From the street, the new addition to the Virginia Museum of Fine Arts follows roughly the same roof line as the rather dour brick structure it adjoins. A large expanse of glass connects the two spaces, and despite the obvious philosophical and aesthetic gulf between the new and old structures, they look content to be side by side. Like almost every museum expansion project in recent years, this is about the institution's public image, its ability to serve a distracted audience and harvest cash from a new bar, restaurant and gift shop. But this expansion successfully cloaks all of that in a generous mantle of calm.

The VMFA wants a larger share of the world's attention, and it deserves it. The museum sits in a lovely part of Richmond, marred only by the obscene presence of Confederate generals on plinths nearby. It has an impressive collection of art, not huge by the standards of New York, London or Paris, but comprehensive and with unusual strength in its Asian and African holdings. Museum leaders have also published a new catalogue, which does an uncommonly good job of explaining art to laymen without sacrificing technical terms and historical context.

And Saturday the museum opens a new wing designed by architect Rick Mather, whose extensive work on cultural institutions in England is formidable and should have earned him a blue chip American commission long before now. Last fall, Mather attracted wide acclaim for his expansion of Oxford's Ashmolean Museum, the first university museum in the world - and sacred cultural ground. Miraculously, Mather managed to double the gallery space in the land-locked building, make it all flow together sensibly, and give light and airy expression to the Ashmolean's larger ambition to modernize its academically balkanized presentation of art and cultural history.

Richmond is his first significant American project and given its serene good sense, it should be the first of many. The 165,000-square-foot new building is the latest in a series of expansions to the VMFA's rather dour 1936 building, a brick and limestone pile in the Georgian style, fronted by a classical pediment and an entryway that immediately requires visitors to ascend a pompously over-scaled staircase. Later additions include flanking brick wings to the 1936 building, and expansions on the back and side of the core structure in 1976 and 1985.

Mather's addition replaces the space added in 1976, which put the museum's main entrance behind the building and near the parking lot. The new wing places the parking under a strikingly canted sculpture garden, reclaims the old parking lot for a landscaped back yard complete with reflecting pool, and situates the main entrance on the side of the building, where it beckons visitors from the street. The old entrance, in the slightly schoolmarmish 1936 building, still functions, which is becoming a rarity in oft-enlarged public buildings. Too often, the old front door becomes a useless bit of ornament after a new one is added, which is to a building what neutering is to a bull.

But the major accomplishment is the new wing's fine temper. Like Mather's addition to the Ashmolean, there is a meditative quality to the new Richmond space. Its dark gray stone floors, white walls and large expanses of glass make you want to take off your shoes, put on a robe, then slip into a tub of warm water. Which is to say, his new wing, which adjoins the old museum along a building-width and sunlit atrium, grounds the visitor. This long, wide corridor provides an axis of Zen in the midst of a highly artificial experience: the contemplation of art from every century, continent and culture that can be crammed into a typical museum visit.

The new space is carefully configured to intersect old sightlines and create new vistas through the larger museum complex. Elevated bridges in the atrium connect Mather's galleries with the older spaces, and give the visitor multiple perspectives on the reconfigured interior and exterior landscape. Above, the ceiling seems to float, tethered by fin-like dark metal supports. Along one wall, a large interior window lets visitors to see into a room dominated by a Roman sarcophagus carved out of marble in the second century.

The rational lines and rectangular profile of the museum's exterior are punctuated with a little humor. At night (if shades aren't drawn), the large ventilation tubes of the museum's conservation shop are visible through a window, like a strange hose creature from the deep suspended in a vitrine. Outside, near the entrance, an 1850 country home that should be wildly out of place on the museum grounds has been cleverly landscaped to look like a little jewel box or folly. Inside the new wing, there's more humor: walls that curve slightly along one side of the atrium, like peelings off a fruit, and a staircase that carves some appealing negative space out of the large gallery reserved for special exhibitions. But these are wry gestures, not laugh-out-loud jokes.

Mather's aesthetic continues a strange but apparently inevitable trend in the history of museum design. Once upon a time, museums were cluttered spaces, with walls loaded choc-a-bloc with paintings, from floor to ceiling. As the museum experience became more pseudo-sacred, as art climbed the existential ladder from image to fetish object, design priorities shifted. Wall colors became more neutral, paintings and sculpture demanded more space, more air between them. And a strange anxiety crept into the visitor, who became increasingly uncertain about how to look at, absorb and think about art.

It's not accidental that the most appealing design details in the new wing of the VMFA sound like they're borrowed from a high-end spa. Art museums have grown, expanded their collections, stuffed their vaults with stuff, without quite explaining to the public what it is one actually does at an art museum. It's not clear that the people who run art museums even know the answer to this question. And so the trend in museum design is to provide appealing space where one doesn't have too worry too much about the expectations and disappointments of looking at art.

Mather's building, which includes new galleries for American, pre-Columbian, South Asian, Native American and 21st-century art, is also a pressure valve, with places to sit, connect to the museum's WiFi network, and have a drink. Its success, as architecture, lies in how well it balances these terrestrial pleasures with the seriousness of its new galleries, and how easily it connects with the existing buildings. Museum directors considering expansion projects may want to study his accomplishment.