But taking on a project like this is no easy task. “Buildings often limit where you can move kitchens, baths and entries,” said Jean Brownhill, the founder and chief executive of Sweeten, a free service that matches homeowners with vetted professionals. Bathrooms typically have to remain stacked on top of each other from floor to floor to ward off leaks and preserve plumbing lines, and gas lines are governed by important safety requirements. “If the two units were updated at different times, they might not share the same flooring, trim, molding, doors, hardware or HVAC systems,” Ms. Brownhill added. “You’ll want to align fixtures and finishes to create a cohesive look, and you will need professional help from a contractor to address floor leveling and mechanical updates.”
“I was terrified at first,” Mrs. Dierks said of her combination project, noting that she and her husband decided to rent the second unit out for a while, work with a design firm to customize the space and take some time to find a rental to move into during construction.
Hiring a designer proved easiest. “We Googled ‘apartment combination and Brooklyn,’” she said, which led to an article about a renovation by General Assembly, a Brooklyn-based firm led by Sarah Zames. A week later, they hired Ms. Zames and began working with her the following month.
“Finding a place to live during the renovations was nerve-racking,” said Mrs. Dierks, who ultimately worked out a deal with a real estate agent to lease an apartment that was proving hard to sell. “He negotiated a six-month lease for us with the contingency that we would need to allow showings on a pretty regular basis,” she said. “It was a win-win for both of us as the apartment was staged with our furniture, making the space look way more livable. They got rent, we got a short-term lease and they were able to sell it within three months.”
After the necessary permits were secured and the couple had been through several iterations of the layout — about six months later — demolition finally began. Construction to turn their two separate two-bedroom, one-bath apartments into a three-bedroom, two-and-a-half-bath duplex lasted six months and cost $500,000.
A large kitchen took the place of a significantly smaller kitchen and living area. They now have an island that seats six, and a pantry — “something I always wanted,” Mrs. Dierks said. Down the hall, a half bath and spacious living room and dining area replaced the original bedrooms and full bathroom. Two sets of French doors open to the deck. A traditional staircase leads to the upstairs unit, now configured as three bedrooms and two baths, with a balcony off one of the bedrooms.
“There’s something about having stairs in an apartment in New York that feels special,” said Ms. Zames, the designer. “It feels like a house or a home.”
But adding a staircase also eats up a lot of space. To maximize every inch, Ms. Zames added storage: a walk-in toy closet beneath the stairs, with shelves to the ceiling and space for a reading nook. “It ended up being really fun,” she said. “Like Julian’s special place.”
Another bonus, Mrs. Dierks noted, is that “you wouldn’t know that kids lived in our apartment unless you opened those doors.”
Other custom features include a shoe-storage bench near the entrance and a built-in oak credenza.
“Since the building was so small, we did get a few noise complaints,” said Mrs. Dierks, who regularly emailed her neighbors to make sure they knew when “loud” days would be and bought lunch for the community health center that rented space directly beneath their apartment on those days. “We also kept our building’s super informed on what work was happening what days, so he could be on site to supervise,” she said.
The family members moved into their expanded space nearly a year and a half after beginning the project. Less than three months later, their daughter, Sloane, was born.
“I’m really grateful that we hired a designer to help us through every single phase of the project,” Mrs. Dierks said, particularly given some of the hiccups they had to navigate along the way, including “changes that we had to make due to new inspection rules” — installing different protectors around the recessed lights, for example, and figuring out where to put electricity in the kitchen island after a change in code.
When everything was done, “We had a big housewarming party where we only invited the neighbors,” Mrs. Dierks said. “They came in for brunch and got to see what it looked like and ask us questions.”
Three neighbors are now renovating their apartments, she said, but no one else has been able to buy a neighboring unit since the Dierks bought the place upstairs: “I’m sure everyone is kicking themselves for not buying it.”