A smoke alarm woke an elderly householder in time for them to alert emergency services.
Fire engines from Borehamwood, Garston, St Albans and Watford attended the detached house, in Oakridge Avenue, Radlett at around 5.46am yesterday morning.
Firefighters used breathing apparatus, two water jets and a hose to tackle the fire, which had spread from the ground floor to the first floor.
Crews were also concerned the fire had spread to the attic, but were able to prevent this and had put the fire out by 6.59am.
Two people were taken to hospital by ambulance.
The cause of the fire has been registered as accidental.