This is a question for a valuer or surveyor who specialises in this area, but if you know the lease length and the current value of the flat and the ground rent schedule, then you can have a go at working this out approximately by using one of the calculators available online, like this one:
http://www.bishopandsewell.co.uk/lease-extension-calculator/
These type of calculation tools come with a health warning as they are only approximate, and cannot consider the unique circumstances of your property, however they can be useful in deciding whether you can afford to go forward with the process, or more importantly if you are thinking about making an offer on a short lease property, what the likely cost will eventually be.
- More information about extending your lease.