Plan the part you can see
The robot cell plant expansion runs for two years. The civil works and steel for the next three months are fully designed. But the layout of the final production area depends on product decisions the customer will make next year.
Planning that area in detail now would be guesswork, and the guesses would need to be redone. Not planning it at all would leave a hole in the budget and schedule.
The answer: plan in detail what you can see, keep the rest at a high level, and refine as you go.