Five CFA units from Clyde, Berwick and Cranbourne responded, supported by FRV.
Rostered Duty Officer District 8, Arthur Haynes, said the single-storey house was fully alight when firefighters arrived.
"The fire started in a double garage and progressed to the house," RDO Haynes said.
"The garage area was totally destroyed but crews did a great job to minimise the damage caused to the house."
Six occupants had safely self-evacuated.
The incident was under control at 1.22pm and a stop was issued at 2.47pm
Fire investigators attended and deemed the cause to be accidental.
The incident is a timely reminder for all Victorians to regularly service and maintain their electrical appliances, including heaters, and ensure working smoke detectors are installed.