The fire burning in the rural area of Clinton Keith and Tenaja roads was first reported about 11:30 a.m. By 3:30 p.m., it had scorched 350 acres. The fire was about 75 percent contained, Riverside County fire officials said.
Authorities said six air tankers, 39 trucks and four helicopters were aiding the firefight, as well as 327 firefighters.
Flames could be seen rising through the roof of the large, two-story house on a hillside. Firefighters tried to put the blaze out with hoses, but the structure appeared to be completely destroyed on the inside.
Three firefighters sustained minor injuries. One was taken to the hospital and another was treated at the scene. The condition of the third was not clear.
Some residents evacuated voluntarily. A temporary evacuation center was opened at Murrieta Valley High School, but it was closed later.
Cal Fire investigators said they believe the fire was started by motorized brush-clearing equipment.
The fire was expected to be fully contained by 8 a.m. Thursday and fully controlled by 6 p.m. Thursday.