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For 30 years, until this summer, the compact central campus of the University of East Anglia in Norwich remained untouched by architects. And with good reason. The campus must be Britain鈥檚 largest, most coherent composition of modernist buildings. As created by Sir Denys Lasdun, architect of the National Theatre, this is inhabited sculpture on a heroic scale, with multistorey blocks shaped as stepped ziggurats and ribbed cubes. Entirely executed in massive, raw concrete, their impact is overpowering.

The pressure to extend the campus, however, could not be more basic: over the 40-year life of the university, its student population has more than doubled from 6000 to 13,500. The impetus to build has until now resulted in architects tiptoeing around Lasdun, with large academic buildings and complexes of student residences stretching into the distance.

Unfortunately, when it came to extending the library 鈥� a project the university has just completed 鈥� there was a problem. The library, being the nerve centre of the university, was located at the heart of the original design, and any extension would risk conflicting with Lasdun鈥檚 consistent composition. Little wonder, then, that planning the proposed extension stretched over 19 years and entailed several abortive schemes by more than one architect.

Fortunately, Lasdun had made provision for the library to expand. There was spare ground between the existing building and the snaking science faculty, which forms one side of the campus. Though this site was attractively landscaped, it was effectively a back-of-house site separate from the mature parkland that forms the green hollow core of the campus. Therefore it was the ideal place for architect Shepheard Epstein Hunter to make the first latter-day intrusion onto Lasdun鈥檚 patch.

Shepheard Epstein Hunter brought to the project a double pedigree 鈥� it had designed the contemporary university campus of Lancaster and was in the middle of refurbishing Lasdun鈥檚 other library at London鈥檚 Institute of Advanced Legal Studies. In essence, its scheme at Norwich was perfectly straightforward, as it entailed extending all five floors of the existing building on the same levels. But the Lasdun factor added complications. The first of these was that the original buildings had entered the hallowed realm of architectural heritage, having been listed grade II in 2003. They were therefore going to be ferociously guarded by English Heritage and the Twentieth Century Society. Pulling in the opposite direction was the fact that architectural style had moved on from the massive gridded concrete structures that Lasdun had adopted 40 years ago. Libraries, too, have metamorphosed from dusty bookstacks and desks to something more akin to internet cafes. The project therefore called for a great deal of architectural finesse in relating old and new.

The exterior of the extension demonstrates this finesse in abundance. The architectural language of Lasdun鈥檚 building is a grid of precast concrete columns and spandrel panels, with the windows inset in two vertical layers to add modelling. Shepheard Epstein Hunter has taken the obvious route of adopting the same grid of structural bays and storey heights, which was largely helped by the practical decision to run the original floors through into the extension. But at the same time it has added to the compositional problem by converting the original structural system of heavy loadbearing precast concrete panels into the more contemporary system of a suspended lightweight window wall.

The conflict has been resolved both practically and aesthetically by adding an outer layer of narrow aluminium louvres. As well as providing solar shading to the window wall, the louvres follow through Lasdun鈥檚 precast concrete spandrel panels at the same horizontal levels and on the same vertical building line. As part of this composition, the louvres are supported on vertical fins that project out from the window wall on the same grid as Lasdun鈥檚 concrete columns. In this way, the double-layered facade offers similar modelling to that of Lasdun鈥檚 building. What is more, the fins are faced in cedar boarding in true contemporary style, though the intention here is that the golden colour will slowly mellow to a silvery grey to match Lasdun鈥檚 weathered concrete.

Internally, the extension combines conventional and modern library spaces. The conventional areas are largely taken up by bookstacks, with a continuous desk lining the perimeter glazed wall along either side. The IT study room is a double-height space with a mezzanine floor taking up half the footprint.

The beauty of the Shepheard Epstein Hunter鈥檚 extension is that all the study areas, both the traditional reading rooms and the modern computer section, are light and airy, bounded as they are by clear-glazed curtain walls. In the Lasdun building, by contrast, many of the reading areas are gloomy places embedded in the heart of the building and reliant on artificial light.

The critical transition zone between the old and new wings has been neatly resolved. On the IT floor, the two buildings flow seamlessly together, with no change in floor level and with reading desks positioned on either side of the dividing line. Here the main and clerestory windows of the original external wall have been removed but the precast concrete panels that supported them have been retained as freestanding sculptural elements.

What Shepheard Epstein Hunter has pulled off is a nice balance of mix and match. The extension relates neatly to its listed neighbour, though not in an obsequious manner, as it is clearly of its own time and with its own presence. As Joseph Saunders, the estates director, says: 鈥淭he subtleties of Shepheard Epstein Hunter鈥檚 design continue to surprise and satisfy. The Twentieth Century Society and English Heritage have both been very supportive.鈥�

The head librarian, Jean Steward, is no less proud of the result, and points out that it has received the ultimate recommendation 鈥� even though it鈥檚 a library, students are actually using it. 鈥淭he students have all been tremendously excited. It is interesting that early in the morning, every seat in the extension is taken, whereas those in the original building are still vacant.鈥