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Call in architect Will Alsop and be prepared to receive an invading herd of blobby-shaped pods strutting around on spindly steel legs. In the Peckham Library, south London, two such pods sheathed in soft bark-like shingles can already be seen entertaining children. And now another four, sleeker versions in smooth fibreglass, have popped up deep inside Alsop Architects' latest completed building – Victoria House, overlooking Bloomsbury Square in central London.

When Victoria House opens for a sneak preview tomorrow in London's Open House event (see the diary on page 18), its four blobby pods can be counted on to bring broad grins to many faces. But the funny thing about Victoria House is that it is anything but a playful building. This stone and bronze former head office of the Liverpool Victoria Insurance Company carries its classical features with grand solemnity and is justifiably listed grade II. Though less than 70 years old and expertly designed in the French beaux arts style by a little-known architect called Charles Long, it is a supremely monumental, imperial building that is as remote from 21st-century sensibilities as the ancient Greek temples and Roman baths it copied. The question begged by Alsop's conversion is whether the deliberately irreverent juxtaposition of the jaunty pods within this solemn temple to commerce is too jarring a culture shock.

In many ways, Victoria House didn't need Alsop Architects to convert it into modern speculative office space. Despite its imperial classical styling, it was nearly there already. More than any other prewar office building, it was the original groundscraper, although it was developed half a century before the term was coined.

No more than eight storeys high, it spread out to cover an entire city block, making it ideal for open-plan office layouts of the computer age. Added to that, it had been purpose-built as a head office with generous storey heights that could accommodate modern raised floors and suspended ceilings, plus a hefty steel frame with plenty of load-bearing capacity to spare. And boasting just one careful owner, it had not been hacked about like so many historic office buildings. Not least, whole suites of rooms on third and fourth floors came complete with sumptuous mahogany panelling, marble fireplaces and brass light fittings to boost the egos of company directors.

Alsop's contribution to the conversion has been the now-standard practice of gleaning more usable office floor space and upgrading the specification for modern support services while keeping English Heritage at bay by conserving the character and much of the fabric of the historic building. In total, usable floor space has been increased by nearly 15%, bringing the gross area of the building up to 42,000 m².

The £60m conversion is actually Alsop's third, and least radical, scheme for the building. The first plan was drawn up in 1998, when Blackfriars Investments was considering Victoria House as a home for the Greater London Authority. For this, the Alsop pods were so powerful that they burst out through the classical envelope of the building. A year later, the building was acquired by the German developer Garbe, which had worked with Alsop's previous Anglo-German practice in Hamburg and planned an upmarket shopping centre. However, being located between the City and the West End, it failed to win support from the retail market. Garbe then switched to, and is just completing, a speculative office development, plus a restaurant and health centre, to replace the old ballroom in the basement and the refurbishment of shops on Southampton Row.

Much of Alsop's design strategy is established practice. In the heart of the building, the two utilitarian light wells with their white-glazed brick walls have been replaced by modern oblong atriums enclosed by glazed roofs and window walls. Not only could the office floors then expand into the heart of the building, but the new window walls funnel daylight into these deep-plan floors. Although all the original grand classical staircases have been retained, vertical circulation and services distribution have been improved by slotting new lift cores and services risers in all eight corners of the two new atriums.

At the top of the building, the sixth and seventh floors have been reconstructed with extra headroom within the old mansard roof. The former plantroom in the central tower has been requisitioned as a conference room, with spectacular views on all four sides. And a new eighth floor has been added to accommodate the plant, stretching across the whole length of the building.

For the most part, all this conversion work has been carried out efficiently and neatly – though this hasn't prevented Alsop's irrepressible sense of fun from popping out here and there to jolly things up. The original fabric, whether in the classical Portland stone shell, the grand marble staircases and central hall, or the mahogany panelling of the so-called "Heritage Suite", has been faithfully restored.

Elsewhere a high-tech style has been used that succeeds in being both uncompromisingly modern and discreet enough not to conflict with the retained classical fabric. For instance, the rooftop plantrooms rise 4 m above the existing cornice to the E E mansard roof and are quite visible from the other side of Bloomsbury Square. Yet they have been discreetly set back behind an access walkway and their continuous slender I-beam cornices and glass louvre walls give them a suitably clean-cut outline on the roofscape.

