The blaze sparked just after 11 Sunday night in the area of Bullard and Maroa, behind St. Anthony's Church. Firefighters said they arrived to find heavy fire and smoke.
Fresno Fire Battalion Chief Todd Tuggle said, "It made a pretty quick entry into the home. It burned out the windows and was able to get into the living space and attic, and started to get into the main fire unit."
It took firefighters 15 minutes to control the fire. Most of the damage was done to the original unit, but most of their belongings had been saved.
Two people in each of the two units affected were displaced, but no one was hurt.
The cause is still under investigation.