Generally, the fewer years remaining on the lease, the greater will be the premium.  Many other factors are taken into consideration.  A surveyor specialising in valuations for statutory lease extensions will be able to advise on a range of valuations which will give you the starting figure for your claim.  The Leasehold Advisory Service has an online lease extension calculator at which can give you a rough indication of the likely premium.  The premium will be valued as at the date of your claim.