Bart Blatstein's mansion brings the Gilded Age back to Rittenhouse Square

Once again there is a mansion on Rittenhouse Square. And yes, there is a regulation-size tennis court in the basement.

And a Parisian-inspired central courtyard with a three-story-high green wall, studded with orchids, bromeliads and other tropical plants.

Overlooked by an infinity pool.

And rimmed by a sequence of airy, open-plan spaces.

Philadelphia has seen its share of large and opulent houses emerge during the recent building boom, but this newest one, built on Rittenhouse Square's southwest corner for developer Bart Blatstein and his family, has attracted an extra degree of public curiosity because of its storied location and unusually protracted construction. After three years of work, involving the same techniques and materials used to erect skyscrapers, it is now an occupied house again — if you can call a sprawling 17,000-square-foot urban villa a house.

In the late 19th century, the genteel square was ground zero for Philadelphia's Gilded Age millionaires, who hired the likes of Frank Furness to build their city castles. But high-rises gradually displaced all but a handful of private homes, and Rittenhouse Square became, if not scruffy, then as threadbare as a dowager's shawl. Such was the exodus of wealth from Philadelphia in the 1970s and '80s that the grandest of the square's mansions, a compound owned by the famed art collector and philanthropist Henry P. McIlhenny, sat empty for nearly 30 years after his death in 1986.

No more. In 2013, Blatstein acquired five house lots that had been part of the McIlhenny estate for $4.2 million. (Two others facing Rittenhouse Square were sold off in the early 2000s and turned back into single-family homes.)

Blatstein — who made his money lining Delaware Avenue with suburban-style strip malls, betting on casino projects, and building the successful Piazza at Schmidts (now Schmidt's Commons) in Northern Liberties — has completely reorganized the properties, adding a substantial addition in the courtyard where McIlhenny, chairman of the Art Museum board, once hosted glamorous parties for his art-world friends.

So expansive is the property that a colleague refers to it as the "Blat Cave."

Blatstein's four-story mansion sprawls across the 4,000-square-foot site, from the southwest corner of Rittenhouse Square to Manning Street. Like McIlhenny's house, Blatstein's mansion is organized around a courtyard, albeit one no longer visible to the public. The space is protected by a retractable glass roof, a kind of miniature version of the Houston Astrodome. One wall of the courtyard is carpeted with a dense array of tropical plants, a design that was apparently inspired by the courtyard of Paris' Pershing Hall, a former palace that was converted into a luxury hotel.

In an interview, Blatstein was circumspect about the project, agreeing to discuss only the exterior architecture and confirm a few details about the interior. But building plans submitted to the city provide ample insight into the design, by Shimi Zakin and Evan Litvin of Atrium Design Group.

Because the main house, a mansard-roofed Victorian dating from the 1850s, is listed on the city's Historic Register, Blatstein was obliged to maintain the facade. Everything else is new.

The exterior's geometric composition will remind many of the high-end houses now going up around the city. As a developer, Blatstein is notorious for his sometimes kitschy tastes. He once proposed to re-create Rome's Spanish Steps on Penn's Landing. When he was hawking a casino for the former Inquirer and Daily News property on North Broad Street, the design featured a Provencal-style village on the roof of the old printing plant.

His Rittenhouse Square mansion is nothing like those historical fantasies.

The exterior is respectful in the extreme, a tasteful contemporary that uses first-rate materials and graciously defers to its historically certified Victorian companion. For the addition facing the square, Zakin and Litvin created an abstracted, mirror-image version of the Victorian, using red brick, brownstone, and flamed black granite.

The proportions of the addition are identical to the Victorian but rendered in a flatter, contemporary language. The architects also reversed the voids and solids. Where the Victorian has a recessed vestibule, the addition has a protruding bay. But while the dimensions of the windows and walls are the same, the addition feels more cloistered because of the immense size of its solid granite panels and imposing entrance. Crafted from solid walnut, the front door weighs 750 pounds and pivots open on a single steel rod.

Building around the Victorian's fragile 19th-century facade proved to be a daunting technical feat, requiring the oversight of Bala Consulting Engineers. The Victorian's old brick walls had to be braced and underpinned to prevent them from toppling over. To make room for the 36-by-78-foot tennis court in the basement, Blatstein had to excavate the entire site, going down nearly 35 feet below the sidewalk.

During construction, passersby often marveled at the size of the pit, which rivaled the one for the new Comcast tower. Indeed, the concrete foundation could easily support a high-rise. The upper stories are framed in steel.

"We always said to Bart, 'We are building you the shortest skyscraper in the city,' " Zakin told me.

Because the house is so open, the number of rooms really doesn't give a sense of its scale. The plans show four bedrooms, including what appears to be a guest suite along the Manning Street side of the courtyard. By my count, there are five kitchen or bar areas and seven bath and powder rooms.

That includes a full kitchen in the four-car garage on Manning Street. "It's designed so Bart can throw parties, and no one sees the caterer," said Eric Blumenfeld, a developer who has visited the house. The two handsome garage doors are crafted of Brazilian Ipe and are a major improvement over the cheap doors McIlhenny had installed.

Besides the full-size tennis court, the most striking luxury is probably the swimming pool on the third floor, overlooking the courtyard. On the plans, the pool area contains a bar, lounge area, and two changing rooms. Around the corner, on the east side of the courtyard, a wing of the house is reserved for a gym.

Built like a small skyscraper, the house probably cost as much as one. Based on its size and normal building costs, several contractors estimated that Blatstein's mansion had to set him back between $8 million and $16 million. The city's property tax website puts the total value of the property at $3.4 million, but a city spokesman said the house would be reassessed next year. Because of the city's construction abatement, Blatstein will pay no property taxes on the improvements for a decade.

Blatstein, 63, acknowledged that the project was a labor of love after a long career pursuing development projects. He is giving up a house in Montgomery County to move back to the city, where he grew up.

"I'm back in the city because I believe in the city," he explained. "It may seem over the top, but this is an important location, and I wanted to do my best."