It happened around 7:10 a.m. on Clay and Rowell Avenues off of Olive and Cedar.
Officials say firefighters responded to reports of a patio fire in one of the units. When crews arrived, they saw flames spread to the second floor of the building.
Fire Battalion Chief Chad Tucker said the fire caused issues with power lines in the area. PG&E crews had to de-energize the lines before fire investigators could enter the building.
Residents of the complex were evacuated. The fire damaged two units, leaving six people without a home.
No one was injured.
The Red Cross is helping the two families find a new place to stay.
The cause of the fire is under investigation.