Six fire engines and around 40 firefighters tackled a blaze in an office block in High Road, East Finchley.
Part of the ground floor was damaged by the blaze. There were no reports of any injuries.
The alarm was raised after a passer-by spotted flames inside the building.
On arrival, crews were faced with a small fire in a ground floor office and the building was heavily smoke logged.
The Brigade was called at 0049 and the fire was under control by 0233. Fire crews from Finchley, Hornsey and Kentish Town Fire Stations attended the incident.
The cause of the fire is under investigation.