Big changes are coming to Washington Square.

About seven months after the Curtis Center was purchased for $125 million by Keystone Property Group and Mack-Cali Reality Corp., plans have been firmed up for a mixed-use redevelopment of the historic building facing the square along Walnut Street between Sixth and Seventh.

Among the changes at the 12-story building will be luxury apartments, a departure from its current primary use as office space.

"We want to create a vibrant pedestrian experience … a 24-hour destination," said Jennifer Cooperman, development director for Keystone.

About 30,000 square feet is available for lease on the ground floor.

A large space on the ground floor on the Sixth Street corner will be offered for lease as a restaurant.

"We thought this is the corner to start where the energy can begin for the building," said Jacob Cooper of MSC Retail, the broker for the property. "The restaurant can not only be an amenity for the entire building, but a destination that can pull residents and tourists and people from around the area."

Cooper said MSC Retail has been talking to several restaurant groups and expects the restaurant to be operated by someone local.

Aside from the restaurant, Cooper said they're looking to add a coffee shop, spa, gym, bar, and other retail stores to the six additional available spaces.

Currently, for phase one, 55 luxury apartments are planned for the upper floors, which Cooperman said will begin construction this spring. She said the apartments will be primarily on the Sixth Street side of the building overlooking Independence Mall, as well as a portion along Walnut Street, overlooking Washington Square.  The Seventh Street side of the building will remain office space.

The building is only one block from what will be the "glass needle tower" - developer Tom Scannapieco's planned 26-story, 40-unit condo tower at Fifth and Walnut Streets. Construction is slated to begin around the same time.

The atrium, one of the Curtis Center's defining features, is also getting a makeover. The developers hired architectural firm Voith and Mactavish to design the space.

"Currently, the atrium serves as a pass-through for pedestrians on a 9-to-5 basis," Cooperman said. "Keystone wants to change that by making the Curtis a central hub."

Cooperman said they will remove the palm trees and fountain, repaint the space, build a bar in place of the fountain, and install archways that span the width of the room.

The space is a popular venue for weddings and other special events under the Cescaphe Event Group. The renovations won't change anything: the company will still operate there.

Another hallmark of the building, the 100,000-piece mosaic called The Dream Garden, will remain in the lobby, Cooper said.

Although some of the windows in the atrium are original from when the Curtis Center was built between 1909 and 1920, much of the room is new.

The structure was built by publisher Cyrus H.K. Curtis as headquarters for his publishing empire, and the atrium served as a loading dock.

The last time the building got a major makeover was in 1980, when $80 million was spent to refurbish office space, common areas, and building systems.

Once complete, Cooper said he believes the area to have the potential of becoming the "second Rittenhouse Square."