Six fire engines and around 40 firefighters tackled a fire at a semi-detached house on Grove Crescent in Colindale.
A loft conversion was destroyed and around half of the first floor was damaged by the fire. Six people left the building before the Brigade arrived. One man was treated on scene by London Ambulance Service.
Control Officers took the first of 20 calls at 1621 and mobilised crews from Hendon, Mill Hill, Wembley and surrounding fire stations to the scene. The fire was extinguished by 1920.
The fire is believed to have been accidental and was caused by an electrical fault.