Hello! to the Hermitage Museum

If buildings had shrinks, I wouldn't like to have Somerset House on my couch. It's a filing cabinet that thinks it's Nero's palace, an 18th-century office block by an architect in denial. Much of it has the charisma of a manila envelope, until you chance upon some leaping staircase or essay in French grace, the incongruous outpouring of the architect's repressed desire to be designing for Cleopatra, not the Offices of the Clerk of the Estreats and the Comptroller of the Pipe, nor the Hawkers and Peddlars and the Hackney Coach Offices. The large courtyard aspires to grandeur, but its surface fissures at the edges to reveal, in deep chasms admitting light to underground catacombs of bureaucracy, the building's true nature.

It has been taboo to say such things this year, since we have been asked to celebrate its opening up to public use, after years when the courtyard was a car park. We have been invited to share the same delusions as its original architect, Sir William Chambers, and see it as London's Louvre, politely ignoring the fact that the dank chasms in the courtyard stand exactly where, if this were a proper palace, there might be spacious colonnades.

This week, with the opening of the Hermitage Rooms, the remaking of Somerset House is complete, for the time being. And I don't begrudge those behind this transformation their fantasies, for they have made the place into something better than it has ever been in its life. The new fountain gives the courtyard - whose true destiny, really, was to be a car park - a certain magic. It's great that you can walk onto the river terrace from Waterloo Bridge, great that you can have a drink on the said terrace, and great to have sheer, car-less, open space in the centre of London. The idea of making it into an ice rink is simply brilliant.

The new Somerset House has also become a treasury of Trinkets of the Rich and Famous, namely the multi-millionaire Sir Arthur Gilbert, and the Empress Catherine the Great of Russia, a sliver of whose wealth has been loaned by the Hermitage Museum in St Petersburg.

The Gilbert Collection, which opened in the spring, shows his phantasmagoric collection of gold, silver, and micromosaics. The Hermitage Rooms are opening with the dinner service Catherine gave her lover Potemkin, after he gave her a cat; the miniature weapons she gave her grandchildren; the not-entirely-honest portrayals of Catherine and her husband as blissful newlyweds; her collection of obelisks more numerous even than her lovers; her chinoiserie; her wig of silver thread. Like the Gilbert Collection, the Hermitage Rooms are saturated with the desire of the rich and powerful to express wealth and domination in incey, wincey, twiddly little thingies.

With these installations Somerset House's analyst would get really busy, for the complex has acquired a whole new set of complexes. It's Hello!, with the Empress of all the Russias inviting us into her gracious home. It's Bond-Street-on-Thames, an art-dealer's dream world. It's like a certain sort of Kensington drawing room, where the accent of the chatelaine is so glass-breaking you wonder how all that crystal remains in tact.

It's also, in a very English way, Las Vegas. Although those responsible may wince at the word, the Hermitage Rooms are themed, as microcosms of the mother ship in St Petersburg. Russian and Armenian craftsmen have installed a decorative floor modelled on those in the original, along with silk curtains and replica chandeliers. The importunate paraphernalia of modern life, such as dehumidifiers, are hidden in boxes in an appropriate style. Computer screens provided to inform the public fold away into a glossy dining table for select dinners, thereby combining the twin pillars of the contemporary museum - education and corporate entertainment - in one natty piece of classically styled furniture.

As you enter, a plasma screen shows you live pictures of St Petersburg, the screen being given a classical housing. Between the rooms is a corridor with a painted ceiling and Etruscan-style lamps, again modelled on an original in Russia. The scale has oddly shrunk, in the transition from palace to office block. Imagine your own office, imagine that the domain of Dave in accounts had been converted into a shrine of gracious living, and you have some idea of the effect.

The Las Vegas connection is less fanciful than you might think, as the Hermitage is also opening a branch there, connected to another interesting psychological case with a waterside location, a hotel/casino that thinks it's Venice. It's all part of the Hermitage's need to raise hard currency, so it can do something about the rotting window frames that protect its art from the Russian elements, or the thick layers of grime that make is Poussins all but invisible. Some might say the Hermitage has learned something from the hookers who hang around its galleries on Saturday afternoons (thereby giving an interesting twist to the notion of a museum shop), but it's a reasonable way of resolving the mismatch between the museum's wealth of art and poverty of cash.

Some might also think that a Las Vegas-style micro-Hermitage in Somerset House is, along with the Madame Tussaud's effigy of Arthur Gilbert in the Gilbert Collection, a bad thing. It's certainly very peculiar to come across these Tourettic twitches of culture and kitsch at inappropriate moments in the great greyness of Somerset House - just as it is to find Chambers's wistful moments of elegance, or a recently installed gilded barge opposite the cloakroom - but there's no law that says all museums have to be about good taste.