Six fire engines and around 40 firefighters were called to a fire at block of flats under construction on Church Street in Marylebone.
Most of a large polystyrene foam slab outside the building was damaged by fire. Around 92 people left the site before the Brigade arrived. There were no reports of any injuries.
The fire is believed to have been caused by hot works which were being carried out.
A London Fire Brigade spokesperson said: "On arrival crews were faced with smoke logging the basement of the building.
"A section of polystyrene was caught when hot works were being carried out on the building site on the ground floor and dropped down into the basement level through a small gap."
The Brigade was called at 0924 and the fire was under control by 1109. Fire crews from Paddington, Euston, Kensington and Soho fire stations attended the scene.