This article originally appeared on PlanPhilly.

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Unexpected houses, cozy little homes tuck into every fold of Philadelphia. Row homes dating back centuries line alleys too narrow to comfortably fit a car. Tiny trinities with one room on each floor hide in courtyards and contribute to the city’s old-world charm.

These diminutive domiciles rose mostly in the 18th and 19th centuries, before cars dominated and when a growing population wanted affordable shelter within walking distance of waterfront docks, markets, and the downtown buildings. In other words, it was a time like 2019.

And now, a new generation of uniquely Philadelphian homes is growing as architects confront a dwindling supply of available land in growing neighborhoods and seek ways to squeeze housing into what architect Brian Phillips, founding principal of the firm ISA, calls the “leftover lots” — the alleys, the side yards, and yes, the courtyards.

“Once developers come through and do all the easy stuff, there’s a bunch of sites left over that are harder to use and require a little energy on the design and construction side,” said Phillips. “We’ve been exploring this idea of the leftover lot, these projects that represent a deeper dive into the fabric of Philadelphia.”

On Friday, ISA will debut a celebration of the city’s most ubiquitous housing typology — the Rowhouse Workshop exhibition — at Cherry Street Pier. The interactive exhibit celebrating four rowhouse blocks comes just two months after ISA won a Housing Award from the American Institute of Architects for a slender new take on the old old Philly row they call the Tiny Tower.

ISA’s tower stands on a narrow back street of Brewerytown, a North Philadelphia neighborhood that’s experienced extensive reinvestment in recent years. Rowhomes line the west end of the block, but vacant lots surround the ISA lot, on the southeastern portion of the block. The roadway is so tight that cars must ease down it, creating a nice quiet block sheltered from bustling Girard Avenue to the immediate north. Developers have shied away from those lots because of their tight confines, but Callahan Ward Companies, the developer that commissioned ISA for the tower, decided to take a risk with a 12- foot-by-29-foot lot.

“Most of the parcels on the south side were so small that the owners haven’t known what to do with them,” said Deb Katz, a principal with ISA who designed the Tiny Tower with Phillips. “Historically, there were trinities on these lots, but they were torn down a long time ago. Most of the lots are vacant and are being used as parking spaces.”

ISA built a house that may not look like the homes that used to stand there, but functions similarly. The Tiny Tower is essentially a trinity, with one room on every level (not counting the bathrooms) and a kitchen in the basement. But it also provides modern amenities, like a roof deck, and its stairway is navigable with contemporary furniture. (Owners of older trinities often have to saw their box springs in half to get them up the tightly winding staircases.)

Other recent projects by ISA and Callahan Ward include a seven-unit apartment building on a sliver of land in Chinatown that previously served as a parking space for two cars. Another 30-unit apartment building the firm is designing in West Philadelphia will rise in an interior lot effectively in the backyards of three separate rows of houses.

Houses that reflect the city’s architectural heritage — and possibilities for its future

By filling in the “leftover lots,” ISA is solving the same problem that back alley rowhouses did 200 years ago.

“When William Penn originally laid out the city, it was not meant to be crowded at all, and every person who bought from the developer would have a house on a lot surrounded completely by a big yard,” said Rachel Schade, author of the city’s Rowhouse Manual. “So the whole rowhouse thing was not part of what Penn envisioned.”

But it quickly became clear that the Philadelphia Penn planned, with big city blocks lined with large houses shrouded by green space, could not accommodate a growing population. Infill was needed, and so alleys lined with trinities and other smaller rowhouses sprouted between the larger homes of Penn’s “green country towne,” allowing working-class residents to live close to their jobs.

For ISA, bringing this kind of design back into vogue doesn’t just allow daring developers to fill out weirdly shaped vacant lots. In a city like Philadelphia, one of the few places in America with miles of walkable neighborhoods and a healthy transit system, Phillips says that nearly every parcel of vacant land should be considered for re-development.

“Philadelphia has one of the most extensive, densest infrastructures in the country and for its fabric to be underutilized is very unsustainable,” Phillips said. “People in cities are shown to have much smaller carbon footprint. A super green house in the hinterlands is less green than a normal house in the city. So we are very committed to creating as may housing opportunities within the existing infrastructure as possible.”

There is a degree of affordability that can be baked into these kinds of projects as well — odd lots typically cost less than prime ones and houses sell by square foot.

Phillips credits their ability to experiment with innovative infill design to Philadelphia’s 2012 zoning code reform, meant to encourage new development and increased density in some areas. The old code encouraged developers to include parking garages that opened onto the sidewalk, an addition that would have made the Tiny Tower essentially impossible to build without a variance.

Several of ISA’s other recent projects would have also been harder to build under the old code. None include parking. Schade said that the buildings reflect the city’s architectural heritage — and the possibilities for its future.

“I’m crazy about the work Brian and his group are doing,” says Schade. “There’s so much crud out there. They are doing something and fresh and new and creative. They do a much better job than most people you see at rowhouse design.”

Over the course of a tour of Tiny Tower, Phillips drops his favorite facts about the city. For instance, we pull more block party permits than any other major metropolis. He attributes these happy quirks to the city’s unique built environment.

“The social contract of Philadelphia is that you give something up to live like this,” said Phillips. “There are other places you can have houses that are 20 feet wide. By giving that up something glorious happens, which is that you share sidewalks and people are walking around.”