An exemption from paying Council Tax, where occupation of a property is legally prohibited. This may be because a property has been sealed off due to ongoing actions e.g. a police crime scene, or because occupation of a property would breach a law e.g. a dangerous building. Properties may also be unoccupied due to a compulsory purchase being carried out.
You need to apply for this exemption. It is not granted automatically when the property becomes unoccupied due to becoming legally prohibited.