Cambridge credo.

A new theatre for Emmanuel College in Cambridge displays an astutely crafted material richness combined with a highly innovative structure.

This is a typical Hopkins building. More than that it is a kind of condensed summary statement of all the formal preoccupations, compositional habits, structural principles and material preferences of Hopkins' mature style. Most obviously, it is a miniature Glyndebourne Opera House (AR June 1994): a free-standing building, oval in plan, basically symmetrical about a longitudinal axis, three storeys high and with a performance space at its heart. And the similarities don't end there. It has a colonnade around the ground floor built of loadbearing masonry, it stands close to a collection of venerable buildings and it has a roof structure of combined timber and steel trusses. In fact, almost every feature of this building has a precedent somewhere in the recent Hopkins oeuvre. The layered lead roof, for example, can be traced back via the Inland Revenue building at Nottingham (AR May 1995) and Glyndebourne to its origin in the David Melior factory. The circular fire escape stair tower, of glass blocks in a steel frame, comes via Nottingham, as does the technique of steel-tensioned masonry (of which more later). One would say that the building was a montage of Hopkins motifs, were it not such a unified, monolithic form - more like a beautifully crafted piece of furniture than a building.

There are also indirect historical precedents. Viewing the building over the railings from Emmanuel Street, its resemblance to the north end of Wren's Sheldonian Theatre in Oxford is striking. The function, too, is similar, though in the Sheldonian, the auditorium seating occupies the curved end of the building, like a Roman theatre, whereas in the Queen's Building, the arrangement is reversed, with straight rows of steeply raked seating facing into the curve. The echo of the Sheldonian is surely not coincidental, given the proximity of the Wren Chapel, the Baroque, porticoed facade of which forms the centrepiece of the college's main quadrangle called Front Court. The chapel is built of Ketton limestone, and this is also the dominant material of the Queen's Building.

The auditorium seats 170 people, for lectures, college ceremonies and chamber concerts. It is a simple, two-storey D-shaped space, surrounded by a narrow gallery with large windows allowing views of the sky, trees and neighbouring buildings. Only three materials are apparent: stainless steel for the tensile members of the exposed roof trusses, stone for the walls, and American White Oak for everything else. Most of the other spaces in the building are single-storey, flat-floored versions of the auditorium. They are designated as common-rooms and reception rooms, but are also potentially little theatres.

Though this is a free-standing building - unusual in Cambridge - it is nevertheless decently scaled and fits comfortably into its surroundings, creating a subtle, formal link with North Court on the other side of Emmanuel Street. North Court is reached via a pedestrian underpass, connected by an arcade to the corner of New Court. Hopkins has refaced this arcade in stone and taken a footpath from one of its arches right through the new building to a gate in the wall of the Fellows' Garden. The entrance to the new building is placed where the footpath crosses the longitudinal axis and is therefore buried in the middle of the plan. There is no canopy or portico of any kind; nothing to disturb the completely smooth, flush face of the monolithic form.

This smoothness is the building's most curious feature, and for once it seems to be unprecedented in Hopkins' work. Curious, though, because this is a massive stone structure, it is also built as a frame of piers and flat arches infilled with non-loadbearing panels. Most architects would have expressed this structural distinction by setting the panels back from the frame, but Hopkins keeps them flush, framing them with narrow incised grooves and changing the bonding of the stone. Ever since he acknowledged the influence of Guarini's Palazzo Carignano on his design for Bracken House, it has seemed legitimate to cite Renaissance precedents for Hopkins' buildings. Here, the precedent might be Alberti's Palazzo Rucellai, a three-storey flush facade on which the Classical orders are 'drawn' in narrow grooves.

As at Glyndebourne and Nottingham, the masonry is loadbearing. This is part of the Hopkins credo: structural honesty and truth to materials at all costs. But the 400 mm thick walls are not capable of resisting the outward thrust of the floors and roof without some kind of reinforcement. They could be buttressed, but that would spoil the simplicity of form. Instead, tensioned stainless steel rods inside the piers counteract the outward thrust with a strong vertical compressive force corresponding to the force exerted by a pinnacle or buttress. This is essentially the same technique that Hokpins used at Nottingham, except that there, the piers were prefabricated and here they were made on site. The rods are anchored at each level in heavily reinforced precast concrete 'kneeling blocks' and are accessible via stainless steel tubes that register on the facade like dowels in a wooden chest. But it is not enough just to strengthen the piers. The arches at the ends of the building are curved in plan and therefore have a tendency to burst outwards. (Wren, incidentally, avoided this problem at the Sheldonian by making the wall faceted rather than curved.) The wall, therefore has to be held in by the concrete floors, by the gallery of the auditorium and, at eaves level, by the precast concrete gutter/cornice that acts like the restraining chains in a Renaissance or Baroque dome.

There are many ambiguities, many 'both-ands', in this building: it is austere, but sumptuous, structurally honest but supported by hidden steel members, massive but delicate, traditional in concept but modern in execution, monolithic but permeable. This is its richness, not just a richness of materials and craftsmanship, but a richness of ideas and imagination.