In London’s Chelsea, a Renovated 1860s House With an Observation Tower and T-Rex Skeleton Sells for $27.5 Million

“This was an extraordinarily unique home with a fun history that was rebuilt from scratch,” said the purchaser of the house, venture capitalist Walter Kortschak.

The sellers were Anthony and Pat Mascolo, according to property records. Mr. Mascolo is a member of the family behind the salon empire Toni & Guy. He co-founded the hair-products company TIGI, which makes Bed Head products and was purchased by Unilever in 2009.

When the Mascolos purchased the house in 2010, the original tower had been demolished and the house had not been touched in decades. They spent years doing a multimillion-dollar renovation, gutting and rebuilding the house with architect Max Broby of BrobyDNA and OLF Construction.

The renovation involved expanding the structure from three floors to five by excavating roughly 62 feet underground, Mr. Broby said. “We couldn’t go higher because of conservation rules, so we went down, as wide and long as possible,” he said. They added two subterranean floors, each with 13-foot-high ceilings and floor-to-ceiling windows. To bring in natural light and air, they dug a light well around the house and designed drop gardens and patios.

The Mascolos also rebuilt the tower. To access it, they installed what Mr. Broby described as a “New York-style fire escape” in a nod to the tower’s original use. A pull-down staircase operates with a “big, old-fashioned cranking handle that you wind,” said listing agent Robert Green of John D Wood & Co. At the top is a small room with 360-degree views. Mr. Broby said the spot is one of the highest lookout points in the area.

The house has four bedrooms and a guest apartment as well as a detached garage. There is a skylight in the center of the house. Just below it, a metal grid holds orb-like light fixtures that hang in an atrium over the central stairway. The stairs have wide landings that the Mascolos treated like a gallery, displaying everything from a motorcycle and a piano to a T-Rex skeleton, which wasn’t sold with the house.

“The exterior looks very traditional,” said Mr. Broby, adding that the entire house was rebuilt brick-by-brick to satisfy local conservation requirements. “But within it and below it, we could then have fun.” A stone staircase is the only piece of the original home that remains, he said.

In the new basement, the Mascolos installed a stainless steel basin for an indoor swimming pool, but then changed their minds. Mr. Broby built a home gym above the basin, which is still there in case a future owner should want a pool.

Mr. Green said his clients decided to sell because the house no longer fit their needs.

When Mr. Kortschak, 62, and his wife, Marcia, saw the house, they had looked at about 80 London properties remotely and some 10 in person. Mr. Kortschak, a former managing partner of private-equity firm Summit Partners, now serves as executive chairman of SignalFire, a venture-capital firm based in San Francisco. After years in Palo Alto, Calif., the Kortschaks now live primarily in Aspen, Colo. They also own two penthouses in London’s Fitzrovia neighborhood, as well as property on Kauai.

They started looking for a larger London home last year, Mr. Kortschak said, to accommodate extended family gatherings and to serve as a “base of operations” while visiting his parents in Geneva, where he grew up. The Kortschaks have two adult children and wanted a place suitable for longer stays for their son, a film producer in Los Angeles who has been collaborating more with partners in the U.K.

They sought a turnkey property that didn’t need to be remodeled, since renovating from overseas would have been challenging, said Henry Sherwood of the Buying Agents, who represented the Kortschaks in the deal.

Mr. Kortschak said they were drawn to this home’s location on Carlyle Square, in a part of London known for its manicured gardens and bohemian roots. They liked the village feel of Chelsea and the uniqueness of the property, which mixes traditional and contemporary design elements.

Inside the house, he said the stairway landings are ideally suited for entertaining. “There is definitely a feel as if you could mingle floor to floor,” he said. They also loved the home’s private garden. The basement level has glass doors to the outside, and in the kitchen, a steel-frame glass wall opens up to the garden. “The terraces have elements of an Italian villa,” he said.

The Kortschaks have listed their London apartments for $10.4 million combined, or $5.2 million and $5.1 million apiece with Alex Ross of Savills and Richard Douglas of London House.

In Hawaii, they own a compound on roughly 18 acres as well as a roughly 3,000-acre ranch where Mr. Kortschak said scenes from the “Jurassic Park” franchise were filmed. The couple listed the Hawaii compound for $35 million in 2018, but they ultimately decided not to sell.

They plan to continue splitting their time between Aspen and London, he said, with trips to Hawaii during cooler months. “There’s definitely a method to the madness,” Mr. Kortschak said. “Having a place in the mountains, near the ocean and in the city is very deliberate.”