Universities' property portfolios boast a range of often-forgotten treasures that are being renovated with spectacular results. Michael North reports.

Mabel Wilson was bewildered by the transformation of the grain warehouse where she worked as a "sack girl" years ago. Where once there were winches and a hive of sweaty-browed industry stood row upon row of bookshelves and computer terminals.

Mabel was in her eighties when she was taken back as the guest of honour to the grand opening of Lincoln University's new library, sited in the old converted warehouse.

Universities are now one of the biggest clients of the UK construction industry as they increasingly renovate old and sometimes listed buildings within their property portfolios. Often they have little choice, as new builds can be out of the question in urban areas where land is at a premium. Many institutions have considerable stocks of property to choose from - Glasgow University, for instance, owns more than 300 buildings.

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The renovation of buildings that have never been used as seats of learning can be challenging. At Lincoln, the university has been building a campus on a disused railway siding for a decade. Beside the old warehouse library stands the new student union - a transformed railway engine shed.

Lincoln is the only university in the UK with its own Royal Institute of British Architects-registered architects' practice, UL Architects, which gives valuable experience to students at the School of Architecture and often employs alumni.

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Principal architect Nigel Stevenson, himself a Lincoln alumnus, says one of the major difficulties of the two recent ?11 million conversions was dealing with the tons of "pigeon guano" that had accumulated in the engine shed over the years. "Pigeons used to sit on the old extraction cowells.

There were 3ft-high piles of the stuff that had to be specially cleaned," says Stevenson.

Other contaminants on the site were dealt with using biopiles - toxic soil mixed with chicken manure and straw. Diesel fuel on the engine shed floor was sealed in with concrete, which is vented to allow gases to escape over time.

Stevenson says his team worked as much as possible with existing materials, such as the original steelwork, to create buildings that echo the past. But there are hyper-modern additions: the glass box that protrudes from the library front and a huge light box where there was once a water tower, which Stevenson says are intended to stand out as state-of-the-art additions rather than "pastiche replicas" of the old.

The buildings have been surprisingly fit for purpose - the thick warehouse walls have "shown excellent thermal cooling performance", which is good for books, while the engine shed has excellent acoustics and is becoming a popular venue for UK bands.

"If we preserve these buildings and they function well then that is the greatest reward," Stevenson says. "One of the most positive things in all this is that, rather than build huge academic monuments, we have put something back into the city."

A similar sentiment is expressed by David Johnson, whose practice Dannatt Johnson Architects won the contract to convert Grade I-listed Wren admiralty buildings for Greenwich University.

Following an unprecedented government advertisement for a tenant for the buildings in Country Life magazine, a long feasibility study was carried out with English Heritage to decide whether thousands of students would damage the historic buildings.

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Johnson says his firm went back to Wren's original plans. "The external work was not huge but internally we put in new services for new technology, power and lighting."

Although severely restricted in the extent to which they could alter the building's fabric - knocking down the panelled partition walls of the one-time hospital was out of the question - the architects have worked with the building. A roof space in the King William building provided an ideal setting for the raked, or rising, seating for a lecture theatre.

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There have been compromises imposed by the age of the building: the catering facilities in the Queen Anne building are far from ideal because the architects were not allowed to put in the necessary ventilation.

But Johnson says this latest use of the historic buildings has unquantifiable benefits for students and staff and for the public who can now roam freely in grounds that were restricted in the days when the Ministry of Defence was the tenant. "It is an elegant setting and also a very calm one. It's a unique experience to be in a 17th-century building and also in a modern teaching environment."

A project that has breathed life into a more contemporary building has been completed in Cambridge, with the renovation of Trinity College's 1969 Wolfson Building by local architects' practice 5th Studio. The building, a brick and exposed concrete ziggurat - a stepped pyramidal design - has been refitted for 90 en-suite rooms, at an undisclosed cost. "Trinity doesn't talk about money," says Tom Holbrook, one of the project architects.

The most striking feature of the conversion has been two glass "hanging rooms" to provide new teaching and social spaces. Other innovations include photovoltaic cells and a heat-recovery system that save energy and minimise the building's carbon imprint.

Holbrook says the building was a "ballsy, robust fresh idea" and oozed modernist confidence when it was built. But it had environmental failings and was cold and dark. New technology solved these problems. Holbrook predicts there will be many more renovations of buildings that went up in the huge university expansion of the 1960s and 1970s.

London institutions are forging ahead with multimillion-pound conversions of their existing building stock. Imperial College London is one of the largest property holders of any institution in the capital with 51,100 square metres of space. The college spends ?120 million a year on new building refurbishment; in 2004, it opened the Tanaka Business School, a project designed by Lord Foster and Partners; it has a ?70 million halls of residence due for completion in autumn 2007 and recently completed restoration of a Grade II-listed Georgian town house in Princes Gate to house the new Institute for Mathematical Sciences, at a cost of ?18 million.

This last project presented architects Tilney Shane with a challenge.

Architect Carl Fisher describes, for example, how a parquet marquetry floor was considered so valuable by English Heritage that separate services had to be provided for floors above and below it to avoid drilling through the floor. Craftsmen were also involved in restoring the ornate plasterwork to its former splendour.

Bill Taylor, an architect at the Michael Hopkins and Partners practice, which has worked for Nottingham and Nottingham Trent universities, says that "practical issues drive the solutions" in the conversion of old buildings. "When I was at Sheffield University, there were 3,500 students; now there are 19,500. The need for more information technology, the size of year groups, the large numbers of courses taught, the need for flexible space - all these have physical consequences. Today, a seminar room for 100 people needs to become three seminar rooms, and then be used for a film video course."

Taylor says the buildings themselves can serve an educational function: at Nottingham's ?40 million campus, which opened on the old Raleigh factory site in 1996, Hopkin and Partners' design focused on sustainability, including wind and solar power and natural and artificial lighting. The practice's lead in this aspect of design has recently won contracts from Ivy League universities in the US, a country that trails the UK in terms of environmentally friendly buildings.

It is also important that the buildings look good. All the architects mentioned say the renovations have had a positive impact on students and staff, a key requirement in a competitive market.

Taylor's firm is currently converting two listed buildings for Nottingham Trent - the 1930s School of Engineering and its neighbour, an 1870s edifice from the Gothic revival - at a cost of ?50 million.

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He says: "The buildings will be a big part of the recruitment strategy, not just for students but for staff as well."