Keystone opens 400 Market Flats, bringing 176 residences to Independence Mall District

Old City office to residential conversion is 40% leased at opening

PHILADELPHIA, PA – Keystone has announced the opening of 400 Market Flats, a newly converted 176-unit residential community in the heart of Philadelphia’s Independence Mall district, offering a first look at one of Center City’s most prominent office-to-residential transformations. The project marks Keystone’s second office-to-residential conversion in Old City, continuing the firm’s track record of repositioning underutilized commercial properties into high-quality rental housing in one of Philadelphia’s most historically significant neighborhoods.

The historic 12-story building is already 40 percent leased, a strong signal of pent-up demand for high quality rental housing in the Independence Mall corridor. Designed by Tantillo Architecture and constructed by Fasttrack Construction, 400 Market offers a mix of modern studio, one-, and two-bedroom apartments with sweeping views of Independence Hall, the Benjamin Franklin Bridge, and the Delaware River waterfront.

At the heart of the 173,700 s/f building is a reimagined lobby that blends its Brutalist roots with a boutique hotel sensibility. A striking concrete sculpture by Costantino Nivola—specifically commissioned for the building as part of the 1970 construction—anchors the space. Wood-clad ceilings, paneled columns, and veined marble soften the structure’s bold geometry, while restored travertine and limestone floors ground the design in its heritage.

Residents have access to a range of amenities, including lounge and co-working spaces on the ground floor, a 2,500 s/f rooftop deck with unparalleled views of the Ben Franklin Bridge and Delaware River waterfront, and approximately 30 underground parking spaces. The building also features pet-friendly amenities, including a dedicated dog wash and a rooftop dog run. PNC Bank will continue to anchor the corner retail space at 4th and Market Streets.

The redevelopment reflects Keystone‘s broader investment in the Independence Mall area, bringing new residential energy to a historically significant and evolving neighborhood. Keystone acquired 400 Market Street in 2024 in partnership with Lubert-Adler.

“400 Market Flats reflects our belief that great historic buildings can evolve to meet the needs of today’s city residents,” said Rich Gottlieb, president and COO of Keystone. “We preserved the character and architectural integrity that make this property distinctive while creating modern homes in one of the most important and dynamic neighborhoods in Philadelphia. The strong leasing activity at opening confirms the demand for this kind of thoughtfully designed housing in Old City.”

400 Market Flats represents the newest chapter in Keystone’s continued investment in the Independence Mall district, where the firm’s portfolio also includes The Bourse, 100 Independence, The Washington, and The Curtis. Keystone is currently adding 44 residential units at The Curtis to bring the property to 130 apartments, while plans at The Washington call for a partial residential conversion that will create 200 additional units.