If plans are grandiose, they sound very important or impressive, but they are not practical — too big, too showy, not thought through. That is how learners usually meet the word, and it is the meaning with an edge. A grandiose scheme promises a new city, a huge reform, a sure success, and people roll their eyes a little. The word can also describe buildings or gestures that are simply very grand in a showy way, not always as a joke — though actually, in modern English the cool, doubtful sense is the common one. Do not mix it up with grand, which can be warm praise, or with grandiloquent, which is about fancy words, not fancy plans.