Robert Cochran, 57, and his wife, Suzanne, 58, have lived in their five-bedroom apartment at 1000 Park Ave. for 17 years. It’s where they’ve raised three children and it is full of memories — and room for their children’s friends to crash when they have summer internships.

This summer, as has been their custom for six years, the family spends weekends in Wainscott, N.Y.

The routine is about to change. Yesterday, Mrs. Cochran was back in the city scouting out fabrics at Donghia for a third home: a 5,200-square-foot loft at 145 Hudson St. in TriBeCa, which they purchased six months ago.

The wealthy have always been something of a rare breed in the city, but there’s a new, even rarer breed emerging: couples who live in elite neighborhoods uptown and are establishing second homes for retreat and fabulousness in the bohemian, chic, and fashionable neighborhoods downtown. Factors driving the trend include the improving quality of real estate downtown, the increasing amount of money couples are earning that must be invested in diverse assets, and the increasing complexity of New Yorkers’ lives.

“I do hear about this,” a vice president at Brown Harris Stevens, Paddington Zwigard, said. “The people doing this are older couples who don’t necessarily want to give up their places uptown. But they want a place downtown because there’s a fun vibe.”

One advantage over a home in the Hamptons or Aspen, Colo.: a shorter commute. Mrs. Cochran does it by foot and subway, taking the no. 4 train to City Hall from 86th Street and walking west. She said it takes her about 30 minutes.

The Cochrans plan to use their loft to throw parties and display the kind of big pieces of contemporary art that until now they’ve held off purchasing because their residences weren’t suited to it. So far they’re acquired a light piece by Leo Villareal and a family portrait in chocolate syrup by Vik Muniz.

“It’s a complete extravagance. But it’s also an investment,” Mrs. Cochran said. “My son Robby said, ‘Look, if you have the money, why not put it into something you can see and use?’ I said, ‘You are so right.'”

The idea to become second-home owners in their own city came to the Cochrans after going to a holiday party last December at an artist’s home on Lexington and 19th Street.

“We were the only Upper East Side people there,” Mrs. Cochran recalled from the living room of her uptown home. “It was all these artists and it was so cool. Bob said, ‘We should think of getting a place downtown.'”

Mrs. Cochran, who describes herself as a real estate junkie, started looking January 5 and closed on the first place she saw.

Mr. Cochran is chief executive of Financial Security Assurance Holdings Ltd. and Mrs. Cochran is a stay-at-home mother and fund-raiser for such institutions as the Central Park Conservancy and the Whitney Museum of Art. She does have concerns about the new housing arrangement. “I can see it being a confusion of where your stuff is and what are you doing. Ultimately, having two places in Manhattan is insanity,” she said.

Nevertheless, they are designing and building their new place as if they are going to live there. The theme of the apartment is industrial, inspired by the building’s past life as a paper plant. “It’s going to be a totally different look from our uptown apartment. It’s going to be machine age, industrial, and modern, but not glitzy or slick. We don’t want any slickness,” she said.

Their Park Avenue apartment has a very compartmentalized floor plan, with views of buildings across the street. Their new place has an open plan with wide banks of windows providing sweeping views of the Hudson River and the cityscape.

Facing south will be a pool table, a piano, a living room, and a dining room, divided by large columns. The floors will be concrete with hardwood accents.

The kitchen will be enclosed, as will Mr. Cochran’s office, the master bedroom, the exercise room, the guest bedroom, and the media room. Stephen Wang is the architect and Tim McDonald the designer, with construction slated to be complete by Thanksgiving.

The Cochrans can’t see giving up their uptown home at the moment. Their plan is to keep that place for at least another five years, which is when their youngest son, Robby, entering his senior year at the Trinity School, would be finishing college. “We’re not sure if we’re downtown people,” Mrs. Cochran said.

The children are excited about the place. Their daughter Lauren, who just started her first job after college, wants to have her 22nd birthday party there.

As for parental entertaining: The Cochrans already threw a “party in the raw” on May 18, to show off the space before construction. Mrs. Cochran plans to host a Christmas party there and a party for the Playground Partners, a group of the Central Park Conservancy.

At their May 18 party, the Cochrans got to see the space at night for the first time. “It was magical. It’s like you’re in a city on a cloud.”

The interior designer Thad Hayes said one of the projects that gave him the most fun of his career was the one-bedroom apartment on the top floor of a pre-war building in the West Village he designed for long-time clients who still live primarily at their East End Avenue home. He completed the project in June.

The apartment is tiny but has charming details: a terrace and a fireplace. His assignment: to design a getaway just for the two of them, a serene retreat, not to be visited by children, guests, or pets.

“Their daughter is a regular at Beatrice Inn these days, but she doesn’t get to stay there,” Mr. Hayes said.

“They’re just so busy. They have people all around them uptown,” Mr. Hayes said. “This is a place they can go to and be like normal people. It is forced relaxation for them.”

So far, the couple has spent only one evening in the apartment. They went out to a play uptown then hopped a cab downtown for dinner at Pastis. In the morning they went out for breakfast at a café.

When Mr. Hayes called his client to see how the evening went, he learned of a slight wrinkle in the couple’s fun. The low-to-the-ground 1950s bed he had selected caused some pain. “I called my client that day, and she said she’d almost broken her back making the bed. Uptown, someone makes the bed for her.”

Examples of the two-homes-on-one-island trend are still few and far between. Some uptown residents establish a toehold downtown by having offices there. The fund-raiser turned fashion designer Lisa Perry initially rented her loft in SoHo as an artist’s studio and place to show art and have parties.

An executive vice president at Prudential Douglas Elliman, Leonard Steinberg, conjectured that most people doing this kind of thing want to keep it under wraps.

“I’d imagine their plan for usage would involve something they want to keep quiet,” Mr. Steinberg said.

He has had “absolutely no one who has ever bought a downtown pad for the fun of it,” he said. But having sold apartments downtown to dozens of clients who were moving from uptown, he said he understands the appeal.

“They don’t want to be dressing in cashmere sweater sets in the lobby and be coiffed all the time, they want a laid back attitude,” Mr. Steinberg said.

The creative director of Domino, Sara Costello, who lives in the West Village, said, “The idea of buying a second home in downtown New York City is unconventional, certainly, but if you are made of cash, then why not? However, I might suggest springing for a weekend at any hotel downtown in lieu of a down payment.” While she doesn’t dismiss the trend completely, she did note, “If you are inclined to think downtown Manhattan is some kind of vacation, you probably need to get out of the city.”