The first thing you notice on crossing the threshold is what the house itself is deliberately turning its back on: the roiling, crashing surf of the Pacific Ocean. Ernest Born, an esteemed Bay Area architect who built the flat-roofed, two-story home for his family in 1950, could hardly have chosen a wilder, more windswept location—then a nearly deserted coastal road running along the westernmost edge of San Francisco—but his austere façade seems content to be merely backdrop for the elements.

The present owner, Tom Lloyd-Butler, first spotted the place after a day riding 20-foot waves on the far side of that road, called the Great Highway. “I was changing, and I looked up and saw this tiny ‘For Sale’ sign,” he recalls. “It was totally different from any other house at the beach,” cloistered by trees and with only two upstairs windows facing the view.

“I thought it would be plain inside,” says Lloyd-Butler, a longtime San Franciscan who runs his own investment company. “And then I went in and was blown away by the architectural motifs and how simple and sophisticated the design was.” Born had worked from a limited palette—Douglas fir, travertine, cork, brick, and aggregate stone near entryways to conceal the sand that eternally swept in—to great effect. “He didn’t use any fancy materials; he chose basic ones and then used them in really interesting ways,” says Lloyd-Butler.

Tom and Diane Lloyd-Butler and their two young sons, Ross and John, moved into not just a house but a design philosophy, complete with furniture and built-in details crafted by Born: a palatial dining-room table, beds, bureaus, wall installations for hanging (and rearranging) artwork, original lighting, original paint colors, even garden furniture. They felt at home with the idiosyncrasies, the personal stamp of an architect who used a ten-penny nail to fasten the door of a strikingly beautiful bathroom cabinet while creating spaces that seemed to reveal an understanding of universal design qualities to which people respond. The spare, double-height living area is grand and intimate at the same time, and its floor-to-ceiling window, which takes up the entire back wall and is sectioned into Mondrianesque blocks, pulls you into the hidden treasure of the house, a deep garden set with stone pine and cypress trees whose top branches catch the wind off the ocean while the atmosphere below is serene, protected. In the front, Born set the kitchen windows high enough to avoid a view of the road but so that they would perfectly frame the sunset.

By 2005, the family’s needs had changed. The Lloyd-Butlers had divorced and Tom kept the house, joined by his new partner, Dan Zelen, who has a design company and a store in Los Angeles called Zelen Home. Ross and John were growing up and clamoring for their own rooms. It was obvious they needed more space, but how to build an addition that wouldn’t compete with or alter Born’s design? After several false starts, Lloyd-Butler found Aidlin Darling Design in San Francisco. “Among other things, the house is remarkable spatially,” says Joshua Aidlin, who took on the project with his partner, David Darling, and colleague Michael Hennessey. “We knew that to simply add to it would be to compromise it.” They also noted how the house pretty much ignored the ocean—escaped from it, even, into the sanctuary of the garden. This standoff between, well, inner peace and outer turbulence became Aidlin Darling’s starting point.

Like Cosimo in Italo Calvino’s The Baron in the Trees, the architects perched in the branches of the cypresses and pines, observing where the canopy was dense and where it was porous, noting various perspectives and view corridors to the ocean. Then they came down again and, removing only one tree in the process, planted a three-story, 24-by-24-foot steel-sheathed glass pavilion next to the house, tethering it by means of a translucent bridge connected at the second stories.

The pavilion’s open plan invites the ocean and the horizon in but stops short of letting them dominate. Like Born, Aidlin Darling subscribes to view “editing.” On the ground floor, a slatted ipe fence lets in sunlight but shields the living space from the street. On the translucent bridge, a horizontal stripe of clear glass at eye level affords just a glimpse of sea and sky. And on the top floor, a large side window is fully translucent, thus directing the eye to an amazing next-stop-Asia view of the ocean through a clear sliding glass panel.

The pavilion can’t help but be much more attention-grabbing than the house next door, but the effect is softened by a careful echoing of materials. “We picked up on the Douglas fir casework and the travertine, and the floors are concrete, crafted in such a way that you see the texture of the troweling,” Aidlin says. “Nothing precious. The interior is mostly Sheetrock, and the skin is Cor-Ten steel, which is great for an oceanside building because it oxidizes and then stops; you don’t have to repair it.”

The addition also echoes the original house’s sense of proportion, radiating simplicity yet organized according to very specific needs and activities. A two-and-a-half-foot-deep storage area, made from Douglas fir, runs the length of the far wall in each of the three rooms. On the ground floor, it’s for surfboards, wetsuits, and a Murphy bed; one floor up, it becomes closets for the master bedroom and bathroom; and on the top floor, where, says Lloyd-Butler, “we all pile on the sofa together after dinner,” it houses a fireplace, electronics, and bookshelves.

Despite the relatively small footprint, each level seems much bigger than it is, because the landscape—particularly the greenscape—is designed as an outdoor room. Sliding glass panels overlooking the garden in the master bathroom, for example, open to put the bathtub essentially in the trees, which are also reflected in an expanse of mirrors. “The wall disappears, and it feels like a tree house,” Lloyd-Butler says.

The partnership between before and after, each distinctly its own design yet in sync with the other, feels natural enough that one imagines Born would have approved. In fact, he may even have anticipated the future possibilities, having secured three contiguous parcels of land way back when. His own residential work was limited; on the faculty of the University of California at Berkeley’s School of Architecture, he focused more on civic building design and restoration, as well as painting and illustration. Born also coauthored the definitive treatise on an enduring architectural enigma: the Plan of St. Gall, housed in Switzerland, is a set of parchment renderings for a ninth-century Benedictine monastery that was never built (and that was drawn at the odd and intriguing scale of 1:192).

One thing is certain: Something significant has changed since Born built his house, and he laid the groundwork to make it happen. The five-foot-tall trees he planted all around the property are now majestic 30-foot elder statesmen, making possible a new, bolder, and more open form of architecture within their intertwined branches.

“We had the advantage of a beautiful layering of foliage, which allowed us to create a glass façade without sacrificing privacy,” says Aidlin. “Born gave us a forest to work with.”