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If I can legally write a number, can I take a building permit refusal instead?
The designer ruled "if you can, you must" in 2018 and 2019, matching the rulebook's "must write" wording.
No. If any of the three combinations lets you place a house legally, you must write a number. A refusal is only for turns when no combination fits, and you cannot choose a refusal voluntarily (not even to end the game early). The one nuance is the Temp Agency: if the printed number does not fit and only an adjusted number would, you may decline the optional effect and take the refusal.