His plan to build a house on the moon is just a flight of fancy.
flight of fancy
idiomAn idea, narrative, suggestion, etc., which is extremely imaginative and which appears to be entirely unrealistic, untrue, or impractical; thinking which is very speculative.
The architect dismissed the client's request for a floating garden as a mere flight of fancy that ignored basic physics.
While the proposal was initially dismissed as a flight of fancy, the researchers eventually discovered that the imaginative concept held a kernel of scientific truth.