The largest penthouse unit at Carl Dranoff's One Riverside condo tower has gone under contract for $7 million, according to a person with knowledge of the deal, making it the most expensive unit sold inside the recently built Fitler Square development.

If the deal closes, the 4,409-square-foot penthouse — which has an additional 1,700 square feet of outdoor space — will become the priciest unit that Dranoff, 70, has developed and sold during his decades-long career. Closing the deal also would mean that Dranoff would come one step closer to selling out the 68-unit tower, which broke ground in 2015 and held its grand opening in May. Only two units are still available.

The identity of the buyer, who put the property under agreement on Feb. 9, was not disclosed. According to the person familiar with the transaction, the buyers are a local couple involved in business. They are expected to settle the purchase sometime in April.

The sale of the One Riverside residence — the largest of two penthouses located in the 22-story tower — comes within just two months of the completion of the unit. In December, Dranoff Properties put the final touches on the two-story penthouse, including installing its glass-walled staircase, before staging the unit in January, Dranoff said last month. The interested buyer emerged just a few weeks later.

Compared with only a few decades ago, when million-dollar sales in the city were a rarity, Philadelphia has been experiencing an unprecedented luxury condo boom in the last few years. No longer is it unusual to see condo units fetch prices as high as $5 million, $10 million, $15 million, or more, according to city property records. In December, developer Tom Scannapieco finalized the sale of the penthouse at 500 Walnut, his luxury condo tower that overlooks Independence Mall. The final purchase price: $17.03 million for the 8,900-square-foot penthouse, the highest residential price recorded in Philadelphia's history.

Developers have attributed those high-end sales to a greater supply of sleek, higher-quality units that allow for more buyer customization — designing a condo's layout or selecting a kitchen's finishes, for example. But Philadelphia has become wealthier, too: Since 2000, the number of households making more than $200,000 annually has jumped 186 percent, according to the U.S. Census Bureau. Plus, with Philadelphia's 10-year tax abatement, which exempts residents from paying taxes on new construction or improvements to existing property, purchasing a luxury unit is cheaper than it ever was before, observers say.

Still, property records show, persuading home shoppers to buy at an ultra-luxury price range takes a lot of work — and patience. Numerous ultra-luxury units have been sitting on the market for months or years, including the $15 million penthouse at 10 Rittenhouse, the $14 million Ritz-Carlton penthouse, and a $14.5 million property on the 46th floor of the Residences at Two Liberty Place.

In addition, despite the uptick in ultra-luxury supply, most condos in the city still typically sell for between $1 million and $2 million, according to data provided by Drexel University economist Kevin Gillen. In 2017, 65 percent of the 144 condos that sold for $1 million or more were in that price range, his data showed.

According to an interview with Dranoff last month, most of the sales at One Riverside have been between $1 million and $3 million. An additional one-story, 3,200-square-foot penthouse at the tower settled last year for $5.2 million.

The penthouse purchased by the couple this month features three bedrooms, a den, and 4½ bathrooms. It includes two roof decks, an in-unit elevator, Calacatta Gold marble, and hardwood floors throughout.

Sometime this year, Dranoff plans to start work on one of two condo projects he has planned, he said last month. One, at Broad and Spruce Streets, is expected to be a hotel and condo hybrid. The other, at Broad and Pine Streets, will be strictly condos.