Affluent young couples are flocking south of Chambers Street—and staying there—perhaps forever transforming the financial district, New York City’s oldest neighborhood.

The Alliance for Downtown New York, the city’s largest business-improvement district, commissioned a study by research firm Audience Research & Analysis. The study found that 42 percent of financial-district households are now couples without children, up from 32 percent of households in 2004, the last year the Downtown Alliance did such a study.

Nearly half of all new households in the financial district since 2004 are childless couples; 15 percent are couples with children. One-quarter are singles; just over 1 percent are single parents.

Basically, according to the study, your typical new financial-district local is a 37-year-old with a live-in significant other, without kids, in a household (probably a rental, but increasingly a privately owned home) with an average annual income of $256,130. Oh, and this local probably wasn’t a New Yorker prior to moving below Chambers—40 percent of financial-district newcomers since 2004 came from outside the city.

Not only are these couples without kids migrating south of Chambers, but they’re coming in with a lot of money, even by Manhattan standards.

The typical financial-district household now has a median household income three times that of Manhattan overall, according to the Downtown Alliance; and that median has increased 47 percent since 2004, to $162,700. About 16 percent of financial-district households now pull in more than $400,000 annually.

“A lot of people that work in the financial district and work in the area are buying,” said Avi Voda, a top broker with Prudential Douglas Elliman, who has listings in the co-op 26 Beaver Street and in 15 Broad Street, one of the more than 30 condo projects in the financial district submitted to the State Attorney General for approval since 2003. “It’s a lot of couples; a lot of them want to be in the bigger apartments, and a lot of the buildings in the financial district are being converted into bigger apartments.”

More than half of the 662 respondents to the Downtown Alliance survey cited apartment quality as a major draw of living in the financial district. And a lot of these apartments come from the over 12 million square feet of commercial space that have been converted since the mid-1990’s to residential in lower Manhattan, according to a fall 2005 report from brokerage CB Richard Ellis. At the same time, several million square feet of new commercial space are underway or planned for the area, including the recently trumpeted 1.3 million-square-foot JPMorgan Chase tower at Ground Zero.

Such development has helped drive the southward seep of the Tribeca demographic into a warren of streets from the 18th century that was barely ever a neighborhood. To borrow overused real-estate parlance, the financial district’s hot.

As many as 22 restaurants have opened there since just 2006, and 15 hotels are planned or under construction there, including Joseph Moinian’s 57-story W New York–Downtown condo-hotel at 123 Washington Street.

Also, top-flight retail has emerged: Thomas Pink, Tiffany & Company and Hermès, a lot of it clustered around Wall Street (why didn’t anyone think of this before?). Plus, Giorgio Armani got into the district, via designing the interiors of 20 Pine: The Collection, a high-rise condo marketed by Michael Shvo, himself young and monied (and, at last check, in a couple), that caters to the styles and tastes of Wall Street’s vernal titans—“a neighborhood rich with landmarks and full of possibilities,” declares Mr. Shvo’s Web site for 20 Pine.

It’s this “full of possibilities” part that might surprise the veteran New Yorker, the cynic (or the realist) from points north of Chambers.

On weekends in the not-too-distant past, the financial district might teem with activity, but much of that sprung from tourists streaming among the landmarks and toward the South Street Seaport. On summer weekends, the winding streets would choke on pasty people, and most traffic only crept along, dodging out-of-towners and the locals dodging them; the Staten Island Ferry Terminal was never busier than on a Saturday afternoon.

Otherwise, forget it. The financial district was where people who had to went from approximately 9 to about 5, maybe 6, on weekdays. A few pubs and restaurants dotted the landscape, enough for after-closing-bell beers and maybe to take a client for a mildly frilly dinner. Otherwise, come dusk, the district crackled with all the energy of New Year’s Day morning. (This reporter lived at Fulton and Gold streets in the neighborhood in 2003 and 2004.)

Now, a swell of fresh locals blurs the perceptions and the boundaries of the district, bleeding it into Tribeca, Soho, even the Lower East Side and Chinatown—all the neighbors that made the transition long ago from edgy and iffy to post-Giuliani, gentrified Manhattan. The financial district’s population has increased by more than 11,000 since 2004 to over 44,700; before Sept. 11, 2001, the district’s population was just under 27,000.

It still is, however, tourist flypaper of the stickiest magnitude and a transient locale for the mighty financial-services industry. Simply too much history packs itself into the district’s blocks and just beyond, on the harbor’s islands; and capitalism’s temple remains Wall Street, whatever the New York Stock Exchange’s electronic future.

And nearly 60 percent of the district’s residents do rent, though the rents have increased steadily; a one-bedroom in a non-doorman building goes for $3,225 a month now, according to brokerage the Real Estate Group New York, up from $3,026 in January and pricier than similar apartments in Soho or the East Village.

Still: When a major survey concludes that nearly one in five financial-district residents stayed there when last he or she moved, then something’s up—a neighborhood, redefined.