The fire was reported at a two-story duplex at 906 E St., near Ninth Street, at about 2:30 a.m. It was controlled by 3:30 a.m., Smith said.
The blaze, which started in a water heater closet on the second floor, damaged both units.
No one was injured, Smith said. He said the residents were fortunate, given that there were no functioning smoke detectors in the upstairs unit.
Smith suspected the fire may have been caused by combustible material near the water heater or a malfunction, but said the blaze is still under investigation.
Roughly 30 firefighters from the Union City, Newark and Fremont fire departments assisted.