Flames began spreading through the residential structures in the 8900 block of Lamar Street in Spring Valley shortly before 1 p.m., according to Cal Fire.
It took crews about 45 minutes to extinguish the blaze, said Kendal Bortisser, a fire captain with the state agency. Medics took one person to Sharp Grossmont Hospital for treatment of smoke inhalation.
The American Red Cross was called in to help the displaced residents -- six adults and three children -- arrange for interim shelter.
The cause of the fire was under investigation. It was not immediately clear which building it started in, Bortisser said.