Fire crews from Llandysul and Carmarthen were called to a blaze in the early hours of Monday morning.
Both crews were alerted to the fire at Rhydargaeau, between Pencader and Carmarthen, at 1.31am on Monday, 10 February.
Mid and West Wales Fire and Rescue Service said: "Crews responded to a building, constructed of corrugated sheeting, that was well alight on their arrival.
Crews utilised four breathing apparatus sets, two hose reel jets and small gear to extinguish the fire.
"No casualties were reported.
"The building was completely destroyed by fire.
"After extinguishing the fire, crews continued to dampen down the area before leaving at 3.32am."
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