It is worth noting that the 90 years extension with a peppercorn ground rent is your legal entitlement but you can reach deals with different years and ground rents outside the law (known as voluntary deals) which can vary the premium you would pay under statute. Also professional costs for a voluntary deal can be cheaper.
The premium calculation is prescribed by statute. Like any calculation the final result depends on the variables that you put into the computation. It is important to note that on 8 January 2020 the Law Commission published a Report on Options to Reduce the Price Payable which, depending on which option is passed by Government, may reduce the premium (please click here for full report). The law change, if at all, is expected in 2021 although keep an eye on this website as this time scale is not set in stone and this page will be updated as and when we know more.
The Leasehold Advisory Service publish a calculator here where you can work out roughly the premium that you will pay.
In addition to the premium you are required to pay your own legal and valuation costs together with the Landlord’s reasonable legal and valuation costs. For more information on costs please see the Lease Renewal Service page on this site. You will also be required to pay any arrears of service charge.