Four fire engines and around 25 firefighters were called to a fire at a sheltered accommodation block in Broadfields, Harrow Weald.
Part of a ground floor flat was damaged by fire.
One woman left the property before the arrival of the Brigade and was treated at the scene by London Ambulance Service crews. A further 20 people left the building before the arrival of the Brigade and one woman was assessed at the scene by London Ambulance Service crews.
The Brigade was called at 1944 and the fire was under control at 2104. Fire crews from Harrow and colleagues from Hertfordshire Fire and Rescue Service attended the scene.
The cause of the fire is under investigation.