The incident in West Lothian happened just before 6pm on Saturday in Rowan Terrace.
Once residents were evacuated, Scottish Fire and Rescue Service (SFRS) crews battled the blaze, which reportedly caused thousands of pounds worth of damage at the premises.
Police Scotland said they were now following leads to catch the wilful fire-raisers.
A spokesperson said: “We were made aware of a fire at a property on Rowan Terrace in Blackburn around 5.40pm on Saturday, January 22. The fire was extinguished by the Scottish Fire and Rescue Service and a 40-year-old woman was assessed at the scene by the Scottish Ambulance Service.
“A neighbouring property was also evacuated. The fire is being treated as deliberate and officers are following a positive line of enquiry
SFRS stated crews extinguished the blaze shortly after 9pm.