The one-alarm blaze was reported at 4:59 a.m. in the 100 block of Greenway Drive, near Ygnacio Valley Road.
Arriving firefighters found the garage, which is attached to the home, engulfed in flames with several cars burning, Contra Costa County Fire Protection District Marshal Lewis Broschard said.
The fire crews attacked the blaze from several angles to keep flames from spreading to the home, and controlled the fire around 5:45 a.m.
Broschard said the garage was destroyed and four cars were "significantly damaged."
Firefighters managed to protect the home's living area, and the family, which consists of two adults and two juveniles, was not displaced, he said.
The blaze caused an estimated $200,000 in damage, including the loss of the vehicles. The cause of the fire remains under investigation.
No injuries were reported.