Ten fire engines and around 70 firefighters were called to a fire at a car workshop and offices on Kings Hall Road in Beckenham.
The first floor of the centre section of the two-storey building and the roof were destroyed by fire. Two cars were also damaged by the blaze. There were no reports of any injuries.
The fire was discovered by firefighters on the way back to their fire station who saw the blaze from the road.
Station Commander David Reed, who was at the scene, said: "The centre section of the building was alight. Firefighters worked incredibly hard to bring the fire under control.
"The blaze was producing heavy smoke, which went in the direction away from residential properties, but those in the immediate area were advised to keep their windows and doors closed.
"Access to Kings Hall Road was limited with road closures in place at the junction of Aldersmead Road and on Bridge Road. We encouraged people to avoid the area."
The Brigade was called at 0141 and the fire was under control by 0431. Fire crews from West Norwood, Bromley, Forest Hill, Croydon and surrounding fire stations attended the scene.
The cause of the fire is under investigation.