Six fire engines and around 40 firefighters were called to a fire at a chalet bungalow on Waring Drive in Orpington.
Most of the ground floor was damaged by the fire and the first floor and roof were destroyed. A woman left the building before the Brigade arrived. She was treated at the scene for smoke inhalation by London Ambulance Service crews and taken to hospital.
A London Fire Brigade spokesperson said: "There were no smoke alarms fitted in the property. The occupant discovered the fire after she smelt burning and tried to tackle the fire herself before leaving.
"This is a timely reminder of just how important it is to have smoke alarms fitted in your home. They give the earliest warning of fire and you should have them fitted in every room where a fire could start.
"You should also never try to tackle a fire yourself. If you discover a fire in your home, raise the alarm to anyone else inside, get out and call 999.
"On arrival, crews found a well developed fire on the ground and first floors.
“There were a lot of possessions and clutter inside the house which made it challenging for firefighters and they tackled the blaze from the outside.
"Hoarding means exit routes can become blocked, making self-evacuation more difficult. Fires can also spread much faster, especially when there are lots of flammable items such as newspapers or cardboard."
The Brigade was called at 1733 and the fire was under control by 1925. Fire crews from Orpington, Biggin Hill, Addington and Woodside fire stations were at the scene.
The cause of the fire has been recorded as undetermined.