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What happens when zombies reach a house that already has a number?
The designers answered each point between 2019 and 2021.
You still write the Z: the number disappears and the space cannot take a new number. An infected house is completely lost: it cannot score as a one-house estate, it cannot sit inside a larger estate (it has fences on both sides), and it does not count as a built house for a City Plan such as "all houses on a street".