As the length of the expired term of the lease falls then so does it's value. If the lease has, for example, more than 90 years to remain under the lease then the decrease may be slow but when it comes to less than 80 years the decrease will become significant. It is worth bearing in mind the following points:
If a lease has less than 80 years unexpired when you seek to extend it then as part of the price you will have to pay what is known as marriage value to the landlord. This cost could be significant. It is the key reason why you should consider extending your lease whilst there are more than 80 years remaining. Paradoxically, most conveyancing lawyers and owners of property do not become concerned about extending their lease until the lease falls below 80 years at which point obviously the price for extending the lease is much greater.
You may encounter difficulties if you want to sell your flat if the unexpired lease is less than 70 years remaining. As stated above, different lenders have different criteria but they are generally looking for a minimum unexpired term of less than 60 years. Sometimes special consideration can apply to flats in Central London areas. If you are extending a lease in Central London it definitely makes sense to use lawyers who are familiar with conveyancing London transactions.