The grand marble public hall, complete with recessed colonnades and upper balconies on all four sides, forms the building's centrepiece and now serves perfectly as the main reception area to the offices above. The only problem was that its marble floor was set 700 mm below the office floors surrounding it, so this has been raised in the interests of disabled access. A shiny modern floor in clear glass has been floated across the entire space, fortunately retaining the bases of the recessed columns and a modicum of mouldings on the pilasters. The resulting effect is of a sybaritic plunge pool in an ancient Roman bathing palace.

Other interventions by Alsop are more risqué. The window walls enclosing the two atriums, for instance, are not vertical or even inclined but bulge out at their midriff.

Alsop likens them to two portly gentlemen having a chat. Slightly less obvious is the commercial benefit brought by the bulging midriffs – larger floorplates on the middle floors.

Needless to say, it is the four curvilinear pods that steal the show inside the building. These are monstrous, but not unfriendly, creatures with smooth white skins that have taken up squatters' rights inside both atriums. The upper two resemble giant larvae whose eight tubular-steel legs are propped off the curtain walls on either side. Their biomorphic forms and outstretched steel legs also bring to mind Archigram's iconic Walking City images of the 1960s. The lower two pods are more like open dinghies bobbing about in the sea. Light reflected off their smooth white skins will change colour throughout the day.

Constructed as double skins of glass-reinforced plastic in a Southampton boatyard, the bizarre pods serve the practical function of meeting rooms. Each of the two upper pods encloses two rooms in double-deck formation, and the two lower pods are open seating areas. They are reached by narrow bridges leading off from the central core or the office floors on either side.

The alien forms, detached locations and intimate interiors of the pods give them a strong sense of apartness from the regimented office space in the building. As they cross the narrow bridges from their routine desk spaces to the outlandish meeting rooms, office workers should feel refreshed and charged up for their discussions. At the same time, the pods give an unmistakable identity to the building as a whole.

For William Avery, director of Garbe UK, the pods are not an amusing optional extra but "absolutely fundamental to the success" of the building. "We could have saved millions by leaving them out and building straight sides to the atriums." He omits to add that, in a building that could not feasibly be extended in any direction, the pods conjure up about 150 m² of lettable floor space out of the empty voids of the atriums, yet scarcely impede daylight streaming through to the office floors on either side.

But what about their jaunty styling, which looks as if it could have lifted from one of Raymond Briggs' children's books, and is so alien to the monumental classicism of the enclosing building? To Alsop's project director, Duncan Macaulay, "the original building makes such as strong architectural statement externally, it needs something to stand up to it internally".

Although Macaulay admits that the local authority planners "took a certain amount of persuading", the conservation authorities eventually swallowed the irreverent Alsop approach entirely. Paddy Pugh of English Heritage, who reviewed the scheme, recollects: "Our view was that, as long as the exterior and the best of the interior was kept and reused, there was an ideal opportunity to introduce modern design into the lightwells. We didn't feel the scheme was controversial and we were happy to approve it."

Even so, there is another problem with conflicting styles in Victoria House. Three contrasting styles can be identified within the developer's base building alone – the classical monumentalism of Long's original building, the high-tech minimalism of most of Alsop's conversion and the cheeky biomorphism of the four pods. The trouble is that this is a speculative developer's building leased to a collection of office tenants, which will all appoint their own interior designers to do their fit-out. As is already evident in the completed offices of recruitment consultant Michael Page, this process introduces yet more architectural styles to the building. When the building is fully let, the result is less likely to be the "dramatic counterpoint" envisaged by Alsop as a mishmash of styles ancient and modern, conventional and bizarre.

Such an outcome would be rather sad. Property developers and office companies have long been perfecting the efficient use of office space. Yet, since they spend most of their waking hours inside them, office workers have a right to demand office architecture that is also inspiring and stimulating. At Victoria House they are given office space that is both efficient and stimulating, and with no holds barred – unless the tenant's fit-out designers walk all over it.