About 20 volunteer firefighters attended a Clyde St home, after a garage underneath the house caught fire.
Balclutha Volunteer Fire Brigade deputy chief fire officer Terry Michie said two appliances from Balclutha and one from Kaitangata were called to the blaze about 4.30am.
Mr Michie said the owner woke to the smell of smoke, found the fire, and got the three other occupants - one other adult and two children - safely out of the house.
The smoke alarms also activated, he said.
Firefighters returned to the house for about 10 minutes at 9.45am after reports of smoke.
Southern region fire risk management officer Stuart Ide said the fire was accidental, with the most likely cause pyrolysis - which in this case involved timber underneath concrete decomposing over several years before igniting.
Mr Ide said the blaze started in the hearth in the lounge, before moving up into the ceiling cavity and into the downstairs garage.
The house was extensively damaged, with smoke damage throughout.
