The fire was first reported just after 4 am on O'Ryan Street near Mitchell Avenue.
Officials said the fire started in a back patio area of the house, then spread to a built-on living area.
Crews were able to keep the fire in that added room but said the flames caused significant damage.
The room did not have a working smoke detector, investigators said, and neighbors had to alert them.
A woman escaped through a window before firefighters arrived on scene.
No one was hurt.