When New Yorkers Christine and James Boyle decided to buy their first home, they took an unexpected route, choosing a two-acre plot—occupied by a couple of wonky shingled cottages, each no more than a few hundred square feet—two hours east of the city. Their wish list for their new abode was short but precise: a clean-lined modern house that could accommodate their family of five, plus room for frequent visitors. And while the preexisting, angular little structures weren’t quite what the Boyles had in mind when they said “modern,” it turns out the cottages had real modernist credibility. The pair was built in 1963 by Andrew Geller, an industrial designer and architect who worked for Raymond Loewy and moonlighted as a builder of modest, eccentric Long Island residences.

Another appealing aspect of the homestead was the lot itself: It was large enough that the couple—he works in finance and she stays at home with their three children—could build a new house, suited to their aesthetic, and turn the Geller cottages into a guesthouse and a pool house. Complicating matters were covenants, put in place by the original owner and passed down through the Peconic Land Trust, that restricted modifications to the midcentury architecture and surrounding yew garden. But the Boyles took the property’s militant rows of Siberian irises (some 400,000 of them) and funky hedges in stride: “The grounds were like a park,” says Christine. “I envisioned the kids exploring it, playing hide-and-seek.”

The preservation and addition project did present a challenge for the first-time home builders. “The only thing we’d ever owned before was a car,” Christine laughs. “We’d never so much as renovated. So, prior to purchasing the property, we started interviewing architects—which gave us confidence that the challenges were not insurmountable. We needed someone with the vision to incorporate [the existing structures] into a new build, to have them make sense.”

Of the four firms the Boyles interviewed, Bates Masi + Architects proved the standout. Based in nearby Sag Harbor, the studio is renowned for executing elevated riffs on the contemporary beach house model. Partner Paul Masi, who spearheaded the project, is also an Andrew Geller fan. “His work is definitely quirky,” says the architect. “But Geller is such a part of the cultural fabric of the Hamptons. You can see the hand of a product designer in his houses, which were like sculptural objects in the landscape. They were often perched on some sort of pedestal, touching down lightly on the ground.”

So lightly, in fact, that they were occasionally uprooted and relocated—as Masi did here, nestling the two structures deeper into the garden. “The pyramidal shape of the yew trees plays so well off their angles, creating a whimsical dialogue,” he observes. As inspiration for the new construction, Masi channeled Geller’s fondness for using raised walkways to tether various structures together. “We latched onto that idea as a way to mesh old and new, while prescribing a path through the landscape,” Masi says. He designed a mahogany boardwalk that snakes, ribbonlike, through the property, starting as a deck in front of the larger Geller structure, passing alongside the bluestone-lined infinity pool, folding into a covered portico, and then running into the new, low-slung house at the rear of the property. “The structure evolved out of the deck,” the architect explains. Though a generous 6,000 square feet, the house looks much smaller; Masi took advantage of the site’s downward slope, slipping in an unobtrusive lower level—clad in black phenolic-resin panels—so the house reads as a single story.

A hallmark of Bates Masi’s work is to develop a simple rule to spark each project’s vocabulary: in this case, the woodwork’s syncopated beat of alternating 10- and 1.5-inch-wide planks. “We had fun with the module, using it as a base to develop various elements, extruding some of the pattern pieces to create wall paneling, light troughs in the ceiling, even the kitchen backsplash and shelves,” Masi explains. Eliding some planks on the facade created a louver effect that modulates light and frames views. The screening device both underscores and abets a connection between indoors and out, as do full-height glass walls overlooking the creek (and, in the case of the master suite’s unfolding bathroom, a soaking tub looking out onto a private shower, open to the sky above).

Masi put the Boyles in touch with Tribeca-based interior designer Damon Liss to help with the furnishings. “The house didn’t want to be overdecorated; the architecture and the quality of light both speak for themselves,” Liss says. While mahogany reigns as the dominant interior finish, he says, “there’s enough movement and varied textures that it doesn’t read at all monotone.” Playing off the faintly pinkish timber are pieces in warm walnut; laid-back profiles, meanwhile, reflect the casualness of the young family’s lifestyle. Midcentury classics and contemporary productions commingle with a selection of vintage Brazilian pieces, such as the living room’s Sergio Rodrigues Eleh bench. “Brazilian design has a tropical, beachy quality that plays well here,” Liss notes. So does the suede-like silk rug, below, in a muted blue-green. “It was clear that Christine wanted pattern and color, but it couldn’t be jarring since the architecture is so soothing.”

With three kids between the ages of nine and 12, durability was also an issue. That guided Liss’s selection of luxe but durable fabrics, like the yellow canvas from BDDW that covers a pert pair of Zanuso chairs. Masi also designed family-friendly built-ins, including a stage for the family room and, in the kids’ rooms, cantilevered bunks with trundle beds and a pull-down bed above a desk. Each room can sleep several children in the event of overnighters—a frequent occurrence, even more so since the Boyles relocated temporarily to Hong Kong. “This is now our residence in New York,” says Christine. She recalls their most recent visit: “We stepped off the plane on Friday, and, by Saturday, there were 24 people here.” The genius of the home’s design is how it seems to expand and contract as needed, elegantly accommodating numerous guests—yet feeling cozy and intimate when it’s just mom, dad, and the kids.