While you may be able to get a rough estimate of the premium to be paid you will need to argue your case for that figure if a Tribunal has to make the final decision, and you could end up paying more than you had planned.  We always recommend, therefore, that you get proper advice on the premium before you start, so that you can budget accordingly.  The legal process of a statutory lease extension is fraught with difficulties as there are very strict deadlines to meet.  If you get it wrong you can lose your right to a new lease immediately, be obliged to pay your landlord’s wasted costs, and will have to wait a year before you can start the process again, by which time the premium will  have increased.