When Norman Sussman’s health began to fail a few years ago, he and his wife, Anita Sussman, faced a difficult decision.

They wanted to remain in the late-19th-century brownstone in Park Slope, Brooklyn, where they had raised two sons and lived for nearly 40 years. But they knew they needed a space better suited to aging — someplace with fewer rooms to maintain and no stairs.

Like many older couples, they found themselves contemplating two options, neither of them very appealing: radically modify their current home or look for a new one.

The solution, it turned out, was staring them in the face. For a decade, they had been renting out the garden apartment. What if they flipped that arrangement, building an addition onto the ground-floor apartment, then moving into it and renting out the top three floors?

Mrs. Sussman, a retired psychoanalyst who is 70, knew that the burden of any renovations would fall on her, as Mr. Sussman is 82 and has heart failure, among other health issues. So she set some ground rules.

Credit…Emily Gilbert for The New York Times
Credit…Emily Gilbert for The New York Times

If they were going to move into the garden apartment, they would need a second bedroom so she could have space for herself and an art studio where she could paint and draw. Mr. Sussman, who had worked in finance, had always handled the family expenses, but now that would also have to change.

“I said, ‘I’m not doing any of this unless I have an extra room and I am in charge of everything,’” Mrs. Sussman said. “He said, ‘All right, that’s fair enough,’ and he has absolutely kept to his part of it.”

And so, after 47 years together, Mrs. Sussman took charge of the most expensive project of their lives. The renovation, completed last spring at a cost of $630,000, was financed with savings and the proceeds of the sale of a modest home in the Hamptons that they had owned since the 1980s and sold about six years ago for about $400,000.

On a recent morning, sitting at the dining table in their newly renovated garden apartment — with Mr. Sussman resting quietly on a recliner behind her — Mrs. Sussman reflected on what it took to get there.

While the work was being done, Mr. Sussman lived in Southampton, in a cottage on property that belongs to one of their sons. She went out on weekends, but stayed in the upstairs apartment during the week, so she could watch what was happening and attend construction meetings.

“There was a lot of dust — it was unbearable,” she said. But now that the work is done, “it’s all a blur. It’s like raising children. You only remember the good things.”

She added: “All in all, it’s much better down here, because it’s easier with him. It’s very pretty, it’s modern, there’s less upkeep. It’s easier on all fronts.”

But the path to a smaller, more manageable life didn’t feel very small or manageable along the way. Here’s how the Sussmans kept going in the face of setbacks that included a grueling public approval process and a major change in plans midway through the project.

Credit…Matthew Williams

For decades, the Sussmans lived in a 2,200-square-foot home spread over three floors. But the garden-level apartment they were planning to move into was just 740 square feet, with a small living and dining area, one bedroom and one and a half bathrooms.

Mrs. Sussman knew they would have to downsize, but they still needed more room than that. “I need space for my grandchildren,” she said. “I need a space that can be transformed if I have more people over.”

And with Mr. Sussman so ill, she also needed a private retreat.

To design an addition, she hired Daniel Kaplan, 45, the owner of Bowerbird Architects in Brooklyn, who grew up three blocks away and, as a child, played with the Sussmans’ older son. “It’s very neighborhoody here,” said Mr. Kaplan, who has renovated several homes for longtime residents in the area.

The Sussmans’ property had limited buildable square footage, but enough for a 272-square-foot expansion that would include a small second bedroom and a larger living area. A deck on the roof of the addition would be an enviable parlor-level amenity.

Mr. Kaplan installed central air-conditioning and closed off the indoor stairwell that connected the garden apartment to the parlor floor, replacing it with a nine-foot-long closet and a shower for the half-bathroom, turning it into a full bathroom.

During the long construction period, Mrs. Sussman pared down the couple’s possessions, giving furniture to one of their sons who had a large house in New Paltz, N.Y., and moving books and memorabilia to the cottage on their other son’s property in the Hamptons. Other items went to charities and thrift stores.

