The three-alarm blaze, however, is not considered suspicious, Contra Costa County Fire Protection District spokeswoman Emily Hopkins said.
Three families, which included four adults and two children, were displaced by the fire and are receiving help from the American Red Cross, Hopkins said.
The fire was reported around 5:30 p.m. on Taylor Road in unincorporated Contra Costa County.
Wind blowing at about 20 mph pushed the fire from one house to another, Battalion Chief Dave George said.
No fire hydrants are on the island, which created another challenge for the nearly 60 firefighters from the county's two fire districts that responded, according to George.
Firefighters shuttled more than 30,000 gallons of water to the island and were able to save the third house, George said.
The fire also sparked several small grass fires in the area, which firefighters also put out, Hopkins said.
The flames were controlled by 7 p.m., George said. A woman was treated for smoke inhalation at the scene, but did not require hospitalization, Hopkins said.
Preliminary estimates put the damage to the houses at about $1.5 million, Hopkins said.