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Do I have to claim a City Plan as soon as I meet its requirements?
No. Meeting the requirements does not force you to claim the plan. You may hold the estates and claim later, accepting the risk that another player claims it first (and drops you to the lower score). Claiming is still immediate once you declare it: you score the plan right then. You can also claim plans in any order, which lets you keep estates free to choose which of two plans to use them for. Whether you may still claim after another player has triggered the game end is not covered by any ruling we found.
“When you claim a city plan you have to score it immediately. But when you meet the requirements, it's not mandatory to claim a city plan immediately.”
— PatatoKen, designer