Four fire engines and around 25 firefighters tackled a fire at a house being used as a suspected cannabis factory on Wellington Road in Wealdstone.
Part of the first and second floors of the building were damaged by fire. There were no reports of any injuries.
A London Fire Brigade spokesperson said: “Cannabis factory fires can be particularly dangerous because of the way criminals use unsafe wiring to illegally obtain electricity in order to grow the plants.
"They're often in top floors or lofts which means when a fire takes hold it spreads, destroying roofs and damaging neighbouring buildings.
"Firefighters can also be exposed to risks when dealing with the fires because of bad wiring and traps at some of the properties.
"It's important that people know the key signs and inform the police so that they can act swiftly to prevent these dangerous fires from happening."
The Brigade was called at 0653 and the fire was under control by 0839. Fire crews from Harrow, Stanmore and Northolt fire stations attended the scene.
The cause of the fire is believed to have been electrical.