Architecture: Act II: a bulky Roman strides in: Jonathan Glancey finds Glyndebourne's new opera house is well tuned for music, but needs time to mature into its country setting

No critic can pronounce on the true merits, or otherwise, of the new opera house at Glyndebourne until 28 May, when the season starts, the orchestra plays, the prima donna hits the high notes and the champagne corks pop. The first performance will be Stephen Metcalf's production of Mozart's Le Nozze di Figaro. Until then, all one can do is walk around the building, consider its Sussex setting and Roman scale, and see how the architect has decided to cope with the enormity of transforming wayward Glyndebourne into a full blown urbs-in-rure opera house.

The pounds 33m project to replace the original has been a considered risk for Sir George Christie, chairman of Glyndebourne. The risk is great, not because anyone expects the venture to fail structurally, acoustically or financially (the combination of Michael Hopkins as architect and Iain Mackintosh of Theatre Projects as design consultant is fairly bullet-proof), but because it fundamentally changes Glyndebourne's character.

That character had been changing over the years, and changed substantially in the latter half of the Eighties. The original 300- seat opera house, the sophisticated plaything of the late John Christie, father of Sir George, opened in 1934. The auditorium, entered through a Tudor marble fireplace, was no more and no less than a curious extension to a rambling country house on the Sussex Downs near Lewes. It was patronised by wealthy opera buffs.

Glyndebourne expanded over the next 50 years and although it was always as much a place to socialise as to listen to music it retained a delightfully dilettante atmosphere until the Eighties, when it went the way of all things Thatcher: commercial, corporate and more than a little flash. It became a grown-up business for wealthy grown-ups and lost some of its special magic.

Hopkins's sobre, red-brick opera house reflects the new seriousness of spirit. This makes sense. Long gone are the days when Baroque orchestras - perhaps no more than 20 players - scraped away at the side of the stage, accompanying castrati and a chattering audience. Opera has grown in every way in Britain (especially since Pavarotti, 'Nessun Dorma' and the 1990 World Cup) and the physical growth of Glyndebourne reflects its brasher and more commercial status.

But no matter how sophisticated the patron and architect, can a 1,200-seat opera house really fit successfully into the folds of the Downs? As it has developed over the past 200 years, the opera house is essentially an urban building type. In the dense weave of the city, its structural excesses and functional excrescences - notably the fly-tower - have been hidden from sight or lost among the clutter of other buildings. At Glyndebourne, Hopkins has not really tried to hide the bulk of the new building.

From a distance its great arcaded form has something of the look of an ancient Roman amphitheatre. Its roofscape is dominated by a chunky and severely trussed lead-covered fly-tower. It is not a thing of beauty. Eventually, ivy will creep across its naked bulk. Hopefully, the bright red bricks of the great horseshoe-shaped auditorium, from which the fly-tower mushrooms, will weather into more mellow colours and textures.

Close up, the building meets its rambling soft-brick and stone neighbours through an arcaded ambulatory (like the Roman Colosseum) and a through a soft-tech tented foyer (much like the tents covering the Mound Stand at Lords, also by Hopkins). It is not the gentlest of meetings, and at no point can one help thinking that the architects decided somewhere along the line that there was no point trying to hide the scale, ambition and function of this new building. But, then, going to Glyndebourne will no longer be the experience it was five years ago.

Inside the auditorium, the project makes near-perfect sense. Mackintosh and Hopkins have produced a big but intimate space. The new house is, at 60ft, twice as high as the old, but at 84ft it is 10ft shallower than its predecessor. This means that everyone is seated close to the stage. The three tiers of seating snake around the house but are free from decorative frills. Although clean-cut, the interior is warm and welcoming - less Modern than elemental. It has as near as one can get to a timeless quality. Hopkins and his team have achieved this effect through simple means: the pine used throughout the auditorium (some of it reclaimed and 100 years old), plain colours, brick walls and basic light fittings.

You might notice, too - not that it will matter to your enjoyment of the performance - that the exterior form of the building is mirrored inside the auditorium. This nice conceit helps to give the building that special elemental quality: the interior appears to have been hewn from those red Roman-style bricks containing it. Normally you expect the auditorium of an opera house to bear little or no relation to the exterior or facade of the building, but here the relationship is more or less direct.

Hopkins sees the interior as a musical instrument and it should prove to be a finely tuned one: the acoustics have been engineered by the hugely experienced Derek Sugden of Arup Associates. The sound, with a reverberation time of 1.4 seconds, should be much warmer than that of the old opera house, and much freer, too, as each member of the audience has been allocated twice the volume of space as before.

So, as an opera house, the new Glyndebourne should prove to be extremely rewarding. As an urban building making a muscular foray into the Sussex countryside, its virtues are questionable. But perhaps we are all too sensitive; every day local authority planners give permission for vile 'vernacular' superstores, executive housing estates and other buildings apparently designed to suburbanise and otherwise foul the countryside.

Hopkins's building has the monumental qualities of a great agricultural or monastic building that enlivens its setting by not trying too hard to please, but is decisive, intelligently thought through and well built. Enjoy the opera from the end of May; give the building a few years to mature.

(Photographs omitted)