When Michael Holtz and Megan Genualdi walk their Havanese Poodle named Bagel on the Hudson River Greenway along the West Side Highway in Manhattan, they often overhear people talking about their apartment.

“Is that a kid’s club?” and “What’s in there?” are common questions. Their doorman regularly turns away strangers who ask to visit their home in person, thinking it is some sort of public amenity space.

The reason for all the attention is what they call their “treehouses”: a wooden structure with two wings that spans the 22-foot high living area with a spiral stairway and netted seating. It is highly visible from the street through massive, floor to ceiling windows in their combined units in the iconic Richard Meier-designed building known as 165 Charles.

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When Mr. Holtz, the 56-year-old founder of luxury travel agency SmartFlyer, first bought four units on the building’s second floor for a combined $9.9 million between 2005 and 2008, he wasn’t sure what to do with the space.

Regarded by some in the New York real-estate market as a serial flipper, because of the numerous apartments in high-end buildings he has bought and sold over the years, Mr. Holtz tried to sell his four units at 165 Charles several times: once for $14 million in 2011, for $12.9 million in 2013 and then for $14 million again in 2015.

“I received some solid offers, but I never really wanted to let it go,” he says.

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In 2017, he hired architect Andrew Heid, the principal of New York-based No Architecture, to create a design for what ended up being a $4.5 million project to integrate two of the apartments into a four-bedroom, four-bathroom, 3,500-square-foot space. The renovation eliminated the partition wall between the two apartments, leaving a cavernous main living area that includes the kitchen, dining room and sitting room.  

Mr. Holtz says his primary goal for the reconfiguration was to create more of a sense of scale. He says before the treehouses, he would sit in one half of what’s now the living room on a sofa, watching TV on a big screen, and feeling like a miniature person in a huge space. “I knew there needed to be something in here, but I didn’t know what,” he says. It was somewhat like a basketball court.

The idea for a treehouse structure stemmed in part from permitting limitations: New York City rules didn’t allow for them to add more square footage to the apartments, including a loft or any other kind of structure with a platform floor. Since the treehouses have only netting, which are considered “soft spaces,” and the entire structure can be moved, it counts as furniture, which passed muster with the authorities.

The treehouses also double as a sculpture, cutting into the space, and adding verticality without being heavy and dark, says Mr. Heid. The higher of the netted seating areas gives a view over the tops of the trees that line the West Side Highway, the Hudson and the buildings in New Jersey.

Mr. Heid’s signature architectural approach is flexible spaces. His work, which includes private residences, museums and other buildings around the country and the world, tends to have what he calls a “matrix plan”: rooms that circle a central area and can either be closed off into separate rooms, such as sleeping spaces for nighttime, or kept open to use as offices and lounges during the day. Such a layout is not only more efficient, but it also helps connect the occupants and create more of a communal style of living, he says.

“The whole idea of bedrooms down a central hallway that you don’t use is outdated,” says Mr. Heid. “This was an opportunity to eradicate that.”

Walking through the front door, the first room to the right has a sofa, a television and thick pocket doors that form a wall when closed. It is what Ms. Genualdi calls the CNN Room because that’s where Mr. Holtz watched nonstop CNN during the first stages of the Covid pandemic.

A passageway next to the windows leads to a guest bedroom, which has a walk-in closet and a bathroom. When there are no guests, the room doubles as Mr. Holtz’s extra work room, because it opens up to the main room—via a sliding wall on the end—where there’s a large open kitchen and a breakfast table where Mr. Holtz usually works when he’s home.  

The treehouses take up a large portion of the main living area, with a dining table beneath them. On the other end of the main living area from the kitchen is a rotating bookshelf and television, which can be swiveled to close off another bedroom, which Ms. Genualdi, who is now an account manager at a New York-based business management firm, uses as an office. Behind that room is the main bedroom, which also opens to the CNN room across the main area via a pocket door.

The rooms have light-colored Douglas fir flooring, large windows with micro shades and midcentury modern furniture. The main rooms are filled with plants (all Ficus plants because they are able to survive the tint on the windows intended to protect artwork).

Mr. Holtz grew up in what he calls a “normal Long Island home” in Oceanside, N.Y., where his dad worked as a patent attorney. Thinking he might want to be a patent attorney, Mr. Holtz studied engineering at Washington University in St. Louis, graduating in 1987. But instead of going to law school, he went to work on Wall Street and then at a high-end travel agency in New York.

In 1990, Mr. Holtz started his own travel company. He met Ms. Genualdi, who is 35 and was working as an account manager at a Phoenix-based travel company, at an industry event in Paris in 2016. The two married in late 2019, and a few months later the renovated apartment was done enough to live in—just before the onset of the pandemic. “We put the wedding gifts to use,” says Ms. Genualdi, who grew up in Grand Rapids, Mich., and majored in business management and communications at Arizona State University. She jokes that since Covid meant Mr. Holtz wasn’t engaging in his usual frenzy of travel, they could have a more “normal relationship.” Pandemic-related supply shortages delayed the finalization of the project until earlier this year.

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Since he started his travel business, Mr. Holtz had been buying and selling apartments in New York, usually in new or iconic buildings. One of his most notable turnarounds was a three-bedroom condo on the 66th floor of a building called the Park Imperial at 230 West 56th Street, in which he bought for $2.985 million in 2003 and sold a year later to Sean “Diddy” Combs (also known as Puff Daddy) for $3.82 million. In 2008, Mr. Holtz sold a 2,520-square-foot unit on the 14th floor of 15 Central Park West for $8.5 million that he’d bought a year earlier for $4.86 million.

Despite his repeated attempts in the past 10 years to sell his units at 165 Charles, Mr. Holtz says the renovation of his apartment, where the couple lives full time when not traveling, and the time he’s spent there during the pandemic, has changed his mind about moving, for now. “We plan on staying here,” he says.