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Can a fence separate a Bis house from the house it copies?
The designer ruled in 2018 that a fence may go anywhere except between a house and its Bis, and the rulebook says the same.
No. In the original game a Bis house must stay in the same estate as the house it copies, so you cannot draw a fence between them, now or later. You may fence anywhere else. Note: the Collector's Edition rulebook says identical Bis numbers can be separated by a fence; that is that edition's own text and does not change the rule for the original game.