All three sites are controlled by Liberty Property Trust, which completed Comcast's sleek, 975-foot headquarters just six years ago.
Since then, Comcast has grown enormously. With its acquisition of NBCUniversal and its move into new digital products, Comcast has filled virtually all 1.2 million square feet in its glass obelisk and needs more office space for its expanding workforce.
There are 3,500 employees in the Comcast Center at 17th Street and JFK, with hundreds of employees scattered in buildings around Center City. The company just signed a short-term lease for 100,000 square feet in Three Logan Square as a stopgap.
Neither Comcast nor Liberty would confirm they are working together on an expansion plan.
In an interview, Liberty vice president John Gattuso said he was intensively studying conditions at 18th and Arch to determine what kind of project would be appropriate. That property was approved for a 1,500-foot skyscraper in 2008, but it seems unlikely that Liberty would aim for such altitude.
"The tower will be as big as it needs to be," he said. "We need to understand the constraints of the site. We need to figure out the program and find a tenant."
Before the recession hit in late 2007, the former owner of the site had floated a proposal for a huge complex called the American Commerce Center that would have included an office tower, hotel, six-story shopping mall, movie theaters, and underground parking.
Gattuso said it was unlikely that Liberty would build anything that grand on the site. At the same time, he said, new construction would not be "just an office building."
Liberty's studies for the property may be more advanced than Gattuso let on. According to a source, Liberty recently requested a meeting with an ad hoc committee of city building officials to work out the logistics of connecting a tower into the concourse serving Suburban Station. Known as the Developer Services Committee, the group helps coordinate very large development projects.
Gattuso denied having any knowledge of such a meeting.
Liberty may be proceeding cautiously because Comcast is still trying to determine its space needs.
The company says it has no desire to relocate NBCUniversal studios or broadcasting operations from 30 Rockefeller Center in New York City to Philadelphia, and it is spending millions to modernize its studios at 30 Rock.
But real estate experts say it would make sense for Comcast to move some back-office workers to Philadelphia, where office rents are half what they are in New York. Even the addition of a few hundred new jobs would be a big boost to Philadelphia's housing market.
Sources who follow Philadelphia's real estate market speculated that Liberty was working on a master plan that would allow Comcast to add office space around its headquarters as the need arises. A source close to the project said the company was interested in creating a campus that might include several towers of varying heights.
Liberty controls three important sites orbiting the Comcast tower: a small parcel at 17th and JFK, the long, skinny site on Arch between 18th and 19th, and a parking lot on the northwest corner of 19th and Arch. All the sites are unoccupied.
Liberty has a reputation for working in a deliberate manner. After acquiring the 1700 block of JFK in 2000, it hired Robert A.M. Stern Architects to create a master plan for the block.
The project was conceived as a Rockefeller Center-like ensemble with a tall tower bookended by Suburban Station and a 17-story tower at 18th and JFK. It wasn't until 2005 that Liberty signed Comcast to be the main tenant in the tower. Any campus that Liberty might build for Comcast would be more than a decade in the making.
It was always assumed that Comcast would eventually occupy the lower building at the corner. But now some real estate experts believe the 17-story building would be too small for Comcast's needs.
More than a year ago, Liberty retained the Philadelphia architecture firm KieranTimberlake to design a 25-story speculative office tower at 19th and Arch. Although at least one tenant expressed interest in leasing space, Liberty abruptly pulled the project off the market a few months ago. Gattuso said that Liberty wanted to rethink its strategy.
In the last few days, sources say, the volume of chatter about Comcast's plans has increased significantly.
On Tuesday, Bradley Maule, a writer for Hidden City Philadelphia, wrote that Foster would be hired to design a new tower for Comcast. A winner of architecture's coveted Pritzker Prize, Foster is known for designing skyscrapers that flaunt their technology, including London's bullet-shaped Swiss Re tower, known as the "Gherkin," and New York's faceted Hearst Tower.
Gattuso has been courting Foster for years, and recently took him on a tour of the Navy Yard, where Liberty has erected five low-rise office buildings since 2006.
Paul Levy, president of the Center City District, a downtown improvement organization, said new Comcast offices would be a "huge positive" because Center City has been standing still for years. He estimated the square footage of office space is the same now as it was in 1990.
Contact Bob Fernandez at email@example.com, 215-854-5897. Contact Inga Saffron at ingasaffron@ gmail.com or 215-854-2702.