Chattanooga firefighters responded at 4 a.m. on Monday to a fire at the Steak 'n Shake at 2296 Gunbarrel Road.
Battalion Chief Lesley Morgan was the first to arrive on the scene and reported seeing fire near the exterior wall near the front door.
Several fire companies arrived moments later and after tearing into the exterior wall, extinguished the fire in roughly 15 minutes. No injuries were reported.
The cause of the fire will be ruled accidental. Chief Morgan said it is likely that a discarded cigarette caught the mulch on fire, and the flames then spread to the exterior wall.
The loss was estimated at around $15,000.