It happened around 7:00 Sunday evening at Fox Run Apartments on the 1000 block of Woodchuck Place.
Fire crews started to battle the blaze and had to shut off all utilities in the building.
About 24 apartment units were evacuated and officials say they hope to let at least half of those residents back by the end of the night.
No injuries were reported but the building sustained water damage.
Authorities believe the fire started in the laundry area and spread into the walls of the complex.
Investigators confirmed Monday that the fire was ruled to be "accidental."
Damage was estimated at about $85,000.