The Philadelphia Museum of Art will undergo the first expansion in its 77-year history, adding 80,000 square feet of gallery space under its front plaza and a 400-car underground parking garage -- a project that will cost an estimated $500 million and take at least a decade to complete.

"We're trying to chart the best course for the 21st century," museum director and chief executive Anne d'Harnoncourt said Tuesday. "It's a plan which we hope to realize over a 10- to 12-year period."

The plan would be organized in five phases that could be completed independently as money is raised from local government and private donors. The first phase is the completion of the art deco Perelman Building, a space across the street from the museum that will house its collection of prints, drawings, textiles and costumes.

That project is expected to be completed in 2007, with other aspects of the plan likely to begin later.

The subterranean galleries under the front plaza would increase the museum's available display space by 60 percent. The underground level would include a larger special exhibition gallery, capable of hosting blockbuster shows such as the recent Salvador Dali retrospective, and high-ceilinged galleries to display large-scale modern sculpture.

"We want to make all of the galleries sing," d'Harnoncourt said. "There is still a lot of unrealized potential for the museum's collections and the public to connect."

Only a small portion of the museum's 226,000 objects are on display at any given time, many because they are light-sensitive and can't be continuously exhibited, but others for lack of space.

D'Harnoncourt said the museum began looking at ways to expand about four years ago.

"We've really run completely out of space for our collections, particularly Asian art," she said, pointing out that resources to accommodate modern art are also strained because new pieces are constantly being produced.