Firefighters responded to reports of a fire at 366 and 376 Flower Ave. at 9:02 p.m., according to the Santa Rosa Fire Department.
Upon arrival, crews found one unit of the duplex engulfed in flames. All of the residents had left the building before firefighters arrived, fire officials said.
A dog tethered to the structure was rescued by firefighters.
The blaze was under control within 30 minutes, fire officials said. One unit was destroyed, and the second unit was badly damaged.
A car in the parking lot of the building sustained major heat damage, and four nearby cars had moderate heat damage.
A nearby gas station on Santa Rosa Avenue was evacuated as a precaution.
Two families totaling eight people were displaced by the fire. One family is staying with relatives, and the other is receiving assistance from the American Red Cross, fire officials said.
Fire investigators said the fire appears to be accidental, but the cause remains under investigation.
No injuries were reported.
The loss is estimated at between $75,000 and $100,000, according to the fire department.