A 33-year-old man inside the property jumped from an upstairs window after being woken from his sleep by flames and smoke, but was uninjured.
Detectives are treating the incident, at around 4.15am on Monday at Longbar Avenue in Beith, north Ayrshire, as attempted murder.
Detective Constable Fraser Murray said: “This fire was wilful and fortunately the man inside the flat was not injured, but he could easily have been.
“We would like to speak to anyone who saw anything suspicious in the area around the time of the incident.
“In particular anyone who might have been driving and have dash-cam footage that might assist with our inquiries.
“If you can help please contact Saltcoats CID via 101, quoting incident number 0308 of Monday May 24 or make a call anonymously to the charity Crimestoppers on 0800 555 111.”