The two-alarm blaze was reported just after 4 a.m. and was out by 5:15 a.m., KTVU-TV reported. Fire officials said one of the two decorative cupolas on the building was gutted.
Twelve fire companies responded to the ranch. No injuries were reported.
Lucas, who began buying land in the area in 1978, owns thousands of acres around the ranch near Nicasio in Marin County. Only about 15 acres has been developed.