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When can I build a roundabout, and can I do it when I cannot place a number?
The original designer ruled on this in 2018 and 2019, and the co-designer confirmed his answers take priority.
At any point in your turn, before or after you write a number and use its effect. It is not an action, does not replace your number, and costs no card. You may build one even when no number fits, but you still take a building permit refusal if you then cannot place a number. It can even end the game. An early statement that you must be able to write a number was withdrawn. The rulebook wording reads "before or after building a house and activating an effect"; the designer's "whenever you want" ruling is broader, although no ruling tests putting one between the number and the effect.