Six fire engines and around 40 firefighters attended a fire at a house converted into flats on Kenton Road in Harrow.
A flat on the second floor was destroyed by fire. Seven people left the property before firefighters arrived, including one person who was treated by London Ambulance Service.
The Brigade's Control Officers took around 12 calls about the fire.
The first call was received at 0124 and Control Officers mobilised crews from Harrow, Northolt, Wembley and Stanmore fire stations to the scene. The fire was brought under control by 0226.
The cause of the fire is under investigation by the Brigade and Metropolitan Police.