The antonyms for "under building" can be "completed," "finished," "constructed," or "built." These words indicate that the building process has already been done and the structure is ready to use. Other antonyms may include "demolished," "destroyed," "ruined," or "collapsed," which portray the opposite meaning of building or construction, indicating that the structure has been taken down or damaged. In some contexts, "vacant," "abandoned," or "empty" can also serve as antonyms for "under building," signifying that there is no structure or activity taking place in the specified location.