The Costellos enlisted the help of the architect Michael Carbine and consulted the original decorator, Gerrie Bremermann — an 87-year-old New Orleans native with whom Sara shared a passion for the interiors of the ’70s modernist John Dickinson — to restore the home on Third Street to its former grandeur. To make it livable for a modern family, closets were built underneath staircases, gutters were added to the exterior and a charred wood stove was removed from the center of the kitchen. “The place had really fallen to wreck and ruin,” says Sara, standing by the open fridge. “But it was a chic disaster,” Paul adds. “Part of the appeal of the house is that the people who lived here had such great style.”

Created for a railroad magnate in the 1860s, the building had only housed three families prior to the Costellos: the Montgomerys, the Heroes and the Reynoirs, the youngest generation of whom had left their mark in the attic by scrawling Bob Dylan lyrics all over the walls. During their first year there, people would constantly come up to the Costellos to reminisce about first kisses on their porch or late nights in their parlor, an airy room with 14-feet walls painted in creamy shades that evoke meringue and mascarpone. An early interaction with their next-door neighbor started with an apology, Paul remembers: “She said, ‘I’m sorry about your garage. I had to punch out the window when I saw that my kid was smoking pot in there. That was my fault. But welcome to the neighborhood!’”

While the Costello family was busy transforming the house, the house was subtly transforming them. The fancy fashion parties of New York society — with their preening crowds and eager photographers — had gotten tedious; here, says Sara, things are much more relaxed: “We stepped into the previous owners’ cool vibe, and imagined it for ourselves.” Nowadays, their social life is as textured as the city itself, with a group of friends that includes the production designer James Chinlund, the musician Solange Knowles and the actress Reese Witherspoon, who has looked at real estate in the area. Ben Jaffe, a bassist and tuba player for Preservation Hall Jazz Band, regularly invites them to all-night parties in the French Quarter. (“We just dance our asses off,” says Paul.) They’ve taken a special liking to Grover Mouton, a gray-haired professor of architecture at Tulane University who has been integral in helping them build a community here. Stately, but with the wicked humor of a John Kennedy Toole character, he is gregarious and gossipy, and just as comfortable discussing seedy massage parlors as local nobility. “The other thing I love here is blending with all age groups,” Sara says. “We hang out with a lot of older people here. You don’t hang out with a lot of 60-year-olds in New York — at least I didn’t.” If their lives feel chaotic, it’s a calm type of chaos — filled with unexpected guests and impromptu get-togethers.

On this particular afternoon in June, Sara is showing me around the house in a flowy white dress and sandals, while Paul tends to his immaculate garden, a luxury the California native missed terribly during the 20 years he spent in New York. Their oldest, 14-year-old Harrison, is in the library eating pizza out of a Domino’s takeout box. Kiki, 11, is outside perfecting the choreography of a hip-hop-inflected swimming routine with one of her classmates. Ruffy, 4, is running naked through the hallways, pursued by the children’s babysitter, while their nine chickens — all named Donut — erupt every now and then into a cacophony of clucks from a coop just outside the house. Everywhere, among the crystal chandeliers and trellised windows, there are signs that this home is meant for living: The dining table is actually a Ping-Pong table, crutches and other storage-room miscellany have been stuffed into a shower on the main floor and piles of magazines and books are stacked on shelves, on counters, on tabletops. The smell of fresh banana bread wafts from the kitchen.