Six homes had to be evacuated after a blaze ripped through a second-hand electrical goods shop.
Four people were treated for the effects of smoke inhalation after the fire on Blackburn Road in Accrington.
Lancashire Police and Lancashire Fire and Rescue Service are treating the fire as a suspected arson attack.
Six fire crews were called to an unoccupied end-terraced property and used an aerial ladder platform to bring the fire under control. Eight firefighters were also equipped with breathing apparatus and hand-held water jets.
Lancashire Fire and Rescue Service said the fire spread through the loft spaces into six adjoining properties and the occupants had to be evacuated.
Police Inspector John Fryer said they believe the fire was started deliberately.
He said: "We have some CCTV taken from a distance which shows a figure approaching the side of the premises and leaving a few moments later before the fire was seen. "It looks pretty suspicious and we are treating it as arson at the moment."
The incident happened at around 7.30am on Saturday, December 13. Anyone with information call police on 101.