This could be the skinniest new apartment building in Philadelphia | Inga Saffron

Like those spies in Mission: Impossible, architect Brian Phillips has never shied away from a challenging assignment. He didn’t blink when an East Kensington developer asked if he could design a rowhouse that could be constructed for $100,000. He managed to slot a group of affordably priced twins onto an overgrown alley in North Philadelphia. His firm, ISA Architects, even found a way to create a relatively quiet apartment house next to the rumbling Market-Frankford El in Fishtown. (Put a false front on it.)

But when a developer approached Phillips and ISA principal Deb Katz about designing apartments for an 11-foot-deep strip of land in Chinatown, they hesitated. “My initial response was, even we’re not crazy enough to try that,” Phillips recalled.

Of course, they could be persuaded.

Making a virtue out of the site’s limitations, they have produced a wafer-thin building that appears to defy both gravity and common sense. Narrower than a standard Philadelphia trinity, but twice as tall, the sliver-slab structure at 12th and Vine somehow squeezes seven apartments onto its 83-foot-long site. ISA, which likes to give its projects names, calls it the XS House, for obvious reasons.

To get an idea of just how small this building is, consider this: The total amount of floor space in the seven apartments adds up to 5,000 square feet, about the size of a single McTownhouse in Center City. On its east and west facades, the building is narrower than the light mast on top of the new Comcast Technology Center by one foot.

ISA’s design is an extreme exercise in packing 10 pounds of stuff into a five-pound sack. Before the 1,100-square-foot strip was acquired for $255,000 by developers Marcus Toconita and Nino Cutrufello of Callahan Ward, it had been used as a two-car parking lot. No one believed that the site, which overlooks the great gulch of I-676, was buildable, even though it is zoned for a skyscraper.

Similar scraps of wasted land exist across the city, often in desirable neighborhoods, yet they sit fallow because no one can figure out what do with them. What if their potential could be harnessed for infill housing? The XS House demonstrates that salvaging leftover parcels isn’t as difficult as it looks. Because these land remnants often go for bargain prices, the sites could even be a prime source of affordable housing.

The I-676 corridor would be a particularly good place to start. During the decades of planning and construction for the crosstown expressway, dozens of buildings were leveled along Vine Street. But the road ended up being narrower than PennDot had intended, and the city was left with an awkward strip of vacant land along the perimeter. While some parcels were later sold to private owners and turned into parking lots, the absence of habitation has served to accentuate the gulf between Center City and the neighborhoods north of I-676. Putting buildings back on Vine Street would make the journey across that highway canyon feel less daunting and help bind together those severed neighborhoods.

That’s not exactly what Toconita and Cutrufello were thinking when the 1,157-square-foot site at 12th and Vine came on the market in 2016. Their instincts simply told them that the location should be more than just a parking lot.

There were already signs that the Vine Street corridor was coming back to life. The opening of the Goldtex apartments in 2014 established a vibrant beachhead on the north side of the expressway. The Chinatown Development Corp. was then starting work on a 20-story tower nearby, at 10th and Vine.

Even though the southern edge of Vine remains rough, Toconita and Cutrufello felt their project could take advantage of the convenient location. It’s an equally short walk to City Hall and the new Rail Park. It was just a matter of figuring out how to build on an 11-foot-wide lot.

Phillips and Katz agreed to take the job once they realized that the apartment house needed only one staircase. Normally, multiunit buildings are required to provide two, the main one and a spare to be used in case of an emergency. But Philadelphia’s code treats small apartment buildings the same as single-family homes. Freed from the burden of cramming a second staircase into the building, they suddenly had a lot more room for living space. But their structural engineers, Larsen & Landis, told them they would have to use a steel frame to ensure the skinny building stood up straight.

ISA kept the interior organization simple and symmetrical. The building is three tall stories, reaching 70 feet. Each floor has two apartments, one on each side of the staircase. Believe it or not, the seventh apartment is tucked into the basement. The unit receives a modest amount of natural light from light wells positioned at the ends of the building.

At less than 500 square feet, the apartments are essentially micro-units, but their double-height ceilings make them feel almost spacious. The units are tall enough to accommodate mezzanines for sleeping lofts. On the exterior, an interlocking arrangement of bay windows brings in ample natural light and extends the rooms.

To create a sense of openness, Phillips and Katz had to be extra disciplined about amenities. Closet space is minimal, and the tiny bathrooms are equipped with shower stalls instead of tubs. “We took a lot of inspiration from Japan,” which is famous for its small, efficient housing designs, Katz explained.

While Toconita and Cutrufello haven’t started renting the apartments, they expect to market the building as a “Center City location with neighborhood prices.”

What the building lacks in size, it makes up for in design sophistication. The facade is dressed in cool gray, fiber cement panels. Narrow channels between the panels and oversize button screws add texture, as do the asymmetrical array of bays. The end walls are done up in corrugated metal, an ISA signature that is an antidote the flatness found on so many new Philadelphia buildings.

Working on small sites is becoming an ISA specialty. Before the firm took on this project, Toconita and Cutrufello commissioned it to design what they call a “mini skyscraper,” a four-story rowhouse that sits on a minuscule 12-by-29 lot on Harper Street, one block south of Girard Avenue in Fairmount.

The site was another bit of waste ground rescued by the developers. Once occupied by tiny trinities, the homes on this stretch of Harper had been gradually demolished and turned into driveways by homeowners on the next block, Cambridge Street. The new house, dubbed “Tiny Tower” by ISA, stands alone amid a row of parked cars.

The developers were able to construct the 1,200-foot-square house for $250,000, including land costs. The most expensive element was a $50,000 staircase, an origami-like ribbon of steel that was fabricated off-site and painted a gleaming white. Pressed against the front facade, the custom structure allowed ISA to keep the rooms open and airy. Like traditional trinity houses, the kitchen is in the basement.

In many ways, the Tiny Tower was a tryout for the XS House. At time when skyrocketing land prices are making it harder to build affordable housing, the two projects offer lessons in how to make the most of that limited commodity.

As Mark Twain once advised, “Buy land; they’re not making it anymore.” And then build a really small, efficient, well-designed house on it.