Roundhouse: the cover version

The first time I went inside the Roundhouse, London's most famous engine shed, was to see The Who at the very end of the 1960s. Railway locomotives had abandoned the place in the 1860s. They were followed by a bump'n'grind of goods wagons, a torrent of gin (the Roundhouse was leased to the distillers W & A Gilbey from 1869 and used as a bonded liquor store until 1963), a thump of rock in the Sixties and an expletive of punk in the Seventies. The building failed to become an arts centre in the Eighties, but - certainly until last week, unless my ears have failed me - was operating as a venue for all-night raves.

The Who were backed by Elton John, who had just released his first album. So I have long known that the Roundhouse is not just a particularly satisfying building to look at - round buildings often are - but a tough one, too. Anyone who remembers just how loud the The Who played will understand; for, if Maria Callas had ever been able to crack a glass chandelier when hitting a high C, The Who would have made an effective firm of demolition contractors. They may have destroyed guitars and a number of eardrums, but the Roundhouse, testimony to confident Victorian engineering and construction, rocked yet never rolled.

Now, after the worst that Sham 69 and The Clash could do to bring down the house, this glorious old building, looking shabbier with each passing year, is about to find a new and dignified role. If the money - pounds 10m of it - adds up by the close of 1998, the Roundhouse will re-open as the British Architectural Library.

This might sound a little stuffy and hushed after the glory years of The Who and The Clash, but the project makes near-perfect sense. Run by the Royal Institute of British Architects (founded in 1834, 12 years before the Roundhouse was built to serve the goods locomotives of the new London & North Western Railway), the British Architectural Library is far more than its name suggests. It contain multitudes - more than 135,000 books (4,000 of them printed before before 1840), 1,400 rounds of old or rare periodicals, 500,000 architectural drawings dating from the 15th century and including such internationally regarded treasures as most of Andrea Palladio's drawings, manuscripts from the 17th century onwards, 400,000 photographs from the birth of the art to the present day, 30,000 slides to hire, plus an assortment of busts and medallions, architectural models and a huge data bank of biographies and technical information.

To date, this sophisticated library - probably the finest of its kind - has been housed in various locations in London and elsewhere. The library proper (books, magazines, afternoon naps) is housed in the RIBA's headquarters at 66 Portland Place, a couple of hundred yards north of Broadcasting House, while the drawings collection is hung, filed and drooled over at 21 Portman Square, an Adam town house near Selfridge's. The lease on the latter runs out in 2002, so the Roundhouse venture is both desirable and necessary.

The architects chosen for the project are Michael Hopkins & Partners, well known for their handsome, if stocky (and just occasionally stodgy) additions to old building complexes; the Mound Stand at Lord's cricket ground, the new opera house at Glyndebourne and the conversion of the former Financial Times building into a Japanese bank in the shadow of St Paul's Cathedral are simply some of their best-known and most-liked designs. The practice has gradually shifted its stance over the past decade from designing flamboyant and memorable Hi-Tech structures such as the Schlumberger research laboratories outside Cambridge to an instantly recognisable style of building that marries sophisticated building technology to a superpowered spin on home-spun English vernacular, epitomised by the new Inland Revenue headquarters in Nottingham.

The Roundhouse is a natural for Hopkins and team. The building will convert readily, with the expert help of Alan Baxter and Associates (engineers), into a library with plenty of archive and exhibition space. The exploded drawing shown here is crystal clear in its presentation of how the new British Architectural Library will look and work. The logic of the Victorian engine shed dictates the plan and everything else stems from that.

The Roundhouse makes a natural store for drawings and other material sensitive to light; its walls are solid and with few apertures. The circular roof, however, which promises to be quite spectacular seen from below, will bring much of the daylight needed into this great brick drum. Because the structure of the building is a given (and cannot be removed: the Roundhouse was listed Grade II* in 1954), the new library will simply slot into place around it as if to the engine-shed born.

The centrepiece of the Roundhouse is a cast-iron coronet circling what was once the turntable and punctuated by 24 openings from where, between 1846 and 1869, railway locomotives once bedded down between duties. The space inside the coronet will be used for exhibitions. These will be open to the public.

The location of the Roundhouse is a good one given the RIBA's wish to be closer to the public. Its present headquarters, although no more than five minutes' walk from Oxford Circus, has always been off the beaten track; Portland Place is a traffic thoroughfare and not a natural pedestrian route through central London. The Roundhouse, in contrast, is located just seconds away from Chalk Farm tube station (one of those liver-coloured Art Nouveau designs by Leslie Green); this is at the heart of crowded Camden Market and in the stomach of dozens of fashionable bars, cafes and restaurants. Location alone will make the new library both far more popular and far more accessible than it has been before.

Whether librarians and curators will welcome this intrusion is another question, but given their noted enthusiasm, they will take it in their stride. Ten years ago, exhibitions of architectural drawings were very much the province of professionals and enthusiasts; today, they command much larger and popular audiences. The Roundhouse, in its latest, post- steam, post-gin, post-punk, Post-Modern role can only stimulate this burgeoning enthusiasm.