The city and state have committed $40 million in grants to the project, officials said. At 59 stories, the new tower will have about 1.5 million square feet of rentable space.
The building will at least initially be called the Comcast Innovation and Technology Center. Its structural core will run the vertical length of one side of the see-through, all-glass tower, instead of through the middle like a more traditional skyscraper.
There will be 13 "sky gardens," or three-story atriums, for software designers, engineers, and product developers to collaborate, and the building will be topped by a blade extending about 150 feet higher than the Comcast Center. Instead of offices, the building will be mostly broken into "lofts."
Comcast and Liberty Property Trust, its partner, have reached an agreement to move the Four Seasons Hotel at Logan Square to the new tower's top floors. One entrance will be designated for the 200-room hotel and a second entrance will be for Comcast employees. Separate banks of elevators will serve both entrances.
Brian Roberts, Comcast chief executive, said at the news conference that he expected the new Four Seasons to be one of the finest hotels in the world. The existing Four Seasons on the Benjamin Franklin Parkway then will not operate as a Four Seasons but could be rebranded.
NBC10 and the local Spanish-language Telemundo TV station (Channel 62), both owned by Comcast through NBCUniversal, will move their studios from Bala Cynwyd to the new building.
In addition, the Suburban Station concourse will be extended a block to the west beneath 18th Street.
The massive project is expected to jolt Philadelphia's lethargic economy with about 6,300 temporary construction-related jobs and an additional 14,400 jobs in Pennsylvania.
To help pay for the concourse extension and other public-space improvements, the state has committed $30 million in economic-development grants, and the city has said it will contribute $10 million, Comcast officials said.
Liberty is expected to seek five ordinances from Council to proceed with construction. Those include recalibrating the commercial zoning on the block, approval for the building height, and the right to tunnel under 18th to extend Suburban Station.
Comcast has agreed to an addition of 1,500 jobs in the city connected to the tower and to lease 75 percent of the building, bringing its Philadelphia payroll to about 6,300 employees. Company officials believe this is a conservative estimate of the projected job growth at the company over the next several years, and pointed to the experience with the Comcast Center.
Comcast initially agreed in the early 2000s to lease only 40 percent of the current Comcast Center. Now it occupies 90 percent and leases offices around Center City for an additional 1,000 employees.
The Comcast Center, a Robert A.M. Stern-designed obelisk that dominates the skyline, was mostly Liberty's project.
The new building's adventurous architecture will be a public statement of the company's evolution from a low-tech cable-TV provider to a technology developer with video streaming, Internet products, and mobile apps.
"This will not be a twin," Roberts said, referring to the Comcast Center. "We wanted to create an environment that's better than anywhere else. We are a different company than we were a few years ago. We are competing for talent all over the world, and we have to offer something special."
Roberts said his vision is for a "vertical urban campus" that could compete with the traditional Google or Facebook suburban-style headquarters.
Foster, who competed for the second Comcast tower along with four other prominent architects, is developing a new Apple Inc. headquarters, Roberts said. Apple manufactures the iPad and iPhone, through which Comcast now streams its Xfinity content.
Some had feared that Philadelphia could be marginalized by New York City, Los Angeles, or Silicon Valley after Comcast's acquisition of NBCUniversal. Comcast bought 51 percent of NBCUniversal in early 2011 and the remaining 49 percent in 2013.
But the new tower emphasizes Philadelphia as Comcast's administrative corporate headquarters and home to its top executives, and consolidates its position as the company's technology hub. Other geographic areas with Comcast technology operations include Denver, Silicon Valley, and Northern Virginia. Comcast also hires software writers in India and China.
In Philadelphia, Comcast has scattered software engineers and technical employees in floors at the Comcast Center and offices around Center City. With the new building, employees can all work and interact together.
Comcast is projected to have more Internet subscribers by next year than cable-TV subscribers. It's also developing WiFi upgrades and "TV Everywhere" streaming.
As part of the building project, which has been discussed since early 2013, Comcast commissioned an independent economic impact study.
"The arrival of the proposed expansion development would enable the retention of Comcast's growth within the city and the commonwealth, the relocation of economic activity into the city and commonwealth, and the provision of sorely needed 'trophy'-level office space within Center City," said the report by Econsult Solutions Inc.
Comcast will own 80 percent of the $1.2 billion project and Liberty will own 20 percent. In 1984, Liberty founder and chairman Willard G. Rouse III proposed breaking the Philadelphia "gentleman's agreement" height limit with One Liberty Place. The limit had been the top of William Penn's hat on the City Hall tower.
William P. Hankowsky, chairman and chief executive of Liberty Property Trust, said he could think of no other U.S. development firm that had redefined a major city's skyline as Liberty has done in Philadelphia with Liberty Place, the Comcast Center, and now the proposed tower. Hankowsky is a partner in the firm that owns The Inquirer.