Glyndebourne.

Glyndebourne Opera, as an institution, is full of ambiguities and contradictions. it brings a cosmopolitan, urban art form to a local, rural setting; it is both formal and informal, both sumptuous and spartan, both professional and amateur. It serves equally the status seeker, out to impress his business clients, and the genuine opera lover for whom the art is everything. Dinner jackets are compulsory, but the dinner is likely to be a picnic on the grass. The gimcrack stalls of a village fete would not look out of place in this setting, though you might have paid 100[pounds] for your seat in the auditorium. And in the old days it was probably a re-used cinema seat. What was going on the stage and in the pit may have scaled the heights of musical and dramatic art, but the acoustics were poor, the seats uncomfortable and the ventilation inadequate. The main task for the architects was to increase the size of the auditorium and improve the physical conditions. But they also had to preserve the ambiguities and contradictions, which are part of the Glyndebourne tradition. A too rational approach might have destroyed them, breaking the tradition and dispelling the magic.

It should come as no surprise, therefore, that the first design decision was taken as much on sentimental as on rational grounds. The new building occupies the same ground as the old, behind the house, though the backstage and front-of-house areas have swapped positions. From the road, the flytower and roof peer over the shoulder of the house, as if hovering reticently in the background. Reputation and exclusivity mean that the building has no need to advertise its presence. Despite its size, it retains an essentially private character. During the season, a passer-by observing the comings and goings might assume that some kind of lavish party was taking place, perhaps a society wedding. Architecturally, the plain lead roofing and the exposed steel trusses of the new building seem to have little to do with the gables, dormers, chimneys and mullioned windows of the old house. It might be some new kind of marquee erected temporarily on the lawn.

An open yard between the house and the opera house serves as a turning circle and dropping-off point for the opera goers. Here the marquee impression is reinforced by the casual relationship between the two buildings. The wing of the old house known as the Organ Room projects towards the curved flank of the new building, almost touching it, but with nothing to reconcile the contrasting forms. There is no portico or front door, just an open ground floor arcade leading off to the right and a tree in the middle of the yard to drive around. The arrangement works perfectly well, but nevertheless has a provisional or temporary character; deliberately so, no doubt. As the main entrance to a 1200 seat opera house, it is just about as informal and understated as it could be. One doesn't so much enter the building as filter into it, both from the drop off point and from the gardens on the other side. The foyer proper turns out to be semi-external space between the main building and the old backstage block, part of which has been converted into a bar. This space is covered by the kind of fabric canopy that has become the Hopkins' trademark. The canopy is placed symmetrically on the longitudinal axis of the building, though the user is hardly aware of this fact. It seems to be nothing more than informal interlude in the perambulation around the building. But this, of course, makes perfect sense, given the peculiar pattern of use of the building. The Glyndebourne season lasts for only six weeks in the summer. There is therefore no need to enclose the foyer spaces. Open arcades and a fabric canopy provide sufficient shelter from the rain and the foyer is free to expand into the gardens.

Some architectural equivalents of those Glyndebourne ambiguities and contradictions are already apparent. Like a marquee, the building is a simple, unified, oval volume. It is formal in the sense that it conforms to a strict axial symmetry, and yet it is placed informally, relating in an ad hoc way to the motley collection of buildings around it. The formality implies permanence, the informality, temporariness. In this rural valley, with its rambling Neo Elizabethan house, one might have expected a more fragmented, articulated form, such as the one proposed by James Stirling in his as yet unpublished competition scheme. Instead, the Hopkins opted for unity and massiveness. True, the building is dug into the slope of the site to reduce its apparent bulk, it is surrounded by smaller buildings and its roof line is broken by the flytower and the raised circular roof over the auditorium. But still, viewed from the car park on the hillside, it is a big object in the landscape. Only the new rehearsal stage, carved out of the rising ground at the north end of the site and linked to one side of the backstage area by a short umbilical corridor, is excluded from the enveloping oval. But if a future phase is built, even this single excrescence will be matched by a second stage on the other side to reestablish the symmetry.

