Many lenders will not lend if your lease has 55 years or less to expiry. Some lenders have a rule of lending so long as the lease term is in excess of the mortgage term + 30 years. If you do not fit this rule you may need to consider a lease extension either with regard to purchasing or selling a property.
How much does it cost to extend a lease?
This depends on the value of your property, the current ground rent, and the original term of the lease – see the following link to calculate a specific value.