The roof of the building on Main Road in Otterbourne, near Eastleigh, collapsed during the blaze, which broke out at about 03:30 GMT.
No injuries were reported at the site, which also contains workshops and offices. An investigation is under way.
The neighbouring convenience store, which was saved, is due to reopen later.
Ty Whitlock, commanding officer at the scene, said: "When we pulled up we were faced with flames coming out of the roof, coming out of the glass doors and windows and obviously huge amounts of smoke coming from the eaves."
Up to 40 firefighters have been tackling a large fire which destroyed a garage in Hampshire.