Step a bit closer and you might instead conclude that you've stumbled onto Central Park West, so furious is the onrushing stream of blondage, 6-inch heels, and glossy shopping bags into the building's very proper lobby. But should you reverse direction and widen the view, expect all your fantasies to puddle like sorbet in the August heat.

Who plopped the discount store on the roof of the historic former Penn Athletic Club?

With the repurposing of this once-dowdy apartment house and hotel into the chic Parc Rittenhouse condos, Philadelphia now has a graphic demonstration of one of the paradoxes of contemporary design: Though interior decorators are capable of re-creating an authentic 1920s Paris bistro right down to the tobacco-stained paint, our architects can't seem to fashion a credible-looking penthouse structure on an American building of the same vintage.

The three-story addition, which overlooks Rittenhouse Square and features three immense raw lofts that are going for $7 million apiece, is papered together with bland stucco panels and has all the aesthetic finesse of a parking garage. It was designed by Jim Garrison of Hillier in a contemporary style, ostensibly so it could be differentiated from the original 1927 art deco building by Zantzinger, Borie & Medary, one of the architects for the Philadelphia Museum of Art.

The new penthouse structure is different all right, but in all the wrong ways. The rooftop box is the sort of disaster, like a particularly bad car crash, that makes you wish you could rewind history, to understand how things got so out of control.

Garrison is a well-regarded preservation architect who has turned out some of the best work in Hillier's Philadelphia office. He presided over the sparkling restoration of Girard Trust Co., now a Ritz-Carlton Hotel.

It's evident from his lovely restoration of the Parc Rittenhouse's original 15 floors that he has a deft hand with historic buildings. He fiddled significantly with the facade's original design, sneaking in French windows, new balconies, steel railings and spandrels. Those elements look absolutely right, even though the balconies and spandrels are high-density urethane - plastic, that is - faux-painted to look like real stone.

Clearly, though, replicating the past requires skills different from inventing something from whole cloth. It may not have helped matters that the city's Historical Commission encouraged a contemporary addition that has virtually no grounding in traditional architecture.

The thinking - standard in preservation - is that new structures should look new, and not attempt to flimflam the viewer by mimicking the past. But Garrison and developer Allan Domb complain that the demand was especially hard to fulfill at the Parc Rittenhouse. At one point, they even submitted a traditional design with rows of indented arches. It was rejected in favor of the adobe-village approach.

Garrison did succeed in borrowing one element from the past: Like the limestone crown on the Rittenhouse Plaza and other early skyscrapers, the Parc's penthouse is a different shade from the rest. But there's not enough contrast and, unlike those old Gothic crowns, the rooftop doesn't reference elements below. It just sits there like a cowboy hat on a bride.

Would things have turned out better if the Parc Rittenhouse had been allowed to go traditional?

Only if designers could have used the same hefty materials and high standards of craftsmanship that were the norm in Zantzinger's day.

The real villain, I'd argue, isn't modern style; it's modern materials. The Parc Rittenhouse's original facade is a rich tiramisu of variegated, caramel-toned brick and terra cotta. The penthouse panels have no depth.

There's no doubt that certain synthetics, like the high-density urethane Garrison used for the balconies, work just fine when judiciously applied. But when an entire structure is clad in an artificial material, our high-priced condos tend to display a family resemblance to big-box stores. The balconies look good because they don't call attention to themselves. Unfortunately, that penthouse won't let us alone.

These last years have been the best and worst of times for Philadelphia architecture, with dozens of new towers and townhouses filling out the city. But rapid increases in material costs and inflated wage rates prompted developers to cut costs where they could - on design. It's bad enough to see the cheapening in isolated subdivisions, but it's an assault on the public realm when the same low standards make their way to Rittenhouse Square.

The Parc Rittenhouse isn't the only offender overlooking our urban living room. Now that the factory-made brick panels are being wallpapered onto Ten Rittenhouse, a luxury condo by architect Robert A.M. Stern, the fakery there is evident. The joints between the panels slice randomly through the limestone-colored lintels and horizontal coursing, reminding us that corners both literal and figurative have been cut.

Philadelphia has never been a city of architectural icons, like New York and Chicago. What it offers is something more down to earth and, in a way, more precious in modern America. Our buildings are the real deal, crafted by men who laid brick and cut stone. Because of their solidity, the fake stuff looks worse here.

This isn't to say that Philadelphia must take refuge in brick, the original bargain material - although brick can be beautiful if used with integrity. Some of our most appealing new buildings, like Skirkanich Hall at Penn and the Murano tower on Market Street, include ample amounts of glass, concrete and metal. What makes those two examples work so well in our city of stone is the same thing that made our historic architecture work: craftsmanship.

Changing Skyline:

Inga Saffron blogs about Philadelphia architecture at http://go.philly.com/skyline.