To make room for what they kept, Mr. Kaplan maximized every inch of usable space. “You try to think of every recess, every nook and cranny,” he said.

In a well along the north wall created by chimney flues, for instance, he installed a built-in bench that not only incorporates shelving for books, but also provides extra seating for dinner parties: Move the dining table from the middle of the living room over to the bench and, with a second table at the end, there is seating for 12.

The garden apartment was small and dark, with ceilings not even eight feet high, and low windows on only the eastern and western sides. That didn’t worry the Sussmans, but Mr. Kaplan had a different perspective. “Light is my thing,” he said.

Low ceilings can make a space “feel oppressively short,” he said, but if you add the right lighting, it will feel more open.

Mr. Kaplan suggested installing a skylight in the addition to brighten up the back of the apartment and satisfy zoning regulations about light and air. Mrs. Sussman was reluctant, as her sister had skylights that leaked. But Mr. Kaplan explained that the city mandated the light, and there was no other way to get it. (To her dismay, the skylight did leak last summer, a problem Mr. Kaplan attributed to a clog in the neighbor’s downspout and damage done when a worker stepped on a paver, messing up the waterproofing. The construction crew made repairs, and the skylight hasn’t leaked since.)

Mr. Kaplan found other ways to bring in light, as well. He installed French doors out to the garden and six-foot-tall windows in the second bedroom. And in the bathroom that shares a wall with the second bedroom, he kept an original transom window that once looked out onto the yard; it is now an interior window, allowing natural light to filter in from the bedroom.

Rather than using a door to close off the second bedroom from the living area, Mrs. Sussman hung a heavy, midnight-blue curtain from Ikea decorated with an abstract painting she did. During the day, when the curtain is pulled back, the light from the bedroom fills the apartment. On sunny days, she said, “I read with natural light.”

The space also had to take into account Mr. Sussman’s mobility issues. He now shuffles when he walks, and falling is a concern: Recently, he fell on the sidewalk and broke a clavicle.

So Mr. Kaplan designed the rooms with low thresholds, and installed handrails in the bathrooms and support structures in the walls, making it easy to add more handrails later on. And in the new bathroom, he designed a shower wide enough to accommodate a walker, with a built-in bench of reclaimed teak where Mr. Sussman can sit while he bathes.

Outside, the garden apartment had two steps leading down from the street to the narrow entrance. Mr. Kaplan rebuilt the entry, widening it and adding extra steps to create a generous space with a more gradual incline.

Credit…Matthew Williams

Initially, the Sussmans planned to renovate only the garden apartment — a project with a $320,000 budget — while leaving the upstairs in its original condition. But midway through the project, as they began feeling more confident about the way things were going, they decided to renovate the entire house.

“Danny gave me numbers, and I looked at what we have and how we could stretch a little bit, and Norman said, ‘We should do it,’ ” Mrs. Sussman recalled.

That decision altered the timeline considerably and added $210,000 to the construction budget — more than it would have cost if they had planned it from the beginning. Architectural, engineering and other fees raised the total by another $100,000.

But the investment, they realized, could translate into significantly higher rental income. The garden apartment had been renting for $2,500 a month; a luxury, four-bedroom triplex in the heart of brownstone Brooklyn could fetch considerably more.

Before the Sussmans bought the property in 1980, for around $70,000, it had been a boardinghouse, and it was in rough shape. The couple renovated the house at the time, putting in a new kitchen and restoring the floors. But the ensuing decades took a toll on those renovations, while the neighborhood transformed from edgy to enviable.

“The house was in good shape, but it needed updating,” said Wassim Fakhereddine, an associate broker with Corcoran and the Sussmans’ leasing agent.

The upstairs apartment, he estimated, might have rented for $6,000 to $6,500 a month as it was, while units at the top of the market — with luxury finishes, in prime Park Slope locations — can fetch as much as $12,500. Renovating this apartment wouldn’t yield that sort of windfall, but it would put the monthly rental somewhere in between.

