Northfield police and fire fighters responded to a fire at 308 S. Happ Rd., at 11:11 a.m. on Dec. 14.
The resident at condo unit 211 discovered fire on the couch last Friday, no one was injured, police said.
The magnifying effect of sunlight and a glass mirror may have caused paper left on the couch to catch on fire, police said. The couch was next to a large glass mirror and an open window on a sunny day.
The cost of the damage is unknown. Northfield Assistant Fire Chief Tom Burke determined that it was an accidental fire, according to police reports.