Four fire engines and around 25 firefighters tackled a fire at a house on Bensham Lane in Croydon.
An outhouse building connected to a semi-detached house was destroyed by the fire. One man left the building before the Brigade arrived and was taken to hospital by London Ambulance Service crews.
The fire is believed to have been accidental and caused by the burning of garden waste, which then spread out of control.
A London Fire Brigade spokesperson said: “We would urge people to consider if their bonfire is necessary and to take care if you’re burning waste at home.
"If you’re having a bonfire never leave it unattended and consider your neighbours when you do decide to have a fire in your garden.
“Make sure you build it well clear of buildings, garden sheds, fences and hedges and never use flammable liquids to start it or burn dangerous items such as aerosol cans. Always keep a bucket of water or a hosepipe nearby in case of fire.
“Always check your local authority’s advice on bonfires and follow our safety tips."
The Brigade was called 1806 and the fire was under control by 1908. Crews from Croydon, Norbury, Woodside, and Mitcham fire stations attended the scene.