Grand opera; Oslo's new opera house is both a dramatic and topographic addition to the city's waterfront.

One of the most difficult projects of our time must be to create a monument to one's own country. This was a chief requirement of the brief for the Oslo opera house, so when the Norwegian/ American practice Snohetta won the anonymous international competition in 2000, the architects had dealt with a problem that has puzzled the profession for a century, one which they had to resolve with one of the most complex building types known outside medicine. But they were undaunted; after all, they were responsible for the amazing Great Library of Alexandria (AR September 2001), where the huge sun-like disc of the reading room roof dominates the smooth waters of the harbour and provides an unforgettable image of the renaissance of the city and its scholarship.

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In Oslo, the relationship of building and sea was not dissimilar, but here, the powerful image is of Norwegian topography. A shining white hill, a snow-covered mountain or iceberg, rises from the end of the city's east harbour (a site chosen after much time-consuming argument). The area used to be grim and run-down where shipping, railway and roads came together in an early twentieth-century industrial mess. But Norway is a very rich country now and the great national fund that invests the state's revenues from the oil industry (which has paid for the opera house) has been upgrading the railways for many years. The white and yellow central station is now cleaned and transformed for the twenty-first century and is beginning to drag the whole area up with it. Even so, a busy and polluting major road still runs along the edge of the bay, cutting off the site of the opera house from the city.

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At the moment, the bay clangs with the racket of pile drivers creating a tunnel under the sea to remove the noxious road. But you have to approach the site from the station along a tatty temporary steel bridge over the motorway. As you come to the seaward side of the bridge, it becomes clear that something impressive is going on. A gaggle of people clusters round the end, marvelling over the great white expanses of Carrara marble that clad the new building in an artificial landscape of precipices, slopes and plateaux. Even on dull days, the new topography swarms with people wandering around, beside and on top of the foyer and auditoria. Freedom to walk anywhere is one of the engrained liberties of Norwegian citizens - for instance, at any time of day or night you can stroll through the unenclosed park that surrounds the king's palace right up to the royal residence.

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The opera house is in a sense a monument to a free country, echoing the line of Ekeberg, the long, low mountain that terminates Oslo to the east. The artificial white landscape celebrates the relationship between land and people in much the way that Mitchell/Giurgola's Australian Parliament in Canberra with its roof over which the public can walk symbolises relationships between legislators, the electorate and their country. Frank Gehry's Disney Concert Hall in Los Angeles is another place that encourages visitors to wander on its roof, but there routes are clearly defined; on top of the Oslo opera, there are no obvious paths, just a wide generous upward spiral from sea to the piazza on top of the foyer on which you can stroll as you wish, just as you can in the natural landscape.

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Approaching the new building, you cross a sort of moat and come to the white artificial island. It rises smoothly, sloping from the sea on granite slabs that lock into marble ones. The surface you walk on is not uniformly smooth, but, evolved with a team of artists, it is textured in different ways to provide grip for shoes, three-dimensionally modelled and creased with strange shallow diagonal crevasses that turn out to be a way of deflecting run-off and collecting dirt (though, for all the people who have wandered over the roof, there is no litter, and no sign of graffiti - Norwegians are civilised).The chunky surface modelling, which (to avoid twisted ankles) would never be allowed in Britain, reveals that the marble is in thick slabs, far too robust to start bending and dishing like the Carrara panels Aalto used on the Finlandia concert hall in Helsinki.

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On the way to the top plateau, you occasionally come to parts of the slopes that are not totally overrun with folk, and the few people around look as if they are in a set that curiously combines the awesome elemental force of Caspar David Friedrich's Sea of Ice with the strange isolated loneliness of De Chirico's towncapes. The mixture of urbanity and arctic waste is created by the emergence of large orthogonal objects like the flytower on the upper levels: these are clad in polished aluminium patterned with small circular bumps and dimples to reflect and scatter light. Resulting patterns look rather like Braille, but are in fact derived from the punched cards that drive Jacquard looms.

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If, instead of climbing the slopes to the top platform, you make towards the entrance doors, the entrance doors, a slot in the granite north wall, the glass walls of the foyer loom overhead. I am no fan of inclined columns, because they are so often architecturally no more than self-indulgent, coarse and expensive gestures but am more or less convinced that the ones in the opera foyer are necessary, because the loads of the roof had to be taken down to a limited number of bearing points. Once inside, the effect is dramatic, with the bulging curve of the timber-clad wall that forms the back of the main auditorium glimpsed through the huge white concrete columns. The curved wall is clad in vertical strips of oak darkened by being exposed to ammonia fumes and made in differing sections to diffuse sound. In fact, the wall is double to accommodate bars and galleries serving the balconies in the auditorium. Long generous horizontal slots in the outer wall offer views (particularly dramatic at night) across the foyer and its huge glass walls to the islands of the fjord, the castle, the station and the east end of the nineteenth-century city (now much restored).

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The grand stairs to the galleries are in oak with beautifully crafted handrails curving round and upwards in three planes almost as if they have been carved out of solid wood (they were made by shipwrights). Fine craftsmanship in wood continues in the auditorium. Traditionally horse-shoe shaped in plan, its intimate dark space (which accommodates some 1400 people) has three balconies with fronts carved out of oak staves glued solid and shaped to enhance sound by reflection down to the stalls. Acoustics can be finely adjusted by altering hangings on the back wall. When I was there at a rehearsal, they were clearly sparkling but resonant. Reverberation time is extended by increasing the volume with a services gallery above the top balcony that extends over the perimeter walls to give a squat T-shaped cross section. Lighting is provided by LED fittings in floor and ceiling and by a great lens-like sound reflecting chandelier made with 5800 hand-cast glass crystals lit with 800 LEDs.

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Modern technology is never obtrusive but is all-pervasive. For instance, instead of sur-titles over the proscenium, each seat has a small screen set into the back of the one in front on which the libretto can be displayed in several languages. The proscenium can be changed in width with mobile towers and the orchestra pit altered in topography with three lifts. Behind the proscenium, stage machinery is as elaborate as can be obtained and occupies an area much bigger than the auditorium, so complete sets can be manoeuvred from left to right, back and forward, up and down.

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Stage and foyer areas and the auditoria (there is a secondary auditorium with flexible configuration that can accommodate up to 400 people, and small black box for occasional use), occupy only half the whole building. In all, the opera house has a staff of almost 600 of many trades, only a small proportion of whom work in the front of house. Beyond a corridor that runs south to north, acting as a sort of green room behind the stage areas, is what the architects call the 'factory' that extends the rectilinear overall plan eastwards. Here are all the dressing and changing rooms, the offices, staff canteens, practice and rehearsal rooms and the workshops that produce everything from scenery to wigs. The resident company offers both opera and ballet, which means that specialised rehearsal and dressing rooms are needed for each, so some 1000 individual spaces have been created.

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In the backstage area, finishes and fittings are much more utilitarian than in the front of house, yet they never descend to the banal: even door handles were designed by the architects. Many of the dressing rooms overlook an internal courtyard, the form of which is projected upwards to make one of the silver-clad forms in the ice landscape above. These walls prevent people looking down into the court, but elsewhere the public is positively encouraged to look at life in the factory through the large windows of the workshops and the glass walls of the ballet rehearsal studio high up at the east end of the whole complex.

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It will take time for the opera house to settle in to the fabric of the city. First, the harbour tunnel must be finished, then the islet to the south of the opera promontory will be converted into a green counterpart of the great white artificial landscape. Gradually, the area between opera house and station will be developed as part of the urban matrix, though no one seems to have a very clear idea of what to do yet. One thing is obvious though: Norway has a new national monument.

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