An elderly couple were alerted to a fire on their doorstep by a passer-by in Pollard Road, Morden.
Police officers are investigating after the blaze beside the front door of the home was thought to be deliberate.
The London Fire Brigade attended the scene to extinguish the fire and police were called. Items found at the location suggested the fire may have been started deliberately.
The passer-by who alerted the elderly couple was driving past the house at about 11.25pm on Wednesday, March 5.
A police spokesperson said nobody was injured as a result of the fire and damage was confined to the door.
A 44 year-old male from Essex has been arrested in connection with this matter and has been bailed to return to a south London police station in late April.