A statutory lease extension gives you the right to add 90 years to the existing unexpired lease term for a flat. If your lease has, say, 75 years remaining, you will be able to extend the term to 165 years remaining. Furthermore, any ground rent will be reduced to zero (‘a peppercorn rent’).
The premium payable to the landlord is calculated according to Schedule 13 of the Act. There’s a useful lease extension calculator here to help you get an idea of overall costs.
In order to qualify, you need to have owned the property for at least 2 years, though there are ways to get around this if you are thinking of buying a short-lease property. If the seller meets the 2-year ownership clause, they can initiate the statutory process and assign the benefit as part of the sale contract. The legislation only applies to long residential leases, not business property or a commercial lease.