Since making his inauspicious debut on South Broad Street in 2007 with the pink-hued, milk-bottle-shaped Symphony House, developer Carl Dranoff has gone on to do something that once seemed improbable: He has resurrected a big stretch of the battered commercial street as a residential boulevard.

A canny developer, Dranoff seems to possess a sixth sense about where the real estate market will go next. He gets his urbanism mostly right, by packing the ground floors with generous commercial spaces and finding tenants to turn the lights on. But architecturally, his growing collection of condos and apartment houses has been a mixed bag.

His follow-up to Symphony House, a mid-rise called 777, drips with Art Deco-inspired bling, while his latest, Southstar Lofts, is shaping up to be a rather staid white box. It's as if his South Broad is still trying to figure out what it wants to be when it grows up. Grand boulevard? Generic apartment row?

Now Dranoff is back with a fourth project near Spruce Street and it strikes a pleasingly different note. At 567 feet, it will be the tallest high-rise built just for residential use in the city. Although it's been clunkily named SLS International, the beanpole of a tower is the most sophisticated design Dranoff has ever commissioned.

If you want proof that architecture follows the money, here it is. Unlike Dranoff's earlier projects, which were on the southern, untested fringe of downtown, it occupies one of Center City's most glittering crossroads, across from the Kimmel Center and steps from the Wilma Theater. The official budget is $200 million.

Dranoff is clearly out to fashion South Broad into a desirable address and recognizes that pink concrete won't cut it anymore. So instead of the usual suspects and the usual designs, he sought out a top New York architecture firm, Kohn Pedersen Fox Associates, which has built skyscrapers around the world.

Known for producing refined buildings with strong, legible shapes - such as Chicago's 333 Wacker Drive and Shanghai's World Financial Center, the tallest building in China - the firm also has left its mark on Philadelphia. The quality here runs the gamut, from the stately Logan Square office-and-hotel complex, to the more ponderous BNY Mellon Bank Center on Market Street, to the airy cube of Children's Hospital's main pavilion.

From indications so far, the long-limbed, 47-story SLS tower promises to be in the refined and elegant group. Like Logan Square, it's a luxury hybrid, combining a 149-room boutique hotel and 125 condos. But instead of separate elements, KPF stacked the pieces vertically, with 28 floors of condos on top of the hotel. Combining two uses into one is what enabled Dranoff to build a tower that hovers above the competition.

KPF follows the current fashion and dresses the exterior in glass, but it's more than a simplistic, straight-up pile of floors.

KPF envisioned the tower as a composition of interlocking boxes. It's a visual trick, achieved by cantilevering the north and south walls out from the central shaft in asymmetrical sections. They jut out only about five feet, but it's enough to provide a sense of depth and scale that you don't get with slick glass walls.

KPF's puzzle box doesn't just sit statically on its podium; it appears to slide over it, like a flash drive plugging into its USB port. The podium, which will house a parking garage and hotel meeting rooms, also gives the impression that it's made of interconnected boxes. Scrims of dark terra cotta will pop out from the glass to screen the parking decks.

It's true there is a whiff of the corporate in the design. The immense hotel insignia that is shown plastered on the podium and near the crown certainly doesn't help. (Is there a brand consultant in the house?) But there is still time for the architecture to settle down into something a bit more homey. As we learned at Symphony House, which was originally depicted in a buff limestone color, material choices will be crucial.

If it's anything like the rendering, SLS will be far better than any of the city's new high-rises. Dranoff and KPF deserve props for positioning the tower a respectful distance from both Spruce Street's Center City One condos and the lacy Gothic facade of the Broad Street Ministry, housed in the former Chambers-Wylie Memorial Presbyterian Church. The advantage of thin towers is that they don't completely block out the sun or views.

It helps that SLS is lean as well as tall. The tower floors will be just 7,500 square feet, making it as leggy as a fashion model. Just a decade ago, Philadelphia condos were far bulkier. The St. James' floor plates are 16,800 square feet, the Murano's 11,200. For all that height, SLS has fewer units than 10 Rittenhouse.

But packing in the same number of units requires more floors. Philadelphia's residential high-rises have been gaining an average of 100 feet in every building cycle - going from 300-footers in the 1980s, to 400-footers in the early 2000s.

Now that we're into the 500-foot range, it's likely that Philadelphia condos will continue to get taller, as they have in New York, where a forest of super-tall, 1,000-foot-plus buildings is rising near Central Park. The demographic for the super-tall condos tends to be older, and they want a full menu of concierge services. The hotel-condo combo makes that easier to provide.

But the mix also makes things more complicated on the ground. To serve the building, Dranoff is seeking zoning changes from City Council to allow big driveways on both Broad and Spruce Streets, as well as a height increase. Because the building's loading dock is planned for narrow Spruce Street, delivery trucks will have to back in, a maneuver sure to cause traffic headaches. The first Council hearing is scheduled for Feb. 12.

Like other hotel developers, Dranoff is seeking a state subsidy - $10 million - as well as the city's usual 10-year property-tax abatement. That's a lot of public subsidy for a building that will serve the one percent.

But at least the rest of us will get to enjoy looking at it.