bâtir des plans sur la comète signifie anglais

build castles in the air▼(to have any desire, idea, or plan unlikely to be realized)
verb
[UK: bɪld ˈkɑːs.l̩z ɪn ðə eə(r)] [US: ˈbɪld ˈkæs.l̩z ɪn ðə ˈer]