It is thought the fire may have been started deliberately and detectives in Lisburn are appealing for witnesses.
A NIFRS spokesman said: "The building has been substantially damaged due to the fire.
“Motorists are advised to seek alternative main routes as Firefighters remain at the scene.
“The initial call to this incident was received at 7:14am and it is too early to determine the cause of the fire at this stage.”
Detective Constable Bell from Lisburn Reactive and Organised Crime said: “We are keen to hear from anyone who was in the area this morning and who may have noticed anything or anyone suspicious or out of the ordinary. Please call 101, quoting reference 206 09/06/16.”