How do I know what premium the leaseholder should be paying me to extend his lease?

It is strongly recommended to obtain a valuation from a qualified surveyor who specialises in this area. They will give you the valuation of what the lease is worth.

The leaseholder is required under the Leasehold Reform, Housing and Urban Development Act 1993 to pay for the legal, valuation costs, transferring the freehold and checking the right to buy the freehold.