It has been 15 years since the Philadelphia Navy Yard was turned into a suburban-style office park. Lured by the promise of free parking and easy highway access, dozens of companies now make their home there, employing some 12,000 people. The development is widely considered an economic success.

If your only experience of the Navy Yard has been a fleeting glimpse from I-95, it may come as a surprise to learn that it is also a design success. Not only has the Navy Yard moved beyond the bland office-park model by creating a formal street plan with real sidewalks, but it also is producing some of the best architecture in Philadelphia - better than most of what we're seeing in Center City or University City.

The credit goes entirely to Liberty Property Trust, which controls most of the Navy Yard's new construction, and its vice president for urban development, John Gattuso. Acting as a latter-day Medici, he has assembled an impressive stable of designers and given them license and money to craft buildings that are both beautifully made and rigorously urban.

How is it, I wondered after a recent visit, that Liberty can make such fine buildings in a lonely, far-flung office park when the quality of the architecture in Philadelphia's booming downtown is so relentlessly second-rate?

After all, Liberty has to contend with the same forces that always get blamed for the nickel-and-diming of new construction: high union-labor costs and the city wage tax. As an office park developer, its bread and butter is the long, rectangular mid-rise - an inherently boring form, especially compared to a soaring tower. What's Liberty's secret?

As the builder of Philadelphia's two Comcast towers and, before that, Liberty Place, the development giant can certainly have its pick of designers at the Navy Yard. It got a lot of attention recently for bagging one of the hot architects-du-jour, Bjarke Ingels Group, a Danish firm that goes by the acronym BIG, to design one of those modest mid-rises.

BIG is probably the last firm you'd expect to be doing office-park fare. It has a reputation for designing incredibly complex, wildly imagined, energy-efficient - and sometimes unbuildable - architecture, like the Copenhagen power plant that incorporates a ski slope on its roof. Its Navy Yard contribution, which broke ground last month, won't be quite so sci-fi, but it's a smart design that does the most with the least. That's the starting point for all good architecture.

Located across from a new, amenity-filled park by James Corner's Field Operations (co-designer of New York's High Line), the building's curved facade was shaped to embrace the circular green. Its front wall dramatically rises up, like an ocean wave about to crash over the park, which includes a round running track and disc-shaped flower beds.

BIG's building actually is being constructed quite efficiently, using low-cost concrete panels. The architect arranged them in an interlocking pattern to make the curve, but the side benefit is a textured surface that will dance with the light. There's no reason the same material couldn't be employed on a downtown tower or even a rowhouse project.

Nice as it is, BIG's design won't rock the design world. Yet, other than the Comcast tower Norman Foster is designing for Liberty, there is almost no developer-built architecture in greater Center City that comes close to it in imagination or materiality.

It's true that most new construction is residential, not office, and that difference imposes different cost constraints. Liberty's buildings command some of the highest commercial rents in the city. Still, architects keep producing variations of the same facade: patchworks of metal, glass, and some kind of masonry material - too often, cheap EIFS stucco. There's no texture, no shadow, no life. The flat surfaces suck in light, like a black hole.

Probably the worst offender is the new apartment building at 19th and Arch, by Varenhorst for PMC, wrapped in a jumble of static, blue-gray metal panels. But it gets stiff competition from recent proposals at Broad and Callowhill, by Hanover for Parkway Corp.; at Fifth and Fairmount, by Harman Deutsch for Sean Frankel; and at 218 Arch, another Varenhorst design for PMC.

Liberty didn't have to go to Denmark to find an architect capable of making a moderately priced building that is handsome and well-crafted. Bordering the new park are three excellent examples designed by local firms, Digsau and Erdy McHenry. Like BIG's project, they started out as speculative buildings, Gattuso said, but picked up tenants before construction began: Iroko, Franklin Square Capital, and a Marriott Courtyard hotel.

The first thing you notice about the trio, which face each other at the intersection of Rouse Boulevard and Intrepid Avenue, is that these are handmade objects, not assembly-line productions.

Digsau's new headquarters for Franklin Square is probably the most seductive. Its dark zinc panels have the rich patina of a much older building. The seams were hand-turned in the field, and the panels were hammered to create the texture of an old copper pot. As the sun passes over, the color shifts from purple to khaki. Vertical fins and sloping windows give the long facade a compelling rhythm, not unlike a muscular old factory building.

While Erdy McHenry clad the Marriott in the now-ubiquitous steel panels, it uses them in a totally new way. Instead of simply wallpapering the panels across the facade, the architects worked out an irregular pattern of narrow and wide panels, interspersed with windows of varying widths. What makes the pattern especially interesting is that some panels were also molded into boxes. They pop out from the plane of the facade, adding sculptural relief to what could have been a flat surface.

All three buildings benefit from having transparent, pedestrian-friendly ground floors. Because Liberty is trying to turn Rouse Boulevard into a real urban street, it provided separate entrances for retail. While the current tenants have, unfortunately, leased the spaces for company cafeterias and gyms, the hope is that the spaces will be freed up for public use in the future, as the Navy Yard evolves into a more urban place.

The Navy Yard will never have the energy of Center City, with its rich variety of old buildings, layered by time. But for a place that started out as a blank slate, it offers lessons for how to build new.