A fire which completely destroyed a caravan and caused thousands of pounds of damage to a house and garage was started deliberately, police said.
Officers said a man was spotted running away from the blaze at the property in Ash Green Road, Ash, at about 2345 BST on Saturday.
The fire, which is believed to have started in a four-berth caravan, caused an estimated £15,000 of damage.
Surrey Police said the elderly woman who lived at the house escaped unhurt.
Det Con Jackie Green, of Surrey Police, said: "Thankfully, the elderly occupant of the house escaped injury, she has been left extremely shaken and upset and we are determined to find the person responsible and ensure they are brought to justice."
He added: "The fire is definitely suspicious although at this present time, we have been unable to pinpoint exactly how it started and there is no clear motive.
"We are speaking to witnesses and have already spoken to one who gave us a description of a man seen running off from the house just after the fire broke out.
"We are also looking into another man who was reported to be knocking on doors in the area earlier that evening, around 10pm."
The man who was seen running away from the fire was white, 6ft tall, aged about 30 years old, with tanned skin and dark greasy hair.
He was wearing beige shorts and a beige zipped-up ski jacket.