Preservation diehards aren't going to like Penn's new Perry World House.

As part of his design for the research institute, New York's David Piscuskas has painstakingly renovated the outside of a charming 19th-century workers' cottage on Locust Walk by the influential Philadelphia architect Samuel Sloan. Everything from the metal ice guards on its faceted mansard roof to its carved wooden porch columns has been polished to perfection.

But here's the thing: After all that effort, the Sloan house - the oldest surviving structure on Penn's campus - can no longer be considered a house. The cottage, which had been used by a fraternity, has been transmuted into a precious object, encased in stone like a fossil, and subsumed by a 21st-century descendant who bears only the faintest family resemblance. All that is really left of the cottage's identity is its front bay, now a mere bulge in the 18,000-square-foot conference center.

The new building born of this unconventional marriage is both a thrilling and deeply thoughtful work of architecture. But it is definitely not preservation as we know it.

In a city that is forever struggling to figure out how to breathe new life into thousands of distinctive, but often obsolete, structures, the design approach used at Perry World House challenges some of our most basic assumptions about what it means to preserve a building. The Sloan cottage now exists in an odd middle ground, neither saved nor destroyed.

Constructed about 1851 by someone named R.D. Work, using drawings Sloan's office provided, the cottage's form has clearly inspired Piscuskas' design. The color of the German limestone facade perfectly matches the cottage's tawny stucco. The new limestone segments are of the same dimensions as the faux blocks Work scored into that stucco. In the biggest tribute to the tiny cottage, the conference center's roof dips and folds to suggest an abstracted mansard.

But that's where Piscuskas' hat-tip ends. Because the two-inch-thick limestone panels are pinned to the building, rather than draped over it like a curtain, the facade is as tight as a drum, the joints between the blocks virtually invisible. The seamless, monolithic container could be an enormous boulder, something conceptual artist Michael Heizer dragged out of the desert, except for the anthropomorphic, metal-framed windows that pop out of the facade. This big mass appears to have gobbled the cottage whole.

Piscuskas, founding principal of the firm 1100 Architects, likens the arrangement to a "collage." As a research institute devoted to global policy issues, funded by Richard and Lisa Perry, Perry World House's mission is to "engage in a dialogue," he says. The architecture reaches for the same result.

Architects often use that phrase when they talk about their preservation work. But in an effort to make peace with the past, designers have come to rely on the same formulas.

If a building's form is particularly malleable, like a 20th-century office tower or a loft building, they will simply renovate it. Once the building is cleaned up and its systems are modernized, it can be adapted for any number of other purposes. In fact, that approach is called adaptive reuse.

In a perfect world, every old building would be renovated when its original use is no longer relevant. But many structures are just too specialized and eccentric and require drastic measures. Inevitably, architects must make compromises, choosing which parts of history to save and which parts to send off to the salvage company.

It's increasingly common for architects to perform a facadectomy on historic buildings, retaining the main facade as a false front for a modern addition. You can see one at the Rittenhouse Club on Walnut Street, now part of the Ten Rittenhouse condos. The facadectomy salvaged a gorgeous beaux arts facade that was once the face of an exclusive eating club. But the operation also masked a bit of fakery, as all the grand rooms, with their fine woodwork and stone mantelpieces, are long gone. Still, the result was far better than a total demolition.

Piscuskas and Penn's campus architect, David Hollenberg, didn't want to perform a facadectomy on the Sloan cottage. For one thing, its facade wasn't at the level of something like the Rittenhouse Club.

At 900 square feet, it was too small to accommodate the research institute. But they were reluctant to raze the cottage, a relic of the days when this part of West Philadelphia was a burgeoning middle-class suburb. Sloan, a frequent contributor to Godey's Lady's Book, America's first shelter magazine, has had a particularly hard time lately, with several of his West Philadelphia buildings succumbing to the wrecking ball.

Piscuskas and Hollenberg found a Solomonic compromise: They decided to retain half the house, the part with the iconic mansard, and demolish the rest. Keeping the cottage's gabled roof, Hollenberg explains, was a signal that the institute would be "an approachable place." It wouldn't be an "austere think tank that had nothing to do with the rest of the campus," he told a Hidden City interviewer last year.

The interior of the $18.5 million research center has been fitted out in cool minimalist style, with only a few hints of the cozy cottage. One is the three-sided window seat tucked into the living-room bay window. The other is the pitched-roof attic. Stripped of its wooden details, and painted white, the room has been reduced to pure abstract form.

Piscuskas offers a nod to the cottage's domesticity in the main conference room, where the glass roof is arranged as a series of gables. But around the back, on the north facade, all evidence of the former house disappears. There, the building becomes a faceted mass rising out of the earth like a rocky outcrop.

This isn't the first time a historic structure has been treated like an object in a museum vitrine. The Swiss architects Herzog & deMeuron gained worldwide attention when they lifted a Madrid power plant off the ground, installed a plaza underneath, and turned it into a museum, the Caixa Forum. In putting the historic building on an actual pedestal, they were making a comment about the way we fetishize old structures.

Caixa Forum is a tour de force, but I suspect most preservationists (myself included) wouldn't be comfortable seeing that approach regularly. It seems pretty unlikely the magic Piscuskas pulled off at Perry World House can be easily replicated on other Philadelphia buildings. But Perry World House accomplishes what a good research center should: It opens our minds. Preservation, we should remember, can take many forms.