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Photos: Mariko Reed

By the time builder/developer Onion Flats broke ground on Thin Flats, an infill project in the Northern Liberties section of Philadelphia, the company--run by brothers Tim McDonald (an architectural designer), Pat McDonald (a master plumber and certified green roof and solar installer), Johnny McDonald (a licensed Realtor), Mike McDonald (a green builder in San Francisco), and their good friend and business partner, architect Howard Steinberg--had already earned a reputation for buildings that were transformative. Their first venture converted a former meat-packing plant into eight funky residential units, putting the company on the map as a catalyst for urban revitalization.

Next came a four-unit structure, which, while honoring the rhythm and scale of neighboring brick row homes, presented a wholly contemporary riff on the skinny building type. Then they went even skinnier—or so it would seem—with Thin Flats. Each of the four duplex units is 18 feet wide (standard width for row homes), but the façade of glass, metal, and wood composite panels plays tricks with exaggerated proportions that make it impossible to tell where one unit ends and another begins.

“It really was a playful investigation of what it would be like if you collected these thin slits of space and pulled them all together,” explains Tim McDonald.

Rent or Own Units in Thin Flats are 65 percent more energy efficient than required by code. Rental units go for $2,000 to $3,000 per month, while the average price tag on a for-sale unit is $680,0000.

Photos: Courtesy Plumbob/Onion Flats

Rent or Own Units in Thin Flats are 65 percent more energy efficient than required by code. Rental units go for $2,000 to $3,000 per month, while the average price tag on a for-sale unit is $680,0000.

In fact, the units aren’t as pencil thin as they appear. The dynamic south elevation is a façade, separated from a second layer of skin by a 3-foot air space. The gap creates balconies, but also serves as a thermal chimney that draws warm air up and out. This ingenious bit of engineering, combined with thermally broken windows, a rainwater catchment system, solar hot water, a green roof, and a handful of other eco-features, snagged props for the project as the first LEED Platinum multifamily building in the state.

Still, eight was not quite enough of a unit count. So last year the team completed construction on Number 9—a single-family corner unit, clad to match, that’s separated from Thin Flats by an existing brick building. It’s a nimble example of an urban fabric that is being stitched back together.

The company's business structure is just as organic. The design-build-development operation, Onion Flats, serves as an umbrella for several offshoot LLCs, including JIG, a licensed general contractor; Plumbob, an architecture firm; and GRASS, a green roof, solar, and stormwater management outfit. The latest addition to the fold, a modular building company called BLOX, is poised to take on some larger, multifamily infill projects.

But it’s unlikely those will be built in the same neighborhoods as the boutique ventures that have preceded them. Having seeded a swirl of revitalization activity in Northern Liberties and in the Old City part of Philadelphia, the team is now looking for its “next sandbox.” “We started our work in Northern Liberties when it was just an abandoned meat-packing plant. But when people really started to come, we knew we needed to leave,” Tim McDonald says. “The sandbox has got to be pretty cheap in order for us to do what we do. Once you see a BMW come in, you gotta go.”

This story has been updated as of July 15, 2011.

Learn more about markets featured in this article: Philadelphia, PA.