So the Sussmans focused their attention, and their budget, on the parlor-floor kitchen, spending $60,000 to install custom cabinets, quartzite counters and a Thermador range and refrigerator. They added a powder room and coat closet, installed central air-conditioning throughout and renovated the third-floor laundry room. In the bathrooms, they installed matte-black penny-tile floors and painted wainscoting. And in the one on the second floor, they reglazed the claw-foot tub, moving it beneath a window overlooking the garden.

But they made sure to retain 19th-century details like the parquet floors, crown molding, millwork and the ornate medallion in the dining room. “I wanted to keep as much of the integrity of the house as possible,” Mrs. Sussman said.

The payoff? After listing the apartment in April of 2019 for $10,500 a month, they rented it three weeks later for $9,000 a month for the first year and $9,500 for the second.

They might have held out for tenants willing to pay more, but Mrs. Sussman was happy to find a family she liked. “The money was not as important as who was taking it,” she said, “because we live underneath.”

Credit…Matthew Williams

When you’re renovating in a crowded city like New York, there are at least two potentially thorny relationships you have to navigate: one with the neighbors who share your walls, and the other with the city agencies that approve your plans. For the Sussmans, that meant appeasing neighbors who had lived next to them for decades and securing approvals for exterior work from the New York City Landmarks Preservation Commission, because the property is in a historic district.

The Landmarks Preservation Commission receives some 14,000 permit applications a year, and the vast majority are approved at the staff level, which does not require public hearings. Some projects, however, require a closer look — like the Sussmans’ renovation, which included an addition visible from a landmark street. The Sussmans had to go through a monthslong approval process and make public presentations to the local community board and the commission.

“It’s exhausting,” Mr. Kaplan said.

Simeon Bankoff, the executive director of the Historic Districts Council, a nonprofit organization that reviews some landmarks proposals, said homeowners run into trouble when they wait too long to show their plans to the commission. “The problem comes in when you don’t talk to them first,” he said. “You should call up and say, ‘Hey, this is what I plan on doing.’”

To avoid a drawn-out review process, Mr. Kaplan designed the addition with an eye toward what the commission would like. He knew that visibility from the street would be an issue, so he kept it to a minimum. The front steps, which he modified to ease access to the apartment, would also need approval.

He presented the plans to the community board first. “They outright approved us,” he said, which came as a relief. When that doesn’t happen, he said, “it’s a black hole, because you have to keep going back and forth, and it can take absolutely forever.”

Meenakshi Srinivasan, the chair of the Landmarks Preservation Commission, said she found the addition modest, and one that blended with neighboring rear extensions, with only a sliver visible from a side street, according to a video of the hearing. The application was approved in November 2017, three months after it was submitted.

The Sussmans also needed to communicate their plans to their neighbors to the north, because an overhang from the neighbors’ roof extended onto the Sussmans’ property and would have to be trimmed to make way for the addition. The Sussmans didn’t need their neighbors’ approval, but did want their blessing. Mary Sue Lindley and Brit Geiger, both 73, had bought their brownstone in 1976, and their son had grown up playing with one of the Sussmans’ sons.

When Mr. Kaplan approached Ms. Lindley to show her the designs, she expressed concern about possible damage and leaks.

“They told me they were going to make the walls contiguous, and I didn’t get it,” said Ms. Lindley, a retired computer science teacher. Mr. Kaplan explained that the work would remove only the overhang and would be secured by new flashing.

Mrs. Sussman was relieved that Mr. Kaplan handled that conversation, as she would not have been able to answer technical questions. “It was less stress,” she said.

Eventually, Ms. Lindley came to see the renovation as an opportunity to envision how she and her husband might modify their own home along the same lines. But downsizing, it seems, isn’t for everyone.

“My husband and I can’t live in the amount of space that Anita and Norm have,” she concluded. “We couldn’t get along.”

For weekly email updates on residential real estate news, sign up here. Follow us on Twitter: @nytrealestate.