Firefighters from around the San Francisco area and as far away as Los Angeles were headed to Napa County to help the 1,000 already trying to get the fire under control. CalFire, the state firefighting agency, said 16 local, county and tribal fire departments were pitching in.
Officials said the weather was warmer Thursday, which could hamper firefighters.
After a long drought, California has had more and larger wildfires than usual in the spring and early summer.
The fire was reported Tuesday. By Wednesday, it had charred more than 3,000 acres in Napa County and in Lake County to the north.
Officials said the fire had destroyed nine buildings, including the two homes. An evacuation order remained in place for 200 houses.
Businesses in the area were also feeling the effects. Chad Frazier, who manages the Markley Cove Marina on Lake Berryessa, said traffic was slow even though the marina is 30 miles from the fire.
"Normally we'll have 30 or 40 launches maybe by noon and today we had a third of that," he told KTVU-TV Wednesday afternoon.