Ten fire engines and around 70 firefighters tackled a fire on Cricklewood Broadway in Cricklewood.
A third floor flat and loft of a building containing a shop and flats was destroyed by fire. Four people and a dog were led to safety from the building by firefighters. A man was treated at the scene by London Ambulance Service crews.
The Brigade's 999 Control Officers received 38 calls about the fire. The fire was extinguished by crews wearing breathing apparatus and a 32-metre turntable ladder was used to help put out the blaze. Road closures were in place for much of the day to allow crews to work safely.
The Brigade was called at 1010 and the fire was under control by 1246. Fire engines from West Hampstead, Kentish Town, Finchley, Willesden, Hendon, North Kensington and surrounding fire stations attended the scene.
The fire is being treated as accidental and is believed to have been caused by a fault with an extractor fan.