Part of the building was damaged by fire. Five people left the building before firefighters arrived and were treated at the scene by London Ambulance Service crews for smoke inhalation.
The Brigade’s 999 Control Officers received 33 calls about the fire due to the amount of visible smoke in the area.
The Brigade was called at 1933 and the fire was under control by 2114. Crews from Mill Hill, Hendon, Finchley and Stanmore fire stations attended the scene.
The cause of the fire is under investigation by the Brigade and Metropolitan Police.