Four fire engines and around 25 firefighters tackled a fire at a block of flats in Apple Grove, Harrow.
Part of the external fascia of the building, from ground to first floor, was damaged by fire, as well as a small amount of two flats. An external storage cabinet was destroyed.
A woman and three children left the flat before crews arrived and were treated on the scene by London Ambulance Crews as a precaution.
The Brigade was called at 1544 and the fire was under control by 1734. Fire crews from Wembley, Northolt and Stanmore stations attended the incident.
The cause of the fire is believed to be the unsafe disposal of smoking materials.