Fire investigators say a family were lucky to survive after a grandmother discovered her sleepout was on fire, despite there being no alarms.

The Turangi shed being used as a sleepout was extensively damaged by the fire, which swept through the building early this morning.

Fire investigator Jon Rewi said it was a close call, as there were no smoke alarms and the grandmother was lucky to get out and help evacuate everybody from the main house. He emphasised the importance of smoke alarms in giving people time to get out of a burning building.

He praised the work of the Turangi Fire Brigade in confining the fire to the sleepout and saving the main house, which was only a metre away. The house was rented out, but insured.

Turangi Fire Brigade chief fire officer Tangonui Kingi said it was initially feared a woman and her grandchildren were inside the sleepout when the fire started just before 1am.

However, the woman was alone in the sleepout and she had managed to get out of the burning building. Mr Kingi said the woman woke her daughter and young grandchildren in the nearby house and made sure they were safely outside.

He too was concerned about an absence of fire alarms in the shed conversion.

He said it was "down to luck" that she managed to wake up and escape the fire.