Comcast Corp. plans to remain a big renter of Philadelphia commercial space even after moving into the new Center City skyscraper being built for the technology-and-entertainment giant, cementing its status as the city's leading office user.

The company plans to increase its footprint at the Three Logan Square office building at 1717 Arch St. by 70,000 square feet to occupy 300,000 square feet. The deal extends its lease at the 57-story, red-granite tower through 2023, spokesman John Demming said in an interview this week.

The roughly one million-square-foot high-rise stands across Arch Street from the Comcast Center headquarters building and diagonally across an intersection from the new Comcast Technology Center tower under construction nearby.

The lease extension keeps Three Logan Square in place as part of a growing campus-like enclave dominated by Comcast at the core of central Philadelphia's main business district.

"The goal for them has always been to create a vertical, integrated urban campus," said Lauren Gilchrist, Philadelphia research director at the commercial real estate firm JLL.

Factoring in the 290,000 square feet that Comcast will continue to lease in other Center City office buildings and in the 1.3 million-foot Comcast Center, the company's office footprint will reach 3.2 million feet next year when it and its affiliates begin using the new CTC tower. That will easily make the company the city's biggest user of office space, Gilchrist said.

Comcast is probably retaining its current Center City offices so space at the CTC can be reserved for the influx of engineering and programming muscle it will need to develop new technologies for expanding businesses, such as its broadband networks, said Paul Sweeney, a media-industry analyst with Bloomberg Intelligence.

Planned for the 60-story CTC are offices and labs for Comcast's growing workforce of technologists and software architects, as well as studios for its local NBC and Telemundo affiliates. Comcast has said about 4,000 people will work at the new tower, similar to the number of workers at Comcast Center.

"The new tower reflects [Comcast's] investment in technology," Sweeney said.

Before its recent extension, Comcast's lease at Three Logan Square had been set to expire at the end of 2018. The tower was built in the early 1990s as a corporate headquarters for Bell Atlantic Corp., one of the companies that eventually merged to form New York-based Verizon.

David Binswanger, chief executive of real estate services firm Binswanger Management Corp., represented Comcast in the lease extension.

Comcast's other offices are at the 709,000-square-foot Two Logan Square building, across 18th Street from Three Logan Square, where it occupies 200,000 square feet, and at the 1.8 million-square-foot Centre Square office complex near City Hall, where it leases 90,000 square feet.

The company is also thought to have a third tower of its own in mind on the block bounded by 19th and 20th Streets, between Arch and Cherry, where it has been acquiring property in collaboration with Malvern-based Liberty Property Trust, developer of the CTC and Comcast Center.

Liberty further expanded its holdings in the area July, when it acquired a four-story office building and adjacent parking lot spanning 1926 to 1936 Arch St. for $15 million, according to records filed with the city.