Four fire engines and 21 firefighters and officers were called to a fire at a house in Mount Pleasant, Harefield.
Half of the first floor and part of the roof of a semi-detached house were damaged in the fire.
Two people left the property before the arrival of the Brigade and one man was treated at the scene for smoke inhalation by London Ambulance Service crews.
The Brigade was called at 1459 and the fire was under control at 1652. Fire crews from Ruislip and Harrow fire stations attended the scene along with colleagues from Hertfordshire Fire and Rescue Service.
The cause of the fire is under investigation.