Four fire engines and around 25 firefighters were called to a fire at a house in Edgehill Road in Mitcham.
Half of a first floor bedroom was damaged by fire. The occupants of the property were alerted to the fire when they heard a smoke alarm sounding.
Three people left the property before the arrival of the Brigade. There were no reports of any injuries.
A London Fire Brigade spokesperson said: “This incident shows just how important it is to have working smoke alarms.
“Smoke alarms give the earliest possible warning when a fire starts Make sure smoke alarms are fitted in rooms where a fire can start except kitchens or bathrooms where heat alarms are more appropriate.”
The Brigade was called at 2157 and the fire was under control at 2226. Fire crews from Mitcham, Norbury and West Norwood attended the scene.
The cause of the fire is believed to be accidental and due to textiles and paperwork being stored too close to an electric heater.