Four fire engines and around 25 firefighters tackled a house fire on South End Road in Hampstead Heath.
Half of the ground floor and part of the first floor of a mid-terraced house under renovation were damaged by fire. Two adults and a teenager left a neighbouring property and were taken to hospital after suffering from smoke inhalation.
The Brigade was called at 0308 and the fire was under control by 0430. Fire crews from West Hampstead, Kentish Town and North Kensington fire stations attended the scene.
The fire is believed to have been accidental and caused by hot works carried out at the property.