It was first reported just after 11:30 p.m. on Belmont and Glenn.
Firefighters say it started in the back of the building, flames moved inside a wall of the two-story building, and then spread to the attic.
Crews are still looking for a cause, but believe it could be electrical. PG&E had to shut off the gas and power to the building.
All of the residents are adults and no one was injured.