When you extend a lease that has less than 80 years to run, there is an additional fee to be paid to the landlord called a Marriage Fee. Hence, it is imperative to extend or buy the freehold prior to the lease falling below 80 years, even by a single day.
Marriage value is the amount of the surplus value produced by taking the value of the extended lease at a peppercorn ground rent and deducting both the existing short leasehold interest held by you as the leaseholder, and the freeholder’s current interest.