Firefighters were dispatched shortly after 11 p.m. Sunday to the 18000 block of Avenida Alondra, where they called for a second-alarm response, said Rancho Santa Fe Fire Department spokeswoman Julie Taber.
A total of 40 to 50 firefighters were deployed, dispatched from Encinitas, Solana Beach and Carlsbad, Taber said.
Crews faced several challenges trying to contain the blaze, including the size of the home, ammunition exploding in the garage and only one hydrant near the mansion.
The homeowner reportedly lit the fireplace before going to bed, but investigators were still trying to determine the fire's cause and assess the damages Monday morning.
The home was a total loss and the Red Cross responded to help the displaced residents find temporary shelter. No injuries were reported.