Six fire engines and around 40 firefighters tackled a house fire on Longfield Avenue in Mill Hill.
Half of the first floor of a semi-detached house was damaged by fire and the loft conversion was destroyed. There were no reports of any injuries.
A London Fire Brigade spokesperson said: “The fire was discovered by an occupant who could smell burning. They attempted to tackle the fire themselves before leaving the building.
“It’s vitally important that you do not attempt to tackle fires yourselves - leave it to the professionals. The quicker you call us, the quicker we can send fire engines to an incident.
“As soon as you become aware of a fire in your property, get out, stay out and call 999.”
The Brigade was called at 2009 and the fire was under control by 2128. Fire crews from Mill Hill, Hendon, Finchley, Stanmore and surrounding fire stations attended the scene.
The cause of the fire is under investigation.