Developer Tom Scannapieco said Thursday that he had an agreement of sale for the penthouse at his ultra-luxury 500 Walnut tower overlooking Independence Square. The price: a Philadelphia record $17.85 million.

If the deal closes when the 26-story, 35-unit high-rise is completed in the spring of 2017, the two-story, 8,900-square-foot penthouse will be the most expensive residence ever sold in the city - more than $5 million more than the penthouse at Scannapieco's 1706 Rittenhouse Square Street, which sold for $12.5 million in April 2010.

Citing a confidentiality agreement, Scannapieco declined to identify the buyer, who - like all buyers to date at 500 Walnut - is from this area.

"I asked, and he declined the opportunity to be identified," the developer said.

The identity of the buyer may not be known until the deed is filed after the penthouse goes to settlement next spring. Calls and emails to all of the major players in Center City real estate turned up no names.

"I can assure you, however, that, contrary to previous rumors, the purchasers were not Beyonce and Jay-Z," Scannapieco said with a laugh.

In March 2015, a Philadelphia real estate agent's blog reported that the entertainers had purchased the penthouse for $20 million - $2.4 million more than the $17.6 million Scannapieco was asking.

Though the Beyonce-Jay-Z rumor caught Scannapieco and his staff off guard, he said it increased interest in the unit in the weeks after the blog item and its denial.

The penthouse's record price proves "our once-sleepy town has surely come of age," said Richard Oller, founder and CEO of GoldOller Real Estate Investments, of Philadelphia. The city "is bursting with culture, convenience, great food, and entertainment scattered throughout our very walkable river-to-river footprint."

At $2,003 a square foot, the penthouse's contract price is more than twice what condo units in 10 Rittenhouse, at 18th and Walnut Streets, were going for after iStar Financial brought developer Carl Dranoff in to market the building in 2012.

Dranoff, who last week topped off his 68-unit, 22-story One Riverside luxury condo building at 25th and Locust Streets, is getting $1,000 a square foot for those units.

Scannapieco said the other units in 500 Walnut are selling for $1,100 to $2,000 a square foot. Each floor has just one or two units, with the full-floor condos averaging about 4,300 square feet.

Both developers readily acknowledge that they are building to two different markets. Scannapieco encourages full customization of units by his buyers, but like Dranoff also delivers finished condos with several floor plans and finishes available.

Scannapieco's prices range from $3 million to $17.85 million; One Riverside's, $715,000 to $6.5 million for the penthouse.

The $180 million 500 Walnut project and the $112 million One Riverside are the only two high-rise condo buildings under construction in the city. Both were designed by Cecil Baker + Partners, of Philadelphia.

The $17.85 million contract price for the penthouse includes two major upgrades, the developer said. One is "a monumental stair connecting the two levels of the penthouse and the roof, which will be a floating, sculptural, concrete element," Scannapieco said. "We also added radiant-floor heating."

The penthouse has a private interior elevator, multiple balconies, fireplaces, and a roof terrace.

Said Dranoff: "I am very pleased for Tom. This sale is yet another powerful validation that affluent residents who can live anyplace are choosing Center City Philadelphia."

So far, 50 percent of the 35 units in 500 Walnut are under contract, valued, including the penthouse, at $90 million, Scannapieco said. More than 60 percent of the buyers will be moving from the suburbs, while the rest are from Center City, he noted.

Those percentages are similar to what Dranoff is seeing at One Riverside, where two-thirds of the buyers of the 40 units sold so far are suburbanites and one-third are from the city.

The building is now four stories high; 500 Walnut's topping off is expected to occur sometime in November.

"The underground parking was tough to do, with the 86-car automated robotic parking system requiring a folded slab depressed in the middle and with high sides," he said.

Digging the foundation was complicated because Dock Creek flows underneath the site.

With the water table just 25 feet below the surface of Walnut Street, parking, which will take 90 seconds using the automated system, was limited to two levels. A total of 263 pressure-grouted concrete pilings were driven into the bedrock to stabilize the foundation.

That work took almost a year, the developer said.

To allow full customization of the units, Scannapieco hired a German engineering firm, WSP Global, which increased the concrete floor thickness from nine inches to 13 inches.

That enabled Baker to eliminate 10 columns to increase flexibility, Scannapieco said.