Firefighters were called out to the incident on Norwich Place at around 3am but the householders were already out of the building.
The fire was contained within the wall cavities of the house and three fire engines attended, along with an aerial ladder and a search and rescue team.
Firefighters were still on the scene at 8am.
Crew manager Shaun Greenhalgh said: "The mission is still ongoing.
"We are not sure where the family is staying but they were out of the building wen we arrived."
The cause of the blaze has not yet been confirmed but will be investigated.