If you own a leasehold flat or just purchased one with 99 years or less unexpired then it’s worthwhile extending the lease before the term falls below the 80 year threshold.
On the flip side if your thinking of selling a flat within the next couple of years then a longer lease will naturally make the property much more saleable and increase the market value of the property.
This is provided you qualify under the terms of the Leasehold Reform, Housing and Urban Development Act 1993. In which case you are legally entitled to purchase a 90 year extension to the existing lease term at a new peppercorn ground rent for the whole new term. So if your flat has 75 years unexpired with a nominal annual ground rent, then the lease would increase to 155 years with a peppercorn ground rent.
Assuming the lessee has no rights under the 1993 Act and the lease is less than 80 years unexpired then the freeholder is entitled to a 50% share of the marriage value, which is the difference between the value of the flat with a long lease and the value of the flat with short lease.
It’s worthwhile having an expert look into extending the lease on flats.