When He Bought the Tiny House, He Was Single. That Didn’t Last.

In 2013, when Nick Gavin was still a bachelor, he found a one-bedroom, 1,700-square-foot cottage on Shelter Island that seemed like the ideal personal retreat. The previous owner, Melvin Dwork, a New York interior designer, had created a casually stylish interior with pine-paneled cathedral ceilings and a floor of lagoon-green ceramic tile that was deeply appealing to Mr. Gavin, now 39, a real estate agent at Compass, in New York.

“It was really a big, open loft space, with 15-foot ceilings,” he said. “There were no walls.”

Other than adding a pool to the yard, he didn’t see the need to change a thing. So when another buyer made a competing offer on the house, Mr. Gavin met with Mr. Dwork to assure the designer that he planned to maintain what Mr. Dwork had created.

That meeting helped, Mr. Gavin said, and Mr. Dwork, who passed away in 2016, sold him the house for $675,000. But life can be unpredictable. Just before closing, Mr. Gavin’s solo existence began to change when he met Katrin Thormann, now 33, a fashion model he married in 2019.

As they began dating, it didn’t take long for Ms. Thormann to see the appeal of the house. “It was so fun,” she said. “We just put our mattress in front of the fireplace and spent so much time there.”

But after the couple had a daughter, Greta, in November 2016, they began to realize that crashing on a mattress in front of the fireplace couldn’t be a permanent arrangement, and that having more than one bedroom might be helpful.

About a year later, they hired the design firm Workstead to create an addition that would contain a new primary suite, and to make selective updates to the rest of the house, including splitting the old primary bedroom into two rooms: one for Greta and one for overnight guests.

“Nick was just really in love with the house,” said Ryan Mahoney, a principal at Workstead. “They had a lot of respect for Melvin as well, so they wanted to treat what was there with a lot of deference.”

Workstead designed a 620-square-foot addition inspired by the original structure, but set apart from it by a glass-walled breezeway. For abundant sunshine, the designers added four pairs of French doors and tall windows in the new structure, and split the bathroom into separate nooks containing the vanity, shower and bathtub, all connected by a central corridor. They also repeated details from the original structure, including the pine-paneled cathedral ceiling in the living room and the interior shutters.

To expand the kitchen — previously just a compact area of countertop with a small, under-counter refrigerator — and make room for full-size appliances, they took space from an adjacent full bathroom and converted it into a powder room. To create new cabinets that looked old, they specified painted, rough-hewn pine doors.

“We basically did gut the kitchen, but replaced it with something quite similar,” Mr. Mahoney said.

Outside, they installed a new cedar-shingle roof and replaced the old siding with new material. They also copied an existing custom cedar-shingle sconce to make more exterior lights and added a few conical copper sconces by Arne Jacobsen that will develop a patina over time, just like the wood.

Geoffrey Nimmer, a landscape designer, added new patios and turf to tie the structure into the landscape, with the goal of making the entire expanded home look like one cohesive unit.

To finish the interior, Mr. Gavin and Ms. Thormann moved a collection of furniture, accessories and art into the home, mixing pieces they owned with suggestions from Workstead. Among their favorites are a vintage Pierre Chapo dining table, which Mr. Gavin said set the tone for much of the other furniture, and a prized painting by Ron Gorchov that hangs above the fireplace in their new bedroom.

They also reinstalled a pair of antique diamond-shaped, black-painted wood louvers that Mr. Dwork had mounted above the living room windows long ago and left with the house.

Construction on the addition and renovation started in February 2019 and took a little more than a year to finish, at a cost of about $850,000. As the final punch-list repairs were being completed, the family moved in for a long-term stay when the pandemic struck New York in March.

“It really was a game changer for us,” Mr. Gavin said. “Katrin and I did this project as a quiet retreat outside the city, as a weekend home and summer house.”

But the moment it was completed, it became a full-time house for six months, he said, and the couple came to appreciate their updated home, and the neighborhood, even more.

“It made us wonder if maybe we could live out there, and have our daughter signed up for school there, because we love it so much,” Ms. Thormann said.

Now that they’re back in Manhattan, they plan to return to the house as often as possible, in all four seasons. It’s a commitment that is sure to continue post-pandemic, Ms. Thormann said, “especially now that we have extra bedrooms, and can bring family and friends.”

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