Four fire engines and 21 firefighters and officers were called to a fire at a house in Clewer Crescent, Harrow on Friday evening.
Half of the first floor and half of the loft conversion were damaged by fire. Three people left the property before the arrival of the Brigade.
The Brigade was called at 2241 and the fire was under control at 0128 on Saturday. Fire crews from Stanmore and Northolt fire stations attended the scene.
The cause of the fire is under investigation.