Four fire engines and around 20 firefighters and officers were called to a fire at a terraced house in Wellington Road in Harrow.
Part of the first floor and part of the roof of one house was damaged by fire.
Nine people left the property before the arrival of the Brigade. One man was treated at the scene by London Ambulance Service crews and three infants were taken to hospital for precautionary checks.
The Brigade was called at 0406 and the fire was under control at 0535. Fire crews from Harrow, Stanmore and Northolt fire stations attended the scene.
The cause of the fire is believed to be accidental and due to an electric fan heater coming into contact with clothing.