Nothing about the opening this month of the Ruth and Raymond G. Perelman Building - the first public addition since the opening of the neoclassical temple in the 1920s - makes the mission less serious. But the 173,000-square-foot expansion represents the museum's only suggestion in decades that its art is not necessarily tied to a building, and that it is ready to come down off its pedestal, literally and figuratively, to become part of a city neighborhood.

Most dramatically, the opening of the Perelman is the museum's first step in a much larger process to substantially capitalize on the many points of entry that art offers its public.

"People are going to invent their own ways of using the building," says museum director Anne d'Harnoncourt.

If Robert Montgomery Scott, the museum's late president, once telegraphed with his Fairmount Park bike tours the idea that the museum was a place both friendly and erudite, the current marketing campaign sends the message that it's friendly, erudite and contemporary, ready and eager to have you no matter who you are. That young man standing before the Jacques Lipchitz sculpture in Art Museum ads - is he black, white, Latino or Asian, straight or gay, 18 or 35? No matter; art is for Everyman (and if you're young and chic, so much the better).

With impending projects totaling $590 million and counting, the Art Museum is gambling that there is a wider audience to be had. The Perelman annex at 2525 Pennsylvania Ave. - né the Fidelity Mutual Life Insurance Co. building, which cost $90 million to acquire and renovate - will make it easier for teachers to access the museum's educational programs; scholars can visit the archives without feeling as if they are hiding in a fallout shelter; art students can examine costume and textiles up close.

The Perelman is a place for the typical visitor to see prints, drawings, photographs, sculpture, costume, and modern and contemporary design in several new, light-filled galleries.

As this facility opens, construction has begun near the main building on a 440-space garage topped with a sculpture garden (the museum prefers to call it a sculpture garden with a garage below). The structure, intended to extend a special welcome to car-based life forms, will cost $34 million.

At the same time, the main building's exterior is being restored and preserved at a cost of $31 million.

And then there is, of course, the reimagination of the main building's interior, a task assigned to the firm of renowned architect Frank O. Gehry, whose titanium- and steel-blossom flourishes have energized the images of Los Angeles and Bilbao, Spain.

Gehry will remake nonpublic space into new underground galleries, but that's just the start. The engagement of this famously expressive architect has created coffeehouse and cocktail-party chatter among the culturati: Will he or won't he conjure a signature stroke on the east plaza above the new galleries, a la I.M. Pei's glass pyramid at the Louvre? Answer to be revealed in a year or two.

"He and his team are diving into it, learning the collections, the museum, the city and the lay of the land," d'Harnoncourt says. "It's going to take a while for them to write up a real design proposal."

Each one of these - the Perelman building, the garage, the restoration of the main structure, and the Gehry project - is a big venture. Taken together, the Philadelphia Museum of Art's next decade or more of expansion and modernization probably represents the most ambitious cultural leap in the city's history. Its price tag will come in at easily twice the cost of erecting the Kimmel Center, heretofore the city's most expensive monument to art.

How much of the $590 million has the Art Museum raised? Officials are unwilling to say. D'Harnoncourt won't even acknowledge that a fund-raising campaign is under way, though chief operating officer Gail M. Harrity said several months ago that $20 million to $25 million more was needed to close the funding gap for the $90 million Perelman.

The museum's big burst of institutional ambition will likely have an ancillary effect: that of keeping d'Harnoncourt, for 21/2 decades the museum's chief artistic voice, in Philadelphia. There's a lot of movement in the leadership of the museum world, and d'Harnoncourt, 64, might be considered a hot property. But she says she "can't imagine any greater pleasure" than helping to realize the museum's dreams for the next decade or more.

At the conclusion of these projects, d'Harnoncourt will be in her mid-70s, raising the possibility that her career as a full-time museum director will end at the museum where she first came to work as an underling in 1967.

"I have no plans to go anywhere," she says. "I'm totally devoted to Philadelphia and the museum and realizing all this potential. There is a lot of art waiting for galleries, a lot of projects waiting for public space, and a lot of the Web site waiting for images."

The Perelman building will reduce the wait for art wanting gallery space. It is not merely an adaptive reuse project, converting office space to a more artful existence. It created more space, a 59,000-square-foot addition designed by Gluckman Mayner Architects. The gap between the old and new buildings creates a terrazzo-floored galleria that, at 35 feet high and 200 feet long, is a nice surprise of air and sun. It holds sculpture, and its glass cap promises one of the city's rare indoor-outdoor pleasures.

In the end, none of the art in the Perelman may be in permanent residence; then again, some of it might be. D'Harnoncourt says no decision has been made.

As important as the play of light, respect for historical architecture, and beauty of display are, the question of how artfully the museum manages its new public plant will substantially determine how successfully it functions. Will nonscholar visitors to the costume collection really be able to call ahead and make an appointment to see, say, Princess Grace's wedding tiara? Will the museum's frequent increases in admission put it out of reach for some visitors? As the Kimmel Center showed by managing public spaces in a way that discourages the public, a lot depends on what you do with your flashy architecture after the opening-day press has gone home.