Verb
build castles in the air (third-person singular simple present builds castles in the air, present participle building castles in the air, simple past built castles in the air or (archaic or poetic) builded castles in the air, past participle built castles in the air)
(transitive, idiomatic) To have any desire, idea, or plan that is unlikely to be realized; to imagine visionary projects or schemes; to have an idle fancy or a pipe dream; to daydream.
Can build castles in the air. Robert Burton
Vote Labor, and you build castles in the air. Vote Conservative, and you can live in them. David Frost
Many who build castles in the air cannot build a hut on earth. German Proverb