The Philadelphia Museum of Art and Frank Gehry on Thursday unveiled an ambitious renovation plan that will add 78,000 square feet of gallery space without altering the exterior of the museum’s landmark 86-year-old building.

The plan, which Gehry Partners has been developing since 2006, is the subject of an exhibition at the museum that runs through September 1. Through large-scale models, site plans, and renderings, it will introduce visitors to Gehry’s plans to promote a more logical flow among its galleries while providing extra space for the museum to display more of its collection, which has grown to include nearly 230,000 objects.

Though Gehry is best known for whimsically expressive, sculptural exteriors, including the Guggenheim Museum Bilbao and the Walt Disney Concert Hall in Los Angeles, his marching orders from the museum were to work almost exclusively within the shell of the landmarked building, which was completed in 1928. (Gehry Partners and OLIN, a Philadelphia landscape architecture and planning firm, will redesign the plaza outside the museum’s west entrance, and skylights will be integrated into the East Terrace to bring natural light to new gallery space to be built below it.)

“The brief from the beginning was, ‘Bilbao effect, don’t touch the building,’” Gehry joked in a question-and-answer session with journalists.

Gehry’s master plan includes:

-The renovation of the museum’s two main public entrances, the Great Stair Hall and the Lenfest Hall.

-A new public space, called the Forum below the Great Stair Hall that will improve circulation by opening an east-west axis at the center of the building.

-The creation of 55,000 square feet of new space for special exhibitions and items from the museum’s collection. The new exhibition space will be carved out by excavating earth and bedrock from beneath the museum’s east plaza.

-The reopening of a third public entrance, on the north side of the museum. This entrance will provide access to a grand, 640-foot vaulted corridor. Long off-limits to visitors, it will serve as a new north-south axis for the building, providing new access points to galleries and intersecting with the Forum.

-The creation of new meeting and event spaces in the center portion of the top floor of the museum’s U-shaped building. Bricks will be replaced with glass to allow for striking views of the city skyline and nearby Fairmount Park.

The museum’s director and chief executive, Timothy Rub, said his predecessor, Anne d’Harnoncourt, who died in 2008, pushed for the selection of Gehry’s firm largely on the strength of its renovation of the galleries at the Norton Simon Museum in Pasadena, California.

“They went to him and said, ‘Frank, can you help us reorganize the building inside?’ Eventually, he remapped and actually restructured the entire Norton Simon inside, and made it one of, I think, the most beautiful and elegant museums in all of the Los Angeles area. But it’s all inside; it’s all within the existing volume. And so that gave her, I think, the greatest confidence that Frank could do something really wonderful here.”

A timetable for the project, which will be completed in phases, has not been set, nor has a budget. The project’s fate rests on the museum’s ability to raise the money that it will need to see the construction work through.

Gehry said he is optimistic that they money will be found.

“I would say it’s been a really interesting collaboration,” he said. “I think it’s going to have a great effect. [It will be] a great asset to the city of Philadelphia, I hope, and it will certainly be a powerful change to the museum.”