Is that a trick question? The negative externality is leaving a usable property vacant when public policy encourages use. You're maintaining this situation: h

Is that a trick question? The negative externality is leaving a usable property vacant when public policy encourages use. You're maintaining this situation: https://squattinglondon.files.wordpress.com/2018/01/ominous-octopus-omnibus-500x294.jpg?w=640 https://squattinglondon.files.wordpress.com/2018/01/ominous-... and it seems obvious to me why that's bad and public policy should not promote it. As a public policy, along the lines of taxing vacant properties, we could also simply permit any vacant or unused property to be lived in and used by anyone who came along.