As far as museum expansions go, the Montreal Museum of Fine Arts's new Claire and Marc Bourgie Pavilion of Quebec and Canadian Art is not much to look at. But herein lies its charm. Seeing the way this discreet, generally unassuming addition fits seamlessly into the museum's existing campus on Sherbrooke Street, you'd swear the new building had been here for years.

Credit for the success of the expansion goes partly to the museum board, who resisted the popular temptation to hire an architect hoping to create a signature building, and partly the firm it chose, Montreal-based Provencher Roy + Associés Architectes, who obviously had a pretty good idea from the get-go of exactly what the museum needed. No surprise there, for the firm's principals Claude Provencher and Matthieu Geoffrion are veteran residents of the city.

The building is more or less a marble-clad cube with rectangular windows punched into the façade on three sides. There is no entrance, for reasons I will shortly explain, and the uppermost level is topped with a glass roof in the shape of an igloo, meant to provide a sensitive context for the display of Canadian Inuit sculpture and prints. The museum also uses this floor for receptions, and the view over the city from up there is spectacular.

But the beauty of the building is in the details. The marble cladding comes from the same Vermont quarry as the marble used on the outside of two of the other three buildings that make up the museum, helping to visually integrate the new building into the cluster. It is an obvious but nice touch. Meanwhile the expanse of windows allows passersby outside to easily see into the building, while visitors inside can look out on vistas of the surrounding city, nicely blurring boundaries between the two.

The sculptures lining Sherbrooke Street and an adjoining road — including a commission by the Montreal-born, New York-based David Altmejd — also help create cohesion between the four buildings, all of which were built at different times and in different styles during the last 150 years. Museum director and chief curator Nathalie Bondil quipped to me in passing that these four buildings create "a kind of miniature museum of architecture." She's right.

Inside the building the exposed concrete walls (which in my view could have been better finished) give way to white sheetrock that wraps elegantly around the interior of the new galleries, which are spread over six levels. There are three identical 2,725-square-foot galleries in the middle, a slightly smaller gallery on the top floor (where the Inuit collection is housed), a larger street level space and, underground, an additional 148-foot-long gallery.

The underground "Mountain Gallery" doubles as a connecting tunnel to the galleries of international contemporary art over in the Moshe Safdie-designed Jean-Noël Desmarais Pavilion which is located across the road and constructed in 1991. The decision to widen the tunnel so that it could be used to exhibit artwork — to serve, in effect, as an additional gallery space — is one of the most creative elements of the design. Art looks good in here.

In chronological order, the upper galleries chart the history of Quebec and of Canadian art from colonization to the 1970s. Some of the floors look better than others, but overall the display of this art is vastly improved: the penultimate-level gallery, organized around the theme of "Founding Identities," presents a beautiful and valuable selection of works from the colonial period (1700-1870s) exploring the origins of Canadian art

The total cost for the project is $42.4 million CAD, though this includes both construction and the purchase and renovation of a deconsecrated historic church on Sherbrooke Street, the nave of which has been converted into a concert hall. Entry to the new pavilion is through the front of the church and a gallery level lies beneath the church floor, which required the architects to excavate an old crypt. I bet this was costly.

The new pavilion, which opens to the public on October 14th, increases the museum's total exhibition space by 20 percent, with a 100 percent increase in the area devoted to the collections of Quebec and Canadian art. More importantly, it allows for a clearer, more coherent display of this material in a purpose-built space. Common sense seems rarely to characterize museum expansions, but for once, simply and beautifully, it prevailed.