The Necker Island home was struck by lightning during a tropical storm at around 0400 on Monday and was completely destroyed by the fire that followed.
The Virgin Group boss, who was staying at a nearby property, said the fire had “completely destroyed” the building.
“The blaze is not yet completely out,” said Sir Richard.
“Around 20 people were in the house and they all managed to get out and they are all fine,” he added.
Sir Richard’s 90-year-old mother Eve and his 29-year-old daughter Holly were also staying at the property, named the Great House, when the blaze broke out in the early hours of Monday.
“We had a really bad tropical storm with winds up to 90mph,” said Sir Richard, who was staying at a nearby property with his wife Joan and son Sam, 25, at the time.
“My son Sam and nephew Jack rushed to the house and helped get everyone out, and many thanks to Kate Winslet for helping to carry my 90-year-old mum out of the main house to safety.”
“The main house is destroyed and the fire is not yet completely out.”
“My office was based in the house and I have lost thousands of photographs and my note books which is very sad. But all family and friends are well – which in the end is all that really matters.”