This is required where the remaining Lease term is 100 years or less. A short Lease term affects marketability and mortgage lenders refuse to accept short term Leases as security for their loans.
That makes a property with a short Lease term tricky to sell. You should get your Lease Extension sorted as soon as possible. The less term remaining the more costly the extension will be.
What Is Involved?
You will need to fund the surveyor's valuation and to pay for your conveyancer AND the Landlord's legal fees.
There are online calculators that will estimate the likely premium based on the relevant legislation. For example https://www.lease-advice.org/calculator/
The below article is also an excellent read if you need to extend your lease.