A grandfather, a father and two children escaped uninjured from the blaze at Kingfield Avenue, Downpatrick.
Paul Reilly said the family were woken up by "an almighty blast", at about 06:00 BST on Friday.
He discovered two fires - one in the eaves and a second downstairs behind the television in the living room.
The roof had partially collapsed and the upstairs bedroom was destroyed.
According to Mr Reilly, the fire service said it was most likely caused by a lightning strike.
"They reckon that the lightning had struck the aerial upstairs, and followed the cabling which went through the eaves of the house which is where the small fire was up in my father's room.
"It came down to the aerial sockets behind the TV in the living room.
"They both blew out and there was a fire in the living room and a fire upstairs.
The Northern Ireland Fire and Rescue Service (NIFRS) said the blaze had also spread to an adjoining property.
The fire has now been brought under control.
A lightning strike was the probable cause of a fire that caused extensive damage to a house in County Down, the Fire Service has said.