Typically, the Hopkins' justification for the single envelope approach is logical and practical. Auditorium, flytower, stage, backstage and wings must all relate to a central focal point of the proscenium arch in a basically symmetrical arrangement. It would be perverse to go against this traditional pattern. Smaller spaces such as dressing rooms, green rooms, studios, offices, staircases and lavatories should all relate as directly as possible to this main event. What could be simpler than to string them out along a corridor, stack them up to three storeys and wrap them around the building? This arrangement offers at least three distinct advantages. First it imparts scale and proportion to what would otherwise be the blank walls of the auditorium and backstage areas, populating them and giving them a human face. Second, it improves the acoustic insulation between the auditorium and the aircraft-infested Sussex skies. Third, and most important, the simplicity of the layout makes it easy to orientate yourself in the building. Whether you are a singer due on stage for the next scene or a member of the audience returning to your seat after the long interval, a glance through a window or at the curve of the corridor wall is enough to tell you which way to go. There are no panic-inducing, windowless rabbit warrens, typical of the backstage (or even the main public areas) of so many theatres. The auditorium is circular in plan, so it is natural for the front-of-house end of the building to be rounded. At the other end, a semicircular plan provides the maximum backstage area for the minimum length of enclosing wall. It all adds up to an economical plan of great simplicity and clarity.

The heart of the plan, of course, is the auditorium. Its seating plan, with a gently raked bank of stalls and three horseshoe-shaped balconies, is thoroughly traditional. According to the architects, however, the form was arrived at more by a rational analysis of distances and sight lines than by a close study of architectural precedents. The aim was to preserve the 'intimacy' that was such an important characteristic of the original 800 seat auditorium. That aim has been achieved, apparently effortlessly. The new auditorium seats 1200, but looks much smaller. At the furthest point, the back wall is actually two metres closer to the proscenium than that of the old auditorium. It seems that keeping as many of the seats as possible as close as possible to the stage gives rise naturally to the horseshoe balcony form. Tradition and rationality are not always at odds. But there was another, equally important influence on the form -- acoustics. In the early stages of development of the design, the acoustics consultant, Derek Sugden, had spoken of the auditorium as a kind of inhabited musical instrument. According to Sugden, the ideal auditorium acoustically would be a cylinder ringed by single rows of seats on balconies so that all members of the audience would be close to the 'sounding board' of the cylinder wall. This idea, rather than precedents such as the Neo-Classical Munich National Theatre, is the origin of the circular auditorium plan, replacing the fan-shaped auditorium of the original design proposals.

But perhaps the musical instrument analogy has had an influence too on the visual character of the auditorium. Wood, mostly reclaimed pitch pine, is the dominant material. The wooden balcony fronts are curved, in section as well as plan. They seem to belong to the same category of man-made-object as traditional musical instruments. Like the curved body of a cello or a lute, their shape has a precise acoustic function. Each has a slightly different profile to reflect the sound from the stage in diffuse patterns around the great volume of the auditorium.

And here is another of those subtle ambiguities. One expects an opera house auditorium to be richly decorated in gilt and plush. It is part of the opera tradition. But the Hopkins are conforming to a different, more local tradition. At Glyhdebourne, the florid, artificial nature of opera as an art form has traditionally been contrasted with the austerity of the surroundings, as if the performance were taking place in something not too far removed from a village hall. The new auditorium, though beautifully crafted, is therefore plain. Floors, walls, seats and balcony fronts are all of natural waxed timber, with no opaque surface coatings of any kind -- no paint, no wallpaper, no carpet, and certainly no gilt or plush. Apart from the occasional glimpse of red brickwork at the top balcony level, the only other visible material in the -- auditorium is the concrete of the balcony soffits and ceiling. Again there is no plaster or paint, but the profiles of these precast units are so precise, and their finish so fine textured, that they seem if anything lighter and more delicate than the timber elements they support. The absence of any decoration and the severely limited palette of materials gives the whole space a kind of purposeful purity, as if its form, like that of a musical instrument, had been honed and simplified over a long period of evolution.

