Coastside Fire Protection District units responded to reports of fire at a one-story house at 12599 San Mateo Road just before 9 a.m., according to Cal Fire.
Arriving firefighters reported smoke coming from the house and found flames in a rear bedroom wall.
Good Samaritans had apparently started to put out the fire before firefighters arrived, and the flames were quickly contained.
No one was injured, according to Cal Fire.
Units from the San Mateo County Fire Department, the Central County Fire Department and the Half Moon Bay Volunteer Fire Department responded.
The cause remains under investigation.