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STEP INSIDE Number 20, SOHO SQUARE, central london, and the elliptical reception desk on a floor of polished marble tells you that you are in Ab Fab land. The 1920s neo-classical building has had a chic £10m makeover to appeal to the style-conscious, high-spending advertising and media set that now congregate in the area. Its architect is that inspired and faithful retainer of the chi-chi classes, Lifschutz Davidson, famous for designing restaurants for upmarket retailer Harvey Nichols in the Thames-side Oxo Tower and, most recently, in its Knightsbridge store.

There is much here for the PRs to coo over: the restored front facade with its stately Portland Stone columns, the gracious oval spiral staircase, and the column-free top floor with its Wimbledon green carpet and slanting window walls. What Patsy and Edina would never realise, though, is that the architect's creativity has been put to an even more demanding if less eye-catching test – that of squeezing in every possible square inch of office space for German-owned developer Asticus UK to reap in rental revenue.

Completed in 1926 as the headquarters of tinned soup company Crosse & Blackwell, the building is not listed but stands in a cramped city centre site in a conservation area. The tricky exercise of conjuring up extra office space has therefore been constrained by tight boundaries, party wall and rights of light legislation and the conservationist demands of Westminster city planners.

In total, some 1010 m2, or 16%, has been added to the building's gross floor area. On top of that, the office space has been upgraded to full British Council for Offices specification, including air-conditioning and raised access flooring.

The largest expanse of new floor space has been created by putting a new top floor on the flat roof of the existing rear extension. "At first the planners wanted a traditional lead mansard, and we wanted vertical walls behind a roof terrace," recalls project architect Brian Reynolds. The planners also pointed out that the extension would jut into one of London's strategic viewing corridors from Parliament Hill to the Palace of Westminster.

What was eventually built was a modern mansard in the shape of a sloping window wall, topped by a low double-pitched roof that conceals the air-handling plant. The result was a clean-cut roofline that satisfied the planners enough to waive the slight infringement on the view. On the inside, the new top floor basks in the daylight that floods through the perimeter window wall. It also commands mesmerising views of the Soho roofscape on all sides.

Not least, the new rooftop extension is a bravura exercise in structural engineering. "We turned the structure into the glazing system," quips Reynolds. What he means is that the 12 m-span lightweight roof is entirely supported at the perimeter on slanting skinny legs of solid steel that look like nothing more substantial than glazing mullions (see "Lightening the load", overleaf).

Another new 3 m extension has been added on all floors along the flank of the existing rear extension. This encroaches slightly on a shared internal courtyard, threatening to cut off rights of light to the adjoining property. Lifschutz Davidson's solution was to slant the new perimeter wall on the top two existing floors by 70°, neatly matching the inclination of the new mansard window wall to the top floor directly overhead. Inclining all these window walls also come with a bonus, in that they are classed under building regulations as roof elements, rather than walls, and therefore avoid the need for fire-resistant glazing.

Fitting modern environmental services and raised access floors into the 80-year-old building's low storey heights was, in Reynolds' words, "a constant fight to create a few more inches of headroom". Unusually in a refurbished building, an advanced, low-energy displacement ventilation system was installed.

Air-handling plant was housed in the basement, except for plant serving the top two floors, which has been fitted into the shallow roofspace above the new top floor. New risers for ducts and cables were imperceptibly fitted into odd corners without windows.

In the displacement ventilation system, fresh air is supplied through circular grilles in the raised access floor tiles and discharged through light fittings recessed in the ceilings. To preserve adequate ceiling heights, the ceiling void was squeezed down to just 75 m, which was too shallow to fit in ducting for the exhaust air. So services engineer Cundall Johnston & Partners punched holes in the ribbed concrete floor slabs in order to discharge exhaust air from below through ducts. The void of the raised floor doubles as a plenum for the air supplied above.

On the sixth floor, where the headroom was the lowest, the solution was even more complicated and, according to Reynolds, quite unprecedented. Here the raised access floor had to be compressed to just 250 mm in depth, which was insufficient to accommodate exhaust air ducts. Instead, a double-decker raised access floor was laid, with the upper void of 150 mm depth serving as a plenum for fresh supply air, and the 100 mm deep lower void a plenum for the exhaust air from the office area below.

Suspended ceilings were required to house the recessed luminaires and lighting cabling. But the architect has steered clear of ugly suspended ceiling tiles and specified smooth drylining instead. A circular access panel, also in drylining, has been included in each bay for access to the lighting control module.

The lift installation also suffered from inadequate headroom because of the extra storey, and Westminster planners would not allow the lift motor room to be extended upwards. This problem was circumvented by specifying Kone's new "Monospace" lifts with slimline motors fixed to the roofs of the lift cars.

Lifschutz Davidson's house style of elegant minimalism is much in evidence throughout, but this has been carefully pitched to complement the neo-classicism of the original building. In the reception hall, for instance, the new polished travertine marble for the floor and internal wall comes in large rectangular panels separated by meticulously chamfered arises. This can be read as a tribute to the stripped neo-classical style of inter-war classics such as Joseph Emberton's Simpson's department store on Piccadilly.

Number 20 Soho Square is an elegant match of old and new that is sure to stimulate the creative juices of its media tenants. It's also an object lesson in how an old building can be radically upgraded sensitively, without committing the ritual architectural murder and embalming of a retained facade fronting a new structure.