Blaze destroys West Berkshire millionaire's Poole house
Dorset Fire and Rescue Service (DFRS) was called to Poole Harbour, at about 7.20am.
The 45 crew members were taken by boat to Green Island (pictured), which was bought in 2005 for £2.5m by The Hon Edward Iliffe, of Yattendon.
On arrival firefighters found a blaze in a four-storey timber building which was still under construction.
Eight fire engines dealt with the incident and four main lines were used to extinguish the fire.
The fire was put out by 1pm yesterday but fire crews will remain on the scene for most of today.
In 2010, Mr Iliffe got permission to build a six-bedroom four storey timbre house on the 45-acre island.
He planned for it to be a retreat for his family.
DFRS say the house involved had been severely damaged by the fire and some parts of the building have collapsed, but firefighters were able to stop the fire spreading to the rest of the island.
The cause of the fire is still unknown.