Four fire engines and around 25 firefighters were called to a flat fire on Stanley Road in South Harrow.
Half of a three-roomed flat on the first floor of a converted semi-detached house and half of the roof of the building were damaged by fire. Two adults and two children left the building before the Brigade arrived. There were no reports of any injuries.
The Brigade’s 999 Control Officers took 10 calls to the fire.
Leading Firefighter Darren Draper, who was at the scene, said: "The fire was very visible, with heavy smoke in the area.
"Firefighters used a turntable ladder to gain access to the roof and remove roof tiles to check for fire spread.
"Crews also carried out salvage operations and prevented the fire spreading to neighbouring properties."
The Brigade was called at 2329 and the fire was under control by 0206. Fire crews from Harrow, Wembley and Northolt fire stations attended the scene.
The cause of the fire is under investigation.