Tucked deep inside Siesta Key's Sanderling Club, behind a wall of tropical foliage and the discreet gates of one of the barrier island's last true architectural enclaves, stands a home that appears less constructed than conjured.

Known as the Ness House, the 4,006-square-foot home on Sanderling Road was built in 1961 by John Lambie Sr. and renovated in 1971 by Sarasota School architect Tim Seibert. Today, it’s on the market for $5.3 million.

“It’s like the goddess of all things,” Sheba Matheu, 53, says of the home. She bought the property in 2014 for $1.75 million with her now-ex-husband and has spent the past decade preserving and shepherding its evolution. “You’re inside yet everywhere you look, there’s nature. When it rains, the whole house comes alive.”

The house has since earned historic designation, a recognition that protects its essential form and can even offer property tax exemptions for renovations and protect the home from demolition or incompatible changes. In the case of Sarasota School homes, the designation preserves their architectural integrity while enhancing long-term value.

For Matheu, the designation was never a constraint. “We didn’t need to change the footprint. Everything about it was already flowing, already intentional,” she says. “The whole house is sliding glass and light.”

Seibert was not just a founding figure of the Sarasota School; he also helped define its ethos. The style is known for its passive cooling, geometric clarity and an integration with the tropical Gulf coast climate. Lambie Sr., its original builder, was known for his poured-concrete “lamolithic” technique, which gives the house its fortress-like durability and sculptural quality.

In 1971, Seibert collaborated with the Ness family on the first of several renovations. His remodel in 1971 added defining features like a circular entry wall and reinforced its philosophical backbone. Decades later, updates executed by Nautilus Homes ushered the house into the present day. Epoxy floors were exchanged for seamless, kiln-fired four-by-four Kerlite porcelain tiles, brushed aluminum baseboards, an outdoor kitchen and shell-inlaid hardscaping that blurs garden and courtyard.

Indeed, walking through the space is like moving through a series of quiet invitations. A reading pod in the front courtyard offers a dry perch during Florida’s long rainy season; an ipe wood treehouse in the back overlooks Heron Lagoon, where Matheu and her daughter Amina fished from the dock “enveloped in mangroves,” she says. In the winter, the family would carry comforters up to the rooftop to stargaze.

“You can see the water from up there, too,” she says. “Even the skylight makes this spot of light inside, and Amina would curl up in it like a cat.”

Before Matheu, the Ness House belonged to Dagan and Gabriella McCann, both architects, who also undertook an earlier renovation with reverence for the home's midcentury roots. “It was flawless,” Matheu says of the McCanns' work. 

To reimagine the grounds, she brought in landscape architect David Young, who collaborated with her on a playful homage to Seibert’s original entry wall, using sculptural concrete circles as a visual echo rather than an exact replica of Seibert's work. She also recreated a fire pit area with the help of Jake Brady.

The interiors—those white porcelain tile floors and warm wood furnishings pulled from original design molds—lean minimal without feeling cold. Matheu, an artist and former model who once worked with the U.S. Department of State on digital outreach, handled most of the design herself. “It was the first time I ever showed my art,” she says. Many of the pieces on the walls were painted by her, guided by sacred geometry, Fibonacci sequences and what she calls “the intersection between numbers and theology.”

The kitchen is clean-lined and recessive, with a dramatic picture window facing native greenery and a counter-to-ceiling terrazzo-pattern backsplash. A long white island stretches across the open dining area, lit by a globe chandelier that echoes the home’s midcentury roots. Nearby, a yoga platform—35 feet by 45 feet of concrete—can host up to thirty guests at a time. The Florida room, enclosed in retractable glass, opens to an outdoor kitchen, lounge and dining zone.

Sustainability, too, is built into the home’s bones. Seibert’s renovation included the home's signature circular concrete wall near the entrance and retained the original flat-roof design with a rainwater collection system that channels runoff into a 1,500-gallon cistern, now used for irrigation. Lambie oriented the house in deference to wind and sun, angling its north wall to blunt gusts coming off the water. Even during last year's devastating hurricane season—when fellow Sarasota School architect's iconic Sanderling beach cabanas were destroyed—damage was minor and involved replacing drywall and fixing some electrical damage. Raised outlets, a steel front door, and concrete slab construction proved prescient.

“It’s a fortress,” Matheu says, “but it never feels closed off.”

Inside, each bedroom has its own view or courtyard. One guest room opens to a hammock behind a patterned breeze-block wall. The pool—a long, deep and narrow strip tucked alongside the house—is wrapped in white concrete and shielded from the sun. “The placement is intentional,” Matheu says. “Even the shade is by design.”

Though it's named for its original owners, it’s hard not to associate the home with Matheu, whose life story, which arcs from New York modeling gigs to policy work to motherhood and artistic expression, feels reflected in its layers. It's an exercise in modern restraint, revived for a new generation—a balance of historic lineage and forward-minded adaptation—and a testament to Seibert’s work. 

Despite all this, Matheu says, it’s time to pass on the house to its next steward. Her daughter is now in high school, and their new neighborhood is closer to the bus stop.

“This house will attract someone grounded,” she says. “Not someone chasing luxury. Someone connected to nature and to themselves.”