The fire on Ribbleton Lane broke out at 1,25am today.
Three fire engines from Preston and Penwortham attended.
It was believed people were still inside the property.
When firefighters got to the scene, they quickly established that no one was left inside the building after sending six teams of firefighters wearing breathing apparatus inside the property.
Firefighters also used two hose reels and two ventilation units to extinguish the fire.
There were no reported injuries.
Lancashire Fire and Rescue Service says the fire, which was in a flat above a shop, is of suspicious origin.