A building is considered in lawful use for CIL purposes when the lawful use is being actively carried out. The fact that a building just has a lawful use which could be carried out isn't sufficient. This follows the High Court Judgment of 'R(Hourhope Ltd) v Shropshire Council  EWHC 518 Admin'.
Whether a building is in lawful use at any time depends on what activities are taking place and what the intentions of the persons using the building are. The amount of activity that you must prove will depend on the type of use. Whether the use has stopped depends on the length of time, reasons for the break in use and the intentions of the property users.
It is your responsibility to provide evidence that any building, or part of the building, has been in continuous lawful use for at least six months in the three years ending on the day planning permission first permits your development.
Evidence may include proof of Business Rates or Council Tax payments, photographs showing the building in the use claimed, sworn statements of people who can confirm the use claimed, or utility or other bills relating to the use claimed.
The council may ask for more evidence of the use if this is not clear. If there isn't enough information to prove that any buildings have been in lawful continuous use for the requisite period, we may not count the floorspace of the buildings as a deduction from your CIL charge.
If you have any questions please contact CIL@portsmouthcc.gov.uk