Six fire engines and around 40 firefighters were called to a house fire on Hillside Road in Pinner.
Most of the roof and part of the second floor of a three-storey house was damaged in the fire.
The Brigade’s fire investigators believe the fire was accidental and caused by an electrical fault with downlights in the bathroom.
The Brigade was called at 1936 and the fire was under control by 2133. Fire crews from Harrow, Ruislip, Northolt, Stanmore fire stations were at the scene.