Four fire engines and around 25 firefighters were called to a fire at a maisonette on Stafford Road in Kilburn.
The whole of the fourth floor of a split level maisonette was destroyed by fire.
Fire investigators found the property didn't have working smoke alarms and the fire was most likely caused by the unsafe disposal of smoking materials.
A London Fire Brigade spokesperson said: "This incident shows just how vital it is to have working smoke alarms.
"It's also a timely reminder for smokers to ensure your cigarette is completely out when you’ve finished smoking it.
“If you don’t, you risk causing a fire which could not only destroy your home, but also cost you your life.”
The Brigade was called at 2118 and the incident was over by 2246. Fire crews from Paddington, North Kensington, West Hampstead and Kentish Town fire stations were in attendance.