For 86 years, the Philadelphia Museum of Art has reigned as the city's most majestic building, a golden crown perched in splendid isolation atop its Fairmount hill. Yet, there is a good reason it is still sometimes called the "Greek garage." Inside, the great temple remains a work-in-progress.

Now, a plan to expand the museum by burrowing deep into its rocky hillside, conceived and designed by celebrated architect Frank Gehry, promises to remedy this unfinished business, completing and polishing the masterpiece in time for its centennial in 2028.

Though no start date has been announced, museum director Timothy Rub revealed Thursday that architects were wrapping up construction drawings, a prelude to actual construction. In a lengthy interview, Rub gave the most extensive outline to date of the museum's plans.

It's been a long time coming. The Art Museum first announced its intent to expand in 2005, when the late Anne d'Harnoncourt was still at the helm, and hired Gehry a year later. The museum has finally decided to unveil the project formally by putting Gehry's architectural models and renderings on display. "Making a Classic Modern: Frank Gehry's Master Plan for the Philadelphia Museum of Art" opens July 1.

Gehry, who turned architecture upside-down two decades ago with the billowing titanium waves of his Guggenheim Bilbao in Spain, is best known for sculptural, otherworldly buildings. But with this design for Philadelphia, he plays brilliantly against type. No evidence of his bravura will be visible on the museum's Kasota limestone walls, and the interior changes will be dominated by straight lines.

This self-effacing strategy has paid off in a big way. It is a fine, thoughtful design, meticulous in its logic and attuned to a city that likes to march to its own drummer.

If the museum can raise the roughly $350 million needed to complete Gehry's laundry list of improvements, the golden temple will emerge both more dignified and more approachable than it is now. All museums are exploring ways to draw in younger visitors, and this design is driven by that goal.

So, entrance doors will be thrown open at street level as a democratizing gesture. Pathways through the enormous, U-shaped building will be made more intuitive by establishing visual connections with the busy city below the hill. Murky spaces, like the Great Stair Hall, will be bathed in natural light. Surface parking lots that mar the west terrace will be banished, replaced by gardens and public space.

The only evidence of Gehry's trademark exuberance may be a coiling glass staircase in a new great hall called "The Forum" and an undulating ceiling in the new galleries under the east terrace.

The main objective of the project is to give the museum more gallery space, especially for the large-scale sculpture and installation art that is so popular among contemporary artists - and the public. Creating new display areas without grafting a significant addition onto the exterior is a complicated undertaking.

It speaks volumes about the museum's ethos that so much time has elapsed since Gehry was hired. The last 10 years have been a hyped-up period of expansion as dozens of institutions announced, financed, and completed dazzling additions. But the Art Museum has preferred to let Gehry's ideas percolate, to move forward at a pace that feels almost geological.

While its counterparts were inaugurating glass jewel boxes, the museum was spending its money on practical improvements, with the long-term aim of clearing the decks for the main show.

It embedded a parking garage into its hill, built a new loading dock, reroofed the main building, and opened a small clutch of galleries in the Perelman annex across Kelly Drive. All the while, Gehry kept building models of the underground galleries.

Although the Art Museum is an axial, neoclassical building - designed jointly by Horace Trumbauer, C. Clark Zantzinger, and Charles L. Borie Jr. - its circulation has always confounded. There are two main problems: Its U-shaped galleries dead-end on the east facade and its two main entrances are on different levels.

If you climb the so-called Rocky steps, you arrive at the second level. If you slog around to the west side, you enter a level below. Meanwhile, you can't walk in a straight line from east to west because an auditorium sits in the middle of the building.

Gehry's plan calls for unclogging the crucial artery by yanking out that blockage. To create the east-west path through the building, he will take down the wall where Chagall's glowing theater backdrop, A Wheatfield on a Summer's Afternoon, now hangs. (It will be relocated.)

With those two moves, the whole museum should feel more open. In place of the auditorium, Gehry will install the Forum, so called because it sits at the crossroads of the new east-west path and an old, long-closed, north-south vaulted corridor. Except for Gehry's coiling stairs and a glass bridge, the aesthetic is straight from the museum's existing Great Stair Hall.

Because the Forum and the north-south corridor are at the level of Kelly Drive, the museum will be able to create a third entrance on the s north side - the first portal that does not require climbing stairs. Visitors will enter through an enormous arched entry into the gorgeous, light-dappled space.

Gehry's hand is more in evidence in the second phase, when the museum excavates under the east terrace to create a new gallery the size of a city block. A saddleback-shaped ceiling unifies the space, while an oculus allows light to pour in.

Gehry's most daring idea involves cutting a chunk of the Rocky steps so a roughly 24-foot-high window can be inserted halfway up. It would form the east wall of the new gallery and offer views of the skyline. Toying with the iconic steps is sure to be controversial, but the museum should be allowed one daring move. The change also would allow a street-level entrance on Eakins Oval.

I was disappointed with Gehry's solution for emergency stair towers at the end of the east-facing wings, required because the museum does not meet current safety requirements.

Because the museum is being forced to break through the walls, it might as well use the occasion to create a strong architectural moment. But Gehry plays it safe, cladding the towers in limestone to match the museum. They look like morose sentries. In an earlier version, he showed a switchback of glass blocks tumbling from the facade. That more playful approach offered a peek at the activity inside the building.

The only other real disappointment is that we will have to wait at least a full decade to see this project completed. Gehry, who is only one year younger than the museum, would be 99 in 2028. Wisely, the museum has invited him to the exhibition opening in July.