The fire at a duplex at 290 White Road was first reported around 1:45 p.m., according to San Jose Fire Capt. Mike Van Elgort.
The fire moved rapidly and the duplex was well involved with flames when firefighters arrived on the scene, Van Elgort said.
Firefighters initially made an interior attack, but were unsuccessful and moved to an exterior defensive attack, as the fire was threatening other structures on both sides, Van Elgort said.
The fire was knocked down around 2:45 p.m. but firefighters were still on the scene an hour later overhauling the duplex and Van Elgort said the fire was not yet under control at that time.
One family member sustained a minor burn to the hand but was not taken to the hospital for treatment. No firefighters were injured.
The duplex sustained major damage to both units and two families, a total of nine people including three children, were displaced by the fire, Van Elgort said.
The fire was caused by plumbing work in the duplex, Van Elgort said.
Hot solder used to connect piping smoldered and then started the fire.