A bedroom was partially destroyed by a fire caused by sunlight reflecting on a mirror.
The blaze started in a house in Third Avenue, Walton-on-the-Naze, Essex, on Sunday morning.
It started after "sunlight on a mirror caused a desk to ignite", Essex County Fire and Rescue Service said.
No-one was injured, but an officer warned: "This incident shows how easy it is for magnifying or reflective objects to cause fires."
He added: "Please ensure there are no such objects on your window sills or anywhere in your home where they are in direct sunlight."