Prompted by a tropical storm, a 4 a.m. lightning strike is said to have set fire to the Great House where Winslet and her children were staying, according to the Associated Press.
The actress was vacationing with her boyfriend, Louis Dowler, and her children Mia and Joe.
Branson, who was also on the premises during the storm — along with his wife and family — said that all guests managed to evacuate the house without injury.
"We'll all stay here for the time being," he told the AP. "There's a lot of damage and we won't be able to stick it back together again right away."
He added that his office was based in the house, and I have lost thousands of photographs which is very sad."
The 60-year-old business magnate purchased the entire island of Neckar in the early 1980s, building an on-site eco-resort as well as his Great House in 1982.