Firefighters responded to a residence at 3399 Sonoma Highway with an engine company after the occupant requested an electrical investigation.
While preparing to walk down the residence's driveway, firefighters smelled smoke and found an adjacent office building located at 3385 Sonoma Highway full of smoke.
The assignment was upgraded to a first alarm structure fire at about 11:10 p.m., with two additional engine companies, a fire truck company and Santa Rosa police officers reporting to the scene.
The 15 fire personnel were able to contain the fire to the office building, which sustained structural damage estimated at $10,000. Damage to the building's contents is estimated at $60,000.
A subsequent investigation by PG&E of the residence led to the discovery of faulty wiring in an underground vault located adjacent to both buildings, according to the fire department.
No one was injured in the fire.