The blaze was reported at about 6:30 p.m. Thursday in a commercial strip in the 800 block of North Elm Street.
Responding firefighters found heavy smoke and fire shooting out of the roof. It took nearly three dozen firefighters from Orange, Anaheim and the Orange County Fire Authority an hour to knock down the flames.
Crews managed to limit the fire to just one of the building's 14 units. No injuries were reported.
The cause of the blaze was under investigation.