The auditorium has a composite structure of concrete and steel within a loadbearing brick drum. Loadbearing brick is the major structural theme of the whole building. Here is yet another ambiguity, for if, conceptually, this is a temporary marquee, then it is a very solid one indeed. Though its unified form is very different from that of the house it hides behind, it is nevertheless built in a matching material: a red, hand-made Hampshire brick, of non-standard imperial size, laid in a modified English bond. The two storey arcades that surround the building, sometimes infilled by windows, sometimes left open, have all the rational simplicity of a Roman aqueduct. They are massive and monolithic. Even expansion joints have been banished by the use of traditional, flexible, lime mortar. The brickwork is all fair face, inside and out. Floors are of precast concrete, with the ends of the main beams showing through on the outer face of the piers. Between the beams, the soffits of the concrete panels are exposed, with no ceilings or surface coatings. Similar in texture and profile to the balcony soffits in the auditorium, they contrast sharply with the rugged brickwork of the arcades. On the top floor the structural system changes. The concrete floor beams project to support round steel columns which in turn support tapered timber beams (steel in the backstage areas). This is perfectly logical, both in planning and structural terms. The spaces at this level require a little more width and there is only the relatively light roof to support. But Michael Hopkins cheerfully admits that this is also a formal device, a satisfying way to cap the facade. He is even prepared to admit the influence of Italian Renaissance architecture -- specifically the jettied top storeys of Michelozzo's fortified villas for the Medici in the Mugello, north of Florence. Such overt historical references would once have been anathema to Hopkins. Now they are readily incorporated into his expanding repertoire.

But, as in all the Hopkins' buildings, structural honesty a nd truth to materials remain the guiding principles. If the context seems to call for traditional brick, then the brick must be used in the traditional way. Brickwork evolved as a loadbearing material, so the flat arches, cleverly integrated with the tapering piers, are real arches, not just the cladding to a hidden frame.

Where does this form come from? Hopkins admits the influence of Louis Kahn, particularly the Exeter Library, but it is easier to trace the origin of the form in his own work. The Mound Stand at Lord's was the turning point in the development of his architecture. Before, in the High-Tech phase of his career, he had always shunned traditional, heavy, site-based building technologies such as loadbearing brick. But at Lord's (another summer building to house a traditional English institution) he was obliged to confront traditional building technology almost for the first time. It made good practical sense to preserve and extend the old loadbearing brick arcade that supports the stand. This turned out to be an enjoyable exercise and it opened .up a new direction for the development of the Hopkins' architecture just when the High-Tech style seemed to be running out of steam. Ever since then they have followed this new path vigorously and inventively. Glyndebourne is the latest version of the loadbearing brick theme. The Inland Revenue headquarters at Nottingham, soon to be completed, will take the theme to the next stage of development.

It is possible to trace the origins of other features of the opera house in previous Hopkins' buildings. Most obviously the shallow pitched roof of lead covered panels, particularly the raised section over the circular auditorium, is borrowed from the little round cutlery factory for David Mellor at Hathersage. The flytower combines exposed steel trusses from earlier buildings with lead cladding from the side walls of the Mellor off ices and showrooms in London. And perhaps it is even possible to perceive in this big, tent-like roof, an echo of the fabric enclosure of the original Schlumberger research laboratories building in Cambridge. At the back and sides of the building, the radiating lattice steel trusses are supported in the centre by suspension rods hanging from the flytower, its four columns corresponding to the four tent poles of the common type of circus big top. We are back with the persistent analogy of the marquee on the lawn -- a particularly appropriate image for a building to house one of the social institutions that grace an English summer.

Formal in its symmetry but informal in its siting, tent-like in its form but massive in its construction, contextual in its colour and texture but alien in its character, austere in its expression but rich in its material quality: the ambiguities and contradictions pile up and the layers of meaning multiply.

A conventional, stereotyped building might have killed off the Glyndebourne tradition. The Hopkins' building has just enough studied eccentricity to ensure that it continues to survive and flourish.