Crews were called to the suspicious blaze at a property in Castle Street, Nelson, in the early hours of this morning.
Lancashire Fire and Rescue were alerted to the fire by the occupants of the house next door due to their smoke alarm sounding.
A police spokesman said: "If anyone saw anyone suspicious in and around the area, specifically the back alley of Castle Street they are urged to contact police.
"Thankfully nobody was injured during the fire.
"Please take this as a reminder to check your smoke alarms and if you don't own any get some fitted. They do save lives."
Anybody with information is urged to contact police on 101 quoting log reference LC-20170310-0126 or email PC 4134 Grounds at [email protected]