Fire early Tuesday morning caused extensive damage to a house that was under renovation on Fond du Lac’s Doty Street. The fire was reported at 1:24 a.m. on the 200-block of Doty. Firefighters found heavy flames coming from the second floor of the two-story home. Extra crews were called in to help. Firefighters went into the house to look for possible victims and found the house was vacant. Fond du Lac Fire/Rescue says most of the damage was to the second floor and roof. What caused the fire is under investigation.