The fire was reported at 6:07 a.m. at a home near 37th Avenue and Edison Street, San Mateo fire Battalion Chief Joe Novelli said.
The small house had been converted to a triplex and there was some confusion when firefighters entered the home and were unable to reach the back portion because it was partitioned off, Novelli said.
There were reports that a tenant was possibly trapped in a rear unit, so firefighters went around to the back of the house but did not find anyone, he said.
The home was packed with accumulated belongings and firefighters had trouble navigating their way through the living spaces, Novelli said.
There were four cats in the home at the time of the fire.
"Three got out, one was not able to make it," he said.
Two residents were displaced and are being assisted by the American Red Cross, he said.
The cause appears to be electrical, Novelli said.
There was at least one smoke detector in the home that did not have working batteries, and Novelli reminded residents of the importance of changing batteries and having functional smoke detectors.