Four fire engines and around 25 firefighters attended a fire on Sydenham Road in Sydenham.
An office at a ground floor commercial premises was damaged by fire. Four people left the premises before firefighters arrived and there were no reports of any injuries.
The property where the fire occurred had no smoke detection fitted.
A London Fire Brigade spokesperson said: "This incident shows just how important it is to have working smoke alarms fitted on every level of your property.
“Smoke alarms give the earliest possible warning when a fire starts and we would urge everyone to make sure they have one fitted in every room where a fire can start except kitchens or bathrooms where heat alarms are more appropriate.”
The Brigade was called at 1333 and Control Officers mobilised crews from Forest Hill, Lewisham, Peckham and West Norwood fire stations to the scene. The fire was extinguished by 1436.
The fire is believed to have been caused by electrical resistive heating.