Firefighters were called to a blaze in a derelict building on East Road in Edgware on Sunday afternoon after passers-by called 999.
The fire badly damaged the single storey building and two people left the property before the Brigade arrived.
The Brigade was called at 1319 and the fire was under control at 1523. Crews from Stanmore, Mill Hill and Hendon fire stations attended the incident. The cause of the fire is under investigation.