the roof without obstructing daylight streaming down from the high-level windows, and walls have been lined with projection screens and more acoustic panels. The glass screens facing the reception area have also been replaced with a semi-transparent etched pattern.

In the warren of rooms beside the reception hall, the original walls and partitions have been cleared out to leave a large open-plan space to house the UK head office. This area features the only new extension to the complex – the filling-in of a former loading bay. Here Aukett has been careful to specify white-finished cladding panels similar to those used by Cullinan.

In the Victorian stable wing, the ground floor of study bedrooms has been converted into a kitchen, servery, cafe-bar, five staff dining rooms and a business lounge, all strung one after the other. The five dining rooms are partially linked by large rectangular holes cut in the original brick cross walls. The fifth occupies a central archway, which had been left open in the Cullinan scheme, but has now been closed off by french doors. Whereas the former restaurant was tucked behind a screen in the entrance hall, the new dining rooms benefit from views out to the largest courtyard on one side and to the surrounding parkland on the other, and they even come with two external brick terraces for alfresco dining in summer. All these amenities back Kilmartin's claim that the relocated staff restaurant "is dramatically more popular than the old one".

As the last piece of the reordering project, doors have been punched through various internal cross walls, turning the whole complex into one interconnected network of rooms and circulation routes.

It would be hard to argue against RMC's claim that the remodelled building functions more efficiently than before.

The international and UK companies have been brought together in closer co-operation. Dead-ends within the building have been opened up to improve interdepartmental communication. Even the now secluded pool is used more frequently than before.

To outside eyes, there is almost no sign of change in this magical office-cum-parkland. This is partly thanks to the sensitivity of Aukett's remodelling, which even extends Fordham's original low-energy ventilation system. It is also partly thanks to the strength and enduring freshness of Cullinan's original design, which 12 years on, looks indistinguishable from the new works.