When Kim Hammond was a teenager in the late 1960s, his parents bought a proud stone house on a hill in Baltimore County, Md., and he immediately recognized it as remarkable.
Built in 1920 by a prominent architect named Laurence Hall Fowler, it originally served as a stately guesthouse for an even grander home that is now part of St. Timothy’s School, a private high school for girls.
“He built these works of art surrounded by nature,” said Dr. Hammond, 67, a veterinarian who was profiled in The New Yorker for his work with fashion designers’ pets, but is also known for tending animals on the sets of films like “All the Pretty Horses,” “The Green Mile” and “Catwoman,” and is involved in efforts to save endangered mountain gorillas in Rwanda, Uganda and the Democratic Republic of Congo. “It’s such a special house, and just one of those homes that you can’t duplicate.”
So after his mother died in 2014, he and his wife, Carol Elerding Hammond, now 50, decided to move in, with their daughter, Stella, now 12.
But first, they wanted to make a few changes.
After nearly a century of wear and tear, the house needed updating and a few spatial alterations to accommodate a more relaxed way of living. As Ms. Hammond said, “The library is formal, the living room is formal, the dining room is formal, but we’re pretty casual people.”
For help, they called on Dr. Hammond’s longtime interior designer, Mona Hajj. Dr. Hammond was one of Ms. Hajj’s first clients in 1994, after he saw a room she designed in Baltimore magazine and hired her to decorate a bedroom on the third floor of his parents’ house.
“I asked her to design me my own English gentleman’s suite, where it’s just lovely to curl up and read a book,” Dr. Hammond recalled. “She designed something that was so reflective of what I wanted, I was just overwhelmed, and we’ve been friends ever since.”
Over the years, that room has remained unchanged. And ever since, Dr. Hammond has called on Ms. Hajj for help decorating whatever home he happened to be living in, including a series of downtown Baltimore apartments. He even credits her with helping him look his best for Ms. Hammond, whom he began dating in 1999.
“Mona’s décor really attracted my wife,” he said. “My wife thought I knew what I was doing.”
By now the secret is out — and has been for some time — so Ms. Hammond had the opportunity to collaborate with Ms. Hajj.
All three agreed that the original windows, woodwork and plasterwork should be preserved. “We didn’t want to jeopardize the essence of the house,” Ms. Hajj said, although “it was not built for the kind of living that they wanted, which was something comfortable.”
The challenge was walking the line between restoration and transformation. Ms. Hajj left most of the house intact, reproducing moldings where they had gone missing and restoring the remaining woodwork to its original luster. But she also proposed a few surgical changes to the floor plan.
On the ground floor, they expanded the kitchen and added a porch along the back of the house, accessible by French doors from most rooms. On the second floor, they converted a bedroom into a bathroom to create an enlarged master suite and added a cathedral ceiling, opening the room up to the third floor, which once housed servants’ quarters.
“It was my mother-in-law’s room, so I wanted it to have a different feel,” Ms. Hammond said.
A substantial addition to one side of the house contains a double-height family room, a powder room and a three-car garage on the ground floor, with a large recreation room above.
To finish the five-bedroom house (which is included in Ms. Hajj’s new book, “A Romance of East and West,” published this month), the designer put together an eclectic mix of furniture and objects spanning centuries and cultures. The living room is decorated with ancient pots from Latin America and Asia, a 17th-century Turkish Ushak rug, Louis XVI-style armchairs, a Cy Twombly lithograph and a tufted sofa that Ms. Hajj had custom made.
“I wanted that look — warm, global, collected — because the house afforded it,” Ms. Hajj said, and because it reflects the way the Hammonds live. As Ms. Hammond put it, “We travel all over the world, and acquired this taste for things that are unique and unusual.”
The design-and-construction process took about two years — the Hammonds moved in toward the end of 2016 — and cost more than $6 million. It also involved significant changes. But at its core, the house remains the one Dr. Hammond has loved for most of his life.
“I look forward to coming home every single day,” he said. “It evolved beautifully.”