When Rick Mather moved to Britain from the US in the early 1970s, Joni Mitchell was riding high in the charts with Big Yellow Taxi, her lament about crass developers. But Mather's museum work stands the singer's famous line. "they paved paradise and put up a parking lot," on its head. The Towner Art Gallery in Eastbourne, which opened in May. is built on what was a car park.

A work in progress the expansion of the Virginia Museum of Fine Art in Richmond, Virginia. takes this much further. The US art museum was surrounded by acres of tarmac. In the future visitors will enjoy a sweeping lawn that rises to a grassy knoll. A cascade of water will flow down its slope while the museum's new car park will be out of sight below the grass and trees. The water feature doubles as a moat. "lf people want to rob a painting they will have to have waders," says Mather.

Back in Eastbourne, on the south coast of England, Mather explains that the original brief was to put the Towner on the opposite side of the Congress Theatre, the town's rather boxy, and to many eyes ordinary-looking, 1960s theatre and conference hall. "(The gallery) was supposed to be suspended above a kitchen, which would have been impractical," he says. "So we moved it to the opposite corner. I liked the idea of coming up to a curve and using that to generate the look of the place."

The art museum's curving side wall echoes the white cliffs of the Sussex coast nearby, but its front takes its cue from the straight-back-and-sides Congress Theatre next door. The Towner seems to play second fiddle to the 1960s building when seen head on. Many architects would have tried to upstage the older one, especially when the brief no doubt demanded something "landmark" and "iconic". "Well, I quite like the Congress Theatre, it has a period charm," says Mather. "We thought it pointless to fight it."

Mather's formula for a museum building is deceptively simple. "Like most of our museums (it) has quite a good collection, although smaller than some. When you have a good collection in a museum you want to be sure that you have good galleries to put it in." Space, and the budget, at the Towner has been used for the galleries, not lavished on a lobby or grand staircase, unlike many new museum buildings and extensions. For a modest-sized museum the galleries are surprisingly large. The museum as a whole seems much bigger inside than seems possible from the outside.

"Every time you are in the corridors you have views of the landscape beyond." This means windows front and back frame the manicured tennis courts in the foreground and the hills of the South Downs in the distance along with the surrounding townscape. Even the lift, a combined passenger and goods lift, is scenic, with a window in the lift shaft. Mather reveals another reason why the museum feels so spacious. "We arranged the doors of the galleries so that they are always aligned with either the lift doors or the cross corridors, so you always have long views." like the Whitney Museum of American Art the Towner makes the most of its corner site, and the Towner's big lift trumps the New Yorker's.

The millennium was a busy time for Mather. He modernised and expanded three London museums: the Wallace Collection, the Dulwich Picture Gallery and the National Maritime Museum. Expanses of glass transformed all three. The glazed courtyard of the maritime museum is one of the largest column-free glass roofs in Europe. But glass is a mixed blessing in a museum: too much results in too much light for the collection's good and not enough wall space. "In Virginia there is a gallery from the 60s that has curved walls and also one big wall of glass," says Mather. "So the museum came in and blocked them off eventually. That is a lesson to an architect. You accept the discipline: Galleries need straight walls. And any time we have a window we really use it. It's there for a purpose."

At the Dulwich Picture Gallery Mather added a cafe, lecture theatre and education room while barely touching Sir John Soane's original early-19th century art museum. Mather extended a garden wall to create a new gatehouse and ran a cloister from it to meet the gallery, the latter running along the side of a chapel. The architect managed to please the Soane fans while keeping the chaplain and the congregation happy. It must have required diplomacy. "We had to convince them that a radical thing - putting a cloister against a church - was alright" he says, revealing a dry sense of humour.

Every other bay of the south-facing cloister has a sliding door that can be opened up in the summer. The lawn becomes another room, which is something we like." At a time when many museums were still consigning education spaces to backrooms or basements, Dulwich's occupies pride of place in the cloister. The lawn becomes an outdoor classroom - an object lesson for other museums.

The way some of the new spaces created by Mather are used has not always been as happy. The glazed courtyard of the Wallace Collection was originally reserved for diners of its posh restaurant. It made nonsense of the architect's intention to link the front and back of the museum through the new space. "That was the whole idea. you could walk through it. It was meant to take the pressure off the side galleries, which were almost traffic jams on wintry days," he says. Thankfully the maitre'd has relaxed and entry for all is now permitted.

Under the great glass roof of the National Maritime Museum, in Greenwich, there is a central square or "deck" on top of temporary exhibition galleries. The glass roof is first glimpsed from a promenade level through a circular opening overhead as you enter through the new entrance. "We brought the building down to ground level, liberating that whole floor so that it could be used as public gallery space and improve circulation," Mather says. "They have a whole new floor of galleries underneath (the deck), which gave them even more space than they ever dreamed possible." Perhaps it was too much space. The first scheme for the upper deck featured a dome and cubes that were meant to suggest maritime activity but didn't - Mather refers to them as "big lumps". Three long showcases of small objects and a few statues have replaced them but the space still feels Marie Celeste-like. "I would have liked to see some more boats and barges," says Mather. "They have some absolutely fabulous barges." He seems bemused that the museum thought it would look like car park instead of a boatyard.

The Great Court of the British Museum was one millennium project that got away: Mather's scheme was the runner up. It would have left the Reading Room standing more or less untouched externally. revealing its underpinnings. "It has a beautiful support that we were going to excavate and expose and have galleries around the side. Like any museum with a good collection what they need is very simple: more space."

The Ashmolean Museum of Art and Archaeology in Oxford is proud of being Britain's oldest public museum but when it reopens in the autumn of 2009 two thirds of its building will be brand new. Behind its grand mid-19th century front, Mather has created several floors of new galleries. From the street, however, the new building will be invisible. "It won't come into its own till you can see the atrium and our bridges and so on."

The museum wants to be more than a collection of palatial rooms housing treasures. How did that shape the design - of the new Ashmolean? "It's all about views of the double-height galleries that  you can follow through (the building). My ideal is a museum where you don't need any signage to see where you want to go. You'll always be able to refer back to these double-height galleries that the single-height ones open off from," Mather seems pleased with the museum's concept of cultural crossings because it fits the architecture, with its bridges and a grand staircase. "You will be always crossing space physically as well as mentally." And when visitors need refreshments a treat lies in store on the roof where a restaurant and terrace will overlook Oxford. "Restaurants should always be in fabulous positions," declares Mather. When museums expand, it is often at the expense of their original character. Mather's non-competitive, light-touch approach studiously avoids this. "If you've got something good in a museum, don't fight it, or destroy it. Leave it alone and make two things that are good."

Javier Pes
Museum Practice