It all started with a 12-foot stepladder, balanced somewhat precariously on the bed of a pickup truck.

Joseph Tanney and Robert Luntz of the Manhattan firm Resolution: 4 Architecture were on Fishers Island, New York, for their first look at the three-acre lot where they were to design a second home for Mimi Parsons and Tom Siebens, an American couple who keep their primary residence in the Notting Hill section of London. They arrived to find that Parsons’s father, Harris, had driven his Chevrolet pickup to a clearing and erected a ladder in the back, hoping to give Tanney and Luntz a sense of the site’s magnificent views of Long Island Sound.

Tanney and Luntz watched as Harris Parsons—82 at the time—scaled the ladder with minimal effort and great enthusiasm. “We looked at each other,” Tanney says, “and realized we were going to have to do this.” They did, with some trepidation, and were mesmerized by what they saw. “From the top of the ladder, we could get a glimpse through the trees of Long Island Sound to the south and Fishers Island Sound to the north,” Tanney says. “We also had a sense that, from the roof, we’d have a view to the east as well. We used this as our starting point, organizing the overall layout of rooms relative to the views.”

Capturing those views was paramount to Parsons, 61, who grew up in Rhode Island and has spent her summers on Fishers Island since childhood, and Siebens, 62, who met his future wife after he came to the island to teach sailing when they were both barely out of their teens. Another priority was to create an open kitchen, dining, and living area large enough to accommodate as many as 40 guests—Parsons’s parents and her four brothers and their families all have houses on the island—at Thanksgiving and Christmas.

The house would also give their 24-year-old twin daughters, Hannah and Mackie, a home base in the United States while their parents are in London. After renting a rambling, shingled house just downhill from their current property for several summers, they bought their lot from one of Parsons’s brothers in March 2007 and began thinking seriously about what to do with it.

Because of the logistical challenges of building on Fishers Island—which is officially part of Long Island but accessible only by infrequent 45-minute ferry trips from New London, Connecticut—Parsons and Siebens reasoned that modular construction would be relatively simple and less expensive. Their research led them to the team at Resolution: 4, which has spent a decade refining what Tanney calls a system of “mass customization”: Prefabricated modules are inexpensively built and outfitted with fixtures in a factory, delivered to a site by truck, and configured to meet a client’s lifestyle and budget.

“We just sat down with sketch paper,” Siebens says of their first meeting with Tanney and Luntz. “As personalities, they’re very simpatico, and the dynamic between them makes a lot of sense. When Joe goes off the rails with creativity, Rob is the guy who says, ‘Joe, that probably violates code,’ or, ‘Where are you going to run the conduit for this?’”

With input from Parsons and Siebens, the architects designed a sprawling house that included a guest wing over the garage with its own kitchen and living area. Then the economy tanked, and Parsons and Siebens put the project on hold. When they revived it a year later, they admitted that the project was simply too big. “So we put the house on a little bit of a diet,” Tanney says.

The revised design called for a 4,500- square-foot structure with a pair of guest bedrooms and a guest bathroom on the ground floor, plus unfinished space in the basement. The sleeping spaces were kept small to maximize the public areas, where Parsons and Siebens could spend time with their daughters and their guests. “The greatest thing about this house is you can have lots of people in it,” Parsons says. It’s ideal for overnight visitors, Siebens adds, “because there are a lot of places to hang out. You aren’t on top of one another.”

Getting concrete mixers on and off the island by boat was virtually out of the question, so Tanney and Luntz specified a prefabricated foundation and arranged to put the slabs on the ferry with the help of Baby Doll, the general contractor. The ferry operator confirmed that the boats could accommodate the 16-foot-wide modules that comply with federal shipping regulations, as long as the modules were arranged two abreast and delivered before Memorial Day weekend—when the island’s population begins its annual surge to around 2,000 from fewer than 300. Tanney credits Siebens with suggesting that they measure the island gangplank, which, as it happened, was too narrow, forcing Tanney and Luntz to reduce the width of the boxes to 12 and 14 feet.

The eight modules were delivered to the island over the course of two days in mid-May 2011. It took another two days to set them in place with a crane. Eleven months later—after the bamboo floors were installed, the exterior clad with cedar siding, the roof deck completed, and the furniture installed—the family spent their first night in their new house.

Since the home was put together, Parsons, who worked in advertising sales, and Siebens, a former finance lawyer, have settled into a retirement routine. They divide their time between London, where Siebens says they expect to live “until we’re too old to get on airplanes,” and Fishers Island. The glorious seascapes and family ties exert a strong pull and make the frustrations of getting there—international air travel and fretting about missing the day’s last ferry while stuck in traffic on Interstate 95—worth every ounce of effort.

“When you finally get here, it’s just calming,” Siebens says. “You’re surrounded by what appeals to most people on a small island, which is the sound of nature—the sea and the birds. And that, to us, is a very good feeling.”