Sculptural street.

Daringly inventive and materially refined, yet mindful of tradition, Henning Larsen's new set of galleries in the nineteenth-century Ny Carlsberg Glyptotek epitomises the best of recent contemporary architecture in Copenhagen.

The Ny Carlsberg Glyptotek is one of the wonders of Copenhagen. In the ninetieth century, the Carlsberg beer company was one of the most successful in Denmark. By a strange combination of commercial acumen, filial hatred and extraordinary amounts of money, the splendid museum to the south of the Tivoli Garden, is one of the wonders of the world. The new (Ny) foundation, which sponsors art, was made as a rebellion by Carl Jacobsen, the son of the founder of the firm, against his father's fund that had been set up to foster science.(1) A glyptotheca is a place for keeping sculpture and Jacobsen must have brought the stuff from Egypt, Italy and Greece by the boatload to make one of the most extensive collections in northern Europe (the more impressive because it was assembled late in the nineteenth century, after the French had looted and the British and Germans acquired so many of the treasures of the ancient world).

The building, off H. C. Andersens Boulevard (the main east-west artery leading to one of the two bridges over the harbour), is in two parts. The front, by Vilhelm Dahlerup, was opened in 1897 and is a cheery exercise in free classicism with a blind arcade of free-standing granite columns against polychromatic brickwork. This part of the museum focuses on the delightful winter garden where tropical palms and creepers flourish under a huge dome of cast iron and glass.

The back part of the museum is altogether more austere. It was built between 1901 and 1906 by Hack Kampmann, who was then moving towards his late (and very daunting) neo-classical style which makes the Copenhagen police headquarters (finished in 1924) look like a proto-Nazi building. But at the Glyptotek, he combined austerity with a sense of fun in the details; the whole grand brick and granite composition is balanced under a stepped pyramidal tribute to the mausoleum at Halicarnassus.

In the Kampmann part, there are two courtyards, intended to bring light down into the galleries that surround them. When the foundation decided to make a new set of galleries to house its collection of French nineteenth-century paintings, it decided to build in one of these courts. (The need was great: if Carl Jacobsen had been sending ships to the Mediterranean in the late nineteenth century, he must have been ordering pantechnicons from Paris in the early years of this one, and the galleries of the Glyptotek were not suitable for displaying them.)

Henning Larsen was asked to make the new part and his insertion rival in quality Norman Foster's work at the London Royal Academy (AR December 1991). Indeed, in many ways Larsen's work surpasses Foster's, because there was a need to make a new building within the Glyptotek complex, rather than connecting and enhancing parts of the existing Academy.

Larsen's building is an awesome white monolith that rises (mysteriously slightly tapering) within the courtyard. It is surrounded by a stepped pale grey Carrara marble ramp with an almost urban scale (Larsen calls it a 'stepped street') that gently(2) leads you up to the three levels of galleries which are entered through the few apertures in the flanks of the white centrepiece. The ramp is flooded with light from simple glazed roofs that span from the monolith to Hack Kampmann's surrounding walls. Larsen thinks of the route as a potential sculpture gallery (the width is ample enough to allow this) but as yet, the opportunity has not been embraced. The rectangular spiral of the ramp ends at ordinary stairs that lead to the terrace, a grey granite platform from which to view the roofs of Copenhagen and the slightly threatening presence of Kampmann's dark stone pyramid. Yet from outside the complex, the new work is entirely invisible.

It is approached from Dahlerup's winter garden along an undemonstrative glazed arcade which bifurcates into the parallel paths of the ramp and the chequered route to the antique galleries. The gleaming white monolith rises with breathtaking force and solidity, its apparently impervious white polished plaster walls gently articulated in abstracted pilasters (the relief is no more than about half an inch). The composition of the walls is topped by a frieze of apertures which hide ventilation grilles.

Stairs next to the lowest entrance to the treasure chest can cut short the long gentle route to the roof (and there is of course a lift). All the details have austerity and clarity in keeping with the exterior of the treasure chest: the handrails of the stairs for instance are simple thin rectangular bronze sections supported on similar dark steel balusters which have glass panels between them. The whole of this outer part is almost as austere as Kampmann at his most fierce: the experiences are Sublime in Burke's sense (the aesthetic sense evocative of death and terror), rather than Beautiful (evocative of sex and sensual pleasure).

Inside, the treasure chest is a warren of small rooms which serve excellently to set off their Beautiful contents. A good gallery stands back and allows you to study the pictures on the walls without itself trying to compete with them. This is exactly what these interlocking spaces do. They are painted in rather muted colours which allow the paintings to speak from their frames: Pompeian red, slate blue and grey, and a rather dusky raw Siena. They have a big white cornice, and a stone edge runs round the oak floors; openings are framed in white marble. The whole effect is grand domestic, and the pictures can be enjoyed as they were painted to be seen: at eye-level and rather close to.

The collection starts with the rooms at entrance level where pictures from the Barbizon School to Manet are hung. In the middle floor are Impressionists and a great collection of Degas bronzes. The top floor (which has rather subdued daylight as well as the artificial illumination used in the lower galleries) houses the Post-Impressionists-particularly the museum's outstanding array of Gauguins, the climax of the entire magnificent collection.

I was there a month ago and everything worked magnificently, but I have doubts about what might happen at the height of the tourist season. The rooms are absolutely right for the artworks they contain, but can they cope with jostling crowds in the height of summer? A queuing system may have to be set up so that the place does not get too congested.

The new piece has been slipped into its court with such grace that the Kampmann galleries that surround it have not been compromised in any way. In particular, the vaulted spaces to the north-east must receive as much light as they ever did: Kampmann's gentle and delicious Arts and Crafts plaster reliefs of daffodils and lillies of the valley, roses and honeysuckle are shown as he intended them to be, but with perhaps a little more clarity because of the horizontal component of light reflected off the polished face of the treasure house.

The Glyptotek is such a complex building, with the plans of Dahlerup, Kampmann and now Larsen interlocking in a sometimes confusing three-dimensional maze, that it is easy to overlook the new Egyptian galleries on the ground and first floors which are reached from the old Egyptian collections. Here, Larsen has created a stepped tunnel to the mummies which is intended to evoke the claustrophobic experience of entering a burial chamber. In the galleries, everything is austere with none of the touches (like oak flooring) which give the French spaces a feeling of grand domesticity. In the Egyptian department, the powerful artefacts are dramatically spot-lit against the dark walls between plain stone floors and white ceilings.

In a sense, the new work at the Glyptotek sums up in microcosm much of what is good about contemporary Copenhagen architecture. It daringly and inventively inserts a major chunk of new building into a well loved setting without compromising either; it provides a new way of looking at the world, and it uses contemporary science and craftsmanship to the full, as did Dahlerup and Kampmann in their time.

As soon as the Ny Carlsberg foundation can find the cash, it should ask Larsen to get on with converting the courtyard at the other side of Kampmann's plan.