Four fire engines and around 25 firefighters were called to a flat fire on Glenmore Road in Belsize Park.
Part of a studio flat on the first floor of a converted mid-terraced house was damaged by fire. There were no reports of any injuries.
The fire is believed to have been accidental and caused by a fault in a toaster.
A London Fire Brigade spokesperson said: “It’s really important that you always check toasters and other appliances are turned off and unplugged when not in use, unless they’re designed to be left on, like your fridge or freezer.
"Keep clutter to a minimum on your kitchen work surfaces especially near toasters and hobs as they can accidentally switch them on or catch fire."
The Brigade was called at 1915 and the fire was under control by 2033. Fire crews from Kentish Town and West Hampstead fire stations attended the scene.