XS House is a six-storey apartment building in the city's Chinatown neighbourhood, alongside a busy highway called Vine Street Expressway.
It is located on a narrow plot of land that measures 11 feet (3.3 metres) by 93 feet (28 metres) and was previously a discrete, disused site for parking cars.
ISA built the rectangular residential complex on the site to include seven apartment units inside, with varying layouts.
"The site's extremely narrow footprint was expanded with strategic use of bays, mezzanines, and bi-level upper units while maintaining a minimal single-stair core layout," said the local studio.
"The existing lot was so marginal and small it was barely noticeable as a development parcel," ISA added.
"Originally used for informal surface parking for two cars, the lot now provides seven apartments."
In the basement is a two-bedroom unit that spans the length of the building. On the two uppermost floors are two, two-storey units.
Both the ground-floor unit and the one above feature mezzanines designed to be bedrooms, and open-plan living and dining rooms on their main levels.
In addition to increasing the number of floors allowed for the building, ISA also optimised the possible width of the structure.
"Despite the tiny scale of the site, facade projections permitted by the city zoning code were utilised to add up to three-feet of width to the horizontal dimension of the building," ISA said. "Mezzanine levels expanded many interior units by about 1/3 of their floor areas."
A circular stairwell is positioned in the middle of XS House to connect all of the levels of the upper units, while lower units enter directly from the street. Each residence also has their own internal staircases.
XS House measures 5,000 square feet (464 square metres) and is clad in black siding on its long section, while its narrow portion is fronted with a pale grey material.
Across the structure are several horizontal and rectangular windows, providing ample natural light inside, while different portions of the building protrude out to the street.
Inside, the apartments are minimally detailed with white interiors and pale wood floors. The public stairwell, however, has bold yellow walls and grey carpet.
ISA, which stands for Interface Studio Architects, is a small architecture studio in Philadelphia's Olde Kensington neighbourhood led by Brian Phillips and Deb Katz.
Photography is by Sam Oberter.
Developer: Callahan Ward
General contractor: Callahan Ward
Structural engineer: Larsen & Landis Structural Engineers
MEP engineer: J+M Engineering
Lighting: Lam Partners