Four fire engines and 21 firefighters and officers were called to a fire at a disused office block in High Street, Wealdstone.
A lift motor room on the third floor of the building was completely damaged by fire.
The Brigade was called at 1159 and the fire was under control at 1323. Fire crews from Harrow, Stanmore and Northolt fire stations attended the scene.
The cause of the fire is believed to be accidental and due to the unsafe disposal of smoking materials.