Four fire engines and around 25 firefighters tackled a house fire on Montrose Road in Harrow.
Half of the second floor of a semi-detached house was damaged by fire. A man and a woman left the building before the Brigade arrived. A woman was taken to hospital by London Ambulance Service crews.
The fire was discovered by a neighbour who could smell burning and came out into the street to see a developing fire in the flats opposite. They made the first of seven calls to the Brigade.
The Brigade was called at 1613 and the fire was under control by 1742. Fire crews from Stanmore, Northolt and Wembley fire stations attended the scene.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.