Four fire engines and around 25 firefighters tackled a fire in a house in Winterbourne Road, Thornton Heath.
Half of a loft conversion was damaged by fire. There are no reports of any injuries.
The Brigade's fire investigators believe the fire was accidental and caused by smoking materials.
A London Fire Brigade spokesperson said: "We’d rather you didn’t smoke at all, but if you are a smoker, it’s absolutely vital you ensure your cigarette is completely out when you’ve finished smoking it.
“If you don’t, you risk causing a fire which could not only destroy your home, but also cost you your life."
The Brigade was called at 1214 and the fire was under control by 1309. Fire crews from Norbury, Croydon, West Norwood and Woodside stations are at the scene.
Safety advice for smokers: