The blaze burned a 1,500-square-foot home at 1995 Capell Valley Road.
On the same property, firefighters found a legal marijuana growing operation in the yard and inside a 1,000-square-foot outbuilding, Munoa said. The outbuilding wasn't burned in the fire.
Munoa said the Napa County Sheriff's Office determined both the owners and the renters of the one-story home had medical marijuana cards, Munoa said.
Neither the owners nor the renters were home when the fire started around 8:30 a.m., Munoa said.
A motorist driving past the house, located off state Highway 128 saw smoke and flames and called the fire department, Munoa said. Cal Fire and the Napa County Fire Department responded.
The fire caused $150,000 in damage to the home and between $50,000 and $75,000 in damage to the contents, Munoa said.
The cause and point of origin are under investigation.