Firefighters responded to a report of fire at a house at 809 Fallon Ave. at about 8:50 a.m., San Mateo Fire Department Battalion Chief Jim Comisky said.
Arriving crews fed hose lines into the house and found no one inside during an initial search, Comisky said.
All nine occupants of the home had evacuated and were located at a neighbor's home.
No one was injured.
The fire was quickly extinguished.
The American Red Cross was working to find temporary housing for the displaced residents.
Investigators were trying to determine what caused the fire, which appeared to be accidental, Comisky said.