At 12,000 square feet—9,600 of it under air—there’s no denying that the green, clapboard Key West-style residence on Siesta Key’s Cocoanut Bayou is big. But the home that designer-builder Rick Oswald and mother-and-daughter interior designers Bonnie and Keffie Lancaster of Lancaster Humma White Studio created for a family of five is actually quite cozy, too—thanks to small, comfy nooks and warm fabrics and finishes.

With a plush, nine-seat home theater with 123-inch screen and popcorn machine, a personal putting green, a coconut palm-flanked lagoon pool, and plenty of bayou frontage to launch their kayaks, there’s plenty to keep the family’s three teenagers active. And with a quiet, light-filled reading room off the master bedroom, a masculine wood-paneled library and a gym equipped with the latest fitness gear, the grown-ups in the family have their own relaxation areas, too.

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The new home stands on the footprint of a much smaller residence the couple bought as a second home 12 years ago while living in Nebraska. When they moved to Boulder, Colo., a few years later, they hired Oswald, of OZ Planning, Design, Construction, to build their home there. In 2005, when it came time to expand and update the Cocoanut Bayou residence, Oswald packed up his drafting board and moved across the country to Sarasota. “My intention was to do this project and go back home,” Oswald, who was born and raised in the West, says with a shrug four years later.

“Living in such a beautiful spot on Cocoanut Bayou, we wanted to take full advantage of the views and the natural beauty,” says the man of the house. And he and his wife also wanted a Key West style, so Oswald traveled to the Keys to study the vernacular. “It was difficult to achieve in a structure this large,” he admits, but he accomplished it by sticking with “pure simplicity”—simple, straight forms, carved details above the doors and windows, a metal roof, powder-coated aluminum sunburst railings and a cypress ceiling on the porch (the only part that will require maintenance, otherwise it was built to be “as maintenance-free as possible,” the builder says.) The exterior mossy green color was chosen so the home would blend into the landscape.

Lancaster came onboard in the beginning and was tasked with finding space for the owners’ extensive collection of paintings and drawings. She created a gallery in a space off the living room, where art, displayed on walls and on easels, is regularly rotated. A sampling of the husband’s hundreds of original classic movie posters lines the hallway leading to the home theater. (“He’s way into his movies,” says Oswald.)

The project took 16 months, including taking down 70 percent of the original house because of rot and mold, reconfiguring the placement of remaining rooms and adding thousands of square feet of living space. (The new master suite alone, with its own oval master bathroom with a light-filled rotunda, is 1,280 square feet). “It went from a remodel to a full-blown whole house rebuild,” says Oswald. “It was deconstructed, not demolished.”

“The house is what we wanted,” says the husband. “One of the reasons the house works for us was the architect’s ability to sit down with the family and understand how we would use the house on a daily basis.” That meant balancing togetherness and privacy, he explains. The house has plenty of spaces where kids and adults can enjoy their separate activities, but it also has common areas where the family can all come together—including those outdoor terraces, where the whole clan loves soaking up Sarasota sunshine and the dazzling Siesta Key views.

 DESIGN TEAM Designer and builder: Rick Oswald, OZ Planning, Design & Construction, (941) 400-6427; Interior designer: Bonnie Lancaster, ASID, Lancaster Humma White Studio, (941) 365-2342; Hand-painted furniture: Keffie Lancaster; Fabrics: Osborne & Little; Outdoor furniture: Oscar de la Renta; Stone and tile: American Tile & Stone, (941) 365-1383; Deconstruction material, recycling: Sarasota Architectural Salvage, (941) 358-7730; Roofing, gutters, guardrails, spiral stair: Mullets Aluminum, (941) 371-3502. For an extended list of suppliers, look below.