The main benefit of extending your lease is the increase in value that it will bring to your property. Due to mortgage requirements, short leases are notoriously hard to assign, therefore if you extend for a further 99 years your flat becomes much more appealing to a purchaser. Not only this, but extending efficiently i.e. before the 80 year limit approaches, gives you the peace of mind that you can extend without being liable to pay any marriage value to the landlord.