Four fire engines and around 25 firefighters tackled a fire at a health centre on Gordon Road in Wood Green.
Part of the ground floor and roof of a single-storey building was damaged by fire. There were no reports of any injuries.
A London Fire Brigade spokesperson said: "The fire started in a staff kitchen area and heat and smoke travelled through the building due to a door wedged open.
"This incident shows the difference a closed door can make. The damage caused by the fire was more extensive than it would have otherwise been if the kitchen door was closed.
"Try to remember to close doors behind you to rooms not in use and when you go to bed to slow down the spread of flames and smoke in the event of a fire."
The Brigade was called at 0647 and the fire was under control by 0745. Fire crews from Southgate, Finchley and Hornsey fire stations attended the scene.
The fire is believed to have been accidental and caused by an electrical fault.