The fire broke out just before 1:15 a.m. in an empty building - formerly a restaurant - at 2153 Oak Grove Road just north of Ygnacio Valley Road in Walnut Creek, the Contra Costa Country Fire Protection District dispatcher said.
Three fire engines, two trucks, and two battalion chiefs were sent to respond to the single-alarm fire, the dispatcher said.
Firefighters battled the flames for more than an hour before the blaze was "knocked down" at 2:29 a.m., meaning the fire was not fully extinguished, but responders had control of it, the dispatcher said.
No one was injured in the blaze, she said.
The fire protection district estimated the damage to the building to be worth $1 million, and declared the structure a total loss, the dispatcher said.
The cause of the fire is undetermined and the case is under investigation, the dispatcher said.
Though the last fire crews left the scene at about 5:25 a.m., an investigator was still at the scene as of 6:40 a.m., she said.