Four fire engines and around 25 firefighters responded to a flat fire on Brecknock Road in Tufnell Park.
Half of the lower-ground floor of a split-level flat, as well as part of the staircase and the ground floor were damaged by fire. There were no reports of any injuries.
The Brigade was called at 1403 and the fire was under control by 1510. Fire crews from Kentish Town, Holloway and Soho fire stations attended the scene.
The fire is believed to have been accidental and caused by an electrical fault in the thermostat of a water heater.