Ten fire engines and around 70 firefighters tackled a flat fire on Netherhall Gardens in Hampstead.
A fourth-floor flat and the roof of a converted detached house were destroyed by fire. Ten people left the building before the Brigade arrived. A man was treated for smoke inhalation and taken to hospital by London Ambulance Service crews.
Firefighters rescued a cat from the first floor of the building.
Two 32-metre turntable ladders were mobilised to the scene and were used as water towers to help extinguish the fire from height. The Brigade's drone team was also in attendance, helping to provide the Incident Commander with a greater situational awareness of the incident.
The fire is believed to have been accidental and caused by unattended cooking.
A London Fire Brigade spokesperson said: “The fire was discovered when an occupant saw their cooking had caught fire. When firefighters arrived, smoke and flames were coming from the front dormer windows on the top floor.
"Fires are more likely to start in the kitchen than any other room in the home.
"Cooking should never be left unattended on the hob or grill - if you have to leave the kitchen, turn off the heat.
“Firefighters also recommend that you have a look at our online Home Fire Safety Checker to make sure there are no hidden hazards in your home.
“The online tool will ask you a series of questions that will help us work out the level of risk in your home, or the home of someone you care for. It only takes a few minutes to get tailored advice to keep yourself and loved ones safe from fire.”
The Brigade's Control Officers received more than 30 calls about the fire. The Brigade was called at 1834 and Control Officers mobilised crews from West Hampstead, Kentish Town, Holloway, Hendon, North Kensington and surrounding fire stations to the scene. The fire was brought under control by 2110.