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A decade later than most private corporations, the British civil service is now in the throes of upgrading its premises to modern, computerised, open-plan offices. The change in working practices for government bureaucrats is radical and the scale of the programme is huge, but the overriding constraint is that the grand classical Portland stone buildings lining Whitehall are all listed or protected by conservation area designation.

Last month, just five months after the conversion of the Treasury building by Foster and Partners, it was the turn of the Cabinet Office, now incorporated into the Office of the Deputy Prime Minister, to reoccupy radically refurbished premises. But whereas the Treasury inherited a purpose-built Edwardian pile at the corner of Parliament Square, the ODPM occupied a raggle-taggle collection of four adjoining buildings towards the upper end of Whitehall. Varying in size and shape, the four buildings were developed by the Admiralty over two centuries – starting with the elegantly classical grade I-listed Ripley ºÚ¶´ÉçÇø of 1726 – and had since been subjected to internal reconstruction following Second World War bomb damage.

The £43m scheme has provided 25,000 m2 of refurbished office space containing 865 workplaces. The architect is the conservation arm of American-owned practice HOK International, which in its original incarnation as Cecil Denny Highton was responsible in the 1990s for the conversion of the Foreign & Commonwealth Office a few doors down the road.

Of the four adjoining buildings, the two oldest were built as residences for admirals, and the other two as offices. Internally, this resulted in a disorientating warren of old rooms of varying sizes and corridors that did not link up. In the words of Jonathan Howe, HOK's vice-president, who was in charge of the project: "The buildings suffered from lack of legibility in circulation: people couldn't understand the map of the building. In the Ripley ºÚ¶´ÉçÇø, for instance, the original notion of six houses had been eroded as new corridors were cut through."

In essence, HOK has rationalised the circulation and unified the disparate building complex by sinking four new cores containing stairs, lifts and bridge links through it. As a rule, these have been located in original light wells, which have been roofed over in glass to convert them into mini atriums filled with daylight. As part of the rationalisation strategy, an original servants' staircase, which was tucked into a relatively inaccessible corner of the Ripley ºÚ¶´ÉçÇø, has been removed.

Howe finds historic justification for his design approach in the existing buildings. "The tiny rooms in the buildings had constantly been knitted together over time by the Admiralty. New corridors, bridge links and infills between the buildings were added," he says. "So we asked ourselves how we could continue that approach – and we think that our atriums are not alien to the building."

The approach is overtly modern in style, with the historic fabric respectfully repaired but new interventions expressed in contemporary architectural language.

The largest of the new circulation cores is a spacious modern atrium containing a staff cafe where there had been a rear service courtyard. "It creates a new heart for the complex," explains Howe. This central atrium offers the biggest opportunity for architectural expression in the entire refurbishment project, and HOK has responded with a variety show of modern and conservation motifs. A horizontal clear-glazed roof suspended from steel trusses oversails the entire column-free space, and a minimalist free-standing staircase with open treads and clear-glazed balustrading climbs dramatically up to a new bridge link on the first floor.

Each side of the atrium is radically different in form. A clear-glazed facade overlooks the Spring Gardens mews at the rear, and the brick external wall plus dormer window of a listed Georgian building lines one side. On the other two sides, the space flows unimpeded into the open-plan floors of the steel-framed 1930s Kirkland House, the rear walls of which have been removed.

The atrium succeeds in providing a suitably lively setting for the social hub of the office complex – so much so, that it is an uneasy space with little sense of containment and coherence. The effect when you walk through the front door of Kirkland House, now the main Whitehall entrance to the whole complex, is even more unnerving. Instead of walking into a neoclassical stone building, as suggested by the facade, the visitor walks directly through to a modern, intensely lit, open-plan space with no visible rear wall.

HOK's original scheme had been to locate a glazed central atrium across the end of the Spring Gardens cul-de-sac, but English Heritage objected that it would intrude on the historic brick elevations bounding the mews. By shifting the atrium partly inside an adjoining building, the historic streetscape has been preserved, though at the expense of the buildings themselves.

When it came to the technical matter of upgrading environmental services and structures, each building was treated according to its particular physical and conservation constraints. The unlisted 1930s Kirkland House, with its relatively flexible steel frame, has benefited from the installation of full displacement comfort cooling and raised access flooring. The plant has been hidden in the basement vaults, which were deepened accordingly. In contrast, in the grade I-listed Ripley ºÚ¶´ÉçÇø, where the original cellular rooms have been retained and restored, Versatemps units have been inconspicuously fitted below windowsills. Each unit houses a heat pump, which can draw both heating and cooling from a constant-temperature water supply.

The most severe structural problem was settlement in the Ripley ºÚ¶´ÉçÇø that was caused by rot in the timber raft that supported it. The solution adopted by structural engineer Alan Baxter & Associates was to X-ray the raft to locate the voids left by this decay, and then to repair them by injecting liquid grout.

Even if the central atrium is a touch frenetic, the refurbished Cabinet Offices on Whitehall are a modern solution to upgrading office space in historic buildings without compromising on conservation. Let's hope that the newly fluid corridors of power will help the government machine run more smoothly.