Photo: Courtesy of One Jackson Square

How could they let that happen?” The indignant thought asserts itself whenever a flashy architectural stranger swaggers into a polite, low-lying part of town. The exhibit “Context/Contrast: New Architecture in Historic Districts” at the Center for Architecture, helps explain. The show, which begins with the passing of New York’s Landmarks Law in 1965, chronicles the ensuing effort to avoid either embalming or obliterating our urban heritage. It makes the point that often, the middle course runs through bold intervention. Nothing throws history into relief like the judicious application of newness.

We have the Landmarks Law to thank for the principle of “appropriateness,” which governs what can be built in historic districts. That’s a maddeningly mushy term, vulnerable to the whims of commissioners, but its vagueness is its strength. Property owners see the commission as obsessively obstructionist, fussing over the minutiae of roofing materials, while preservationists see it as weak-kneed, unwilling to let a big-money real-estate deal die. Proponents of new architecture detect a creeping blight of nostalgia at work as Landmarks continues to expand and carve out new historic districts.

But Landmarks doesn’t just enforce the status quo. Preservation and innovation aren’t mutually exclusive, and never were. No sooner was Brooklyn Heights designated historic in 1965 than the fledgling commission green-lighted Ulrich Franzen’s elegant, sere, and implacably modern design for the Jehovah’s Witnesses. The commission realized early on—though at times it has forgotten—that copying ye olde dormers or pasting shards of antique façades onto new buildings threatens to turn the city into a reliquary, killing old architecture with unthinking conservatism. A flexible definition of appropriateness recognizes what makes a district historic. Soho, for instance, retains its industrial bones and allows for the new, like Jean Nouvel’s 40 Mercer and Aldo Rossi’s Scholastic Building. Both play with homage to their cast-iron ancestors.

Far better—in certain areas, at least—to sharpen the borders of history with a keen glass edge. That’s what’s happening at the Central Park Precinct, where the courtyard in the stables from the 1870s is getting a modern canopy. The new condo at One Jackson Square twists around the edge of Greenwich Village with grid-defying bravado and plenty of unquaint glass. The design defines the border of a historic district, turning a more or less arbitrary line on a map into an urban fact.

The designation of a historic district can be an invigorating force for modernity, because it insists that architects think beyond the borders of their single building and engage the city at large. This can happen only if the commission matches its regulations with a muscular advocacy of good new design. Often, it sends mixed signals, urging courage but rewarding caution and accepting mediocrity. The current development ice age gives New York a chance to consider what kind of city it wants to be when the money starts oozing and the earthmovers slouch again. We should use the pause to take a hard look at the city’s protected neighborhoods, not to savor their timelessness, but to imagine how we want them to change.

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