Four fire engines and around 25 firefighters tackled a fire at a suspected cannabis factory on Greenway in Harrow.
Part of the ground floor of a semi-detached two-storey house was destroyed by fire.
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 1231 and the fire was under control by 1349. Fire crews from Stanmore and Wembley fire stations were in attendance.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.