A land use may be technically illegal or absolutely illegal. A technical illegality is one where the valuer is confident that the use could be allowed upon application to the appropriate authorities. In this case the value is the market value at that use less the cost and risk of gaining approval.

On the other hand an absolutely illegal use is one that is illegal under common law or statute nd there is no hope of that use being approved. For example, a property being used as an illegal casino. The value of the property is its legal use less any cost of “make good” ie reverting to its original condition.