Six fire engines and around 40 firefighters tackled a fire at an assisted living accommodation block on Western Parade in Hillingdon.
Part of the roof of the three-storey property was damaged by fire. Around 17 people left the building before firefighters arrived and there have been no reports of injuries.
A number of roads at the junction of Western Parade and Long Lane were closed to allow firefighters to deal with the incident safely.
The Brigade was called at 0328 and the fire was under control by 0634. Crews from Northolt, Ruislip, Hillingdon and surrounding fire stations attended the scene.
The fire is being treated as accidental and is believed to have been caused by an electrical fault.