A investigation will aim to establish how a set of bedroom curtains caught light.
Firefighters from Peterlee and Seaham were called to a house in Colling Avenue, Deneside, Seaham, at 6.20pm today.
When the crew arrived, they found the family had already evacuated their home.
Two officers wearing breathing apparatus were sent in to put out the fire using a hose reel, with a fan then clearing the house of fumes.
The blaze has left the bedroom damaged and the rest of the first floor affected by smoke.
No one was injured in the incident.