But it turned out that just as Mr. McMahon was contemplating his future, Mr. Farrar and Ms. Chapman were thinking about theirs. Ms. Chapman mused about how the couple might eventually need to move into the parlor-level apartment, if the day came when they were no longer comfortable climbing stairs.
“Somehow, it became a conversation,” Ms. Chapman said, and she and Mr. McMahon began discussing a renovation and expansion plan for the apartment that would allow the designer and his daughter to remain more comfortably in place for the time being, while making the space more accessible and safer for the owners to use in the future.
Ms. Chapman and Mr. Farrar, they agreed, would cover the cost of materials and labor, and Mr. McMahon would contribute his services to design and manage the project, in exchange for a five-year rent freeze at $2,250 a month.
After much planning, Mr. McMahon got started on the two-year effort in 2014. He and his team of contractors gutted much of the apartment, tore up the linoleum floors, exposed a brick wall and shifted the placement of the kitchen, bathroom and living room to bring in more light from the backyard.
Mr. McMahon frequently uses reclaimed materials in his projects and maintains a stockpile of wooden boards and beams torn out of old buildings. For the new kitchen, he topped Ikea cabinets with Brazilian walnut counters made from reclaimed New Jersey boardwalk and installed open shelves made from heart pine beams that came from another building in Brooklyn.
For the bathroom, he installed a double marble vanity that Ms. Chapman bought from an antiques dealer in Saugerties, N.Y., and built a sliding barn-style door from mismatched boards he pulled out of a Dumpster across the street. Inside, he designed a curbless shower with a sliding glass door, a folding seat, a hand shower and blocking behind the walls so that grab bars would be easy to add in the future.
“I was making it for me,” he said. “And for them.”
Off the back of the house, where there was previously a crumbling stone and cement foundation with a deck, he added a second bedroom with large sliding glass doors looking out to the garden Ms. Chapman tends, with its grape vines, flowers and fig trees. He also put in slate floors with radiant heating, a heart-pine desk and a flip-up, removable platform for a bed.
As the project progressed, it also expanded. Before it was finished, Mr. McMahon and his team had also repaired the building’s facade and mansard roof. The total cost came to about $150,000.
Construction projects add stress to any relationship, but the friendship between tenant and owners held fast. “Not that we didn’t butt heads now and then,” Ms. Chapman said.
She envisioned hexagonal ceramic tile for the bathroom floor where Mr. McMahon wanted marble, so they compromised on hexagonal marble tile. She also initially disagreed with Mr. McMahon’s decision to move the bathroom to open sightlines to the backyard, but has since come around.
Otherwise, “you would never have this sweep and this flow,” she said on a recent sunny afternoon, while enjoying a latte and blueberry muffin that Mr. McMahon had baked. “I love being down in the flowers.”
She is also happy to keep Mr. McMahon and his daughter nearby. “I really want a neighbor downstairs; I don’t just want a tenant,” she said. “I think neighbors are close to the most important people in your life.”