Rittenhouse Square is one of five original squares planned by William Penn in 1683 to serve as an open-space oasis. Now, the 2010 opening of 1706 Rittenhouse Square, a block away, is generating some of the highest-ticket sales the city has ever experienced.

The 31-story 1706 Rittenhouse Square has full-floor, 4,200-square-foot units on the sixth floor and above. In 2010, money manager Theodore Aronson, head of AJO, bought the building's duplex penthouse for $12.459 million, breaking the city's previous record of $7.7 million for the highest price paid by an individual for a single residence.

Tom Scannapieco, president and CEO of Scannapieco Development, left, co-developed 1706 with Joseph S. Zuritsky, CEO of Philadelphia-based Parkway.

The building's layout is designed to maximize privacy for residents. The subterranean parking garage allows residents to drive directly in and out. Owners can call for their cars from the elevator, after which the car is brought to ground level. The three elevators are also programmed so they don't stop at intermediate floors. Shown here is the entrance lobby.

Dan and Luana Neducsin, 70 and 66 years old, respectively, purchased a unit in 1706 for $5.435 million in 2010. Mr. Neducsin is the owner of Neducsin Properties, a real-estate development company. Shown here is one of the common areas of their unit.

The Neducsins used to live in a brownstone but decided to downsize after becoming empty-nesters, which is the case for many of 1706's owners. The average age of occupants is 57, says Mr. Scannapieco, of Scannapieco Development.

Shown is the dining room of the Neducsins' unit. Other residents in the building include Cliff Lee and Jonathan Papelbon, both pitchers for the Phillies.

John Paz and his wife, Rachel Moore, shown here, paid $6 million for a unit in 1706 and live there with their 16-month-old son Wesley.

Shown is the kitchen of Mr. Paz and Ms. Moore. 'We're in the city for food, fun, the buzz,' says Mr. Paz, who also owns homes on the Jersey Shore and in Naples, Fla.

Inside the units, the plumbing stacks and structural columns are designed so owners can customize their floor plan with their own architects and designers. Craig Rogerson, CEO of specialty chemicals company Chemtura, and his fiancée Irene Chan, shown here with their two standard poodles, have a unit with a master suite, two bedrooms, a media room, an office and 3½ bathrooms.

The building of 1706 Rittenhouse Square is part of a condominium boom that began in the early 2000s, prompted in part by low interest rates and generous tax abatement for real-estate improvements, says economist Kevin Gillen, senior research consultant with the University of Pennsylvania's Fels Institute of Government. Shown here is Rittenhouse Square.

Pictured is the exterior of 1820 Rittenhouse Square where H.F. 'Gerry' Lenfest, founder of Lenfest Communications—which was later sold to AT&T and then Comcast—paid $1.4 million for a unit.

Antoine-Louis Barye's bronze sculpture, 'Lion Crushing a Serpent' is displayed in Rittenhouse Square in downtown Philadelphia.

Stephen Burke, CEO of NBCUniversal, paid $5.85 million for this townhouse, far right, at 1817 Delancey Place in 2010.

The six-bedroom, 5½-bathroom townhouse was recently listed on the market for $5.6 million.

According to the listing, the townhouse features six bedroom suites, fireplaces, a grand staircase and four terraces. Laurie Phillips with Prudential Fox and Roach Realtors holds the listing.

Shown is one of the bathrooms in 1817 Delancey Place.

Shown is one of the terraces at 1817 Delancey Place.

New construction and proponent groups of the square, like Friends of Rittenhouse Square, Center City District and Rittenhouse Row, helped clean up the square and re-establish it as a desirable place to live. Now, prewar apartment buildings, contemporary hotels and restaurants stand side-by-side.