"Reasonably certain" is evidence, not a feeling. Here is what counts.

The single judgement that moves a lease liability most is the lease term. A ten-year lease with a break at five is either a five-year liability or a ten-year one, and the difference is a large part of the figure.

FRS 102 sets the rule in two lines. The term is the non-cancellable period plus any option period the lessee is reasonably certain to take (paragraph 20.38). Where there is a choice between shorter and longer, it is the shorter unless the lessee is reasonably certain of the longer (paragraph 20.40).

"Reasonably certain" is not "probably". ACCA calls it "a high hurdle to pass"; S&W's technical alert puts it as "quite a high bar, higher than a probable threshold". The first-year audits now being planned will test it, because it is the easiest number to challenge and the hardest to defend with words alone.

The standard lists what to weigh (paragraph 20.41):

1. The rent for the option period against market rates. A cheap renewal is an incentive to stay; a market-rate one is not.
2. Fit-out and leasehold improvements that still have value when the option falls due. Money spent on the premises is the strongest evidence there is.
3. What leaving would cost: relocation, penalties, lost trade.
4. How important the asset is to the business. A head office and a satellite office are not the same judgement.
5. Any conditions attached to exercising the option.

And past practice: how long the business has typically kept similar premises (paragraph 20.42).

Note what is on that list and what is not. A director's expectation is not on it. A fit-out capitalised last year with a useful life running past the break is. A costed relocation plan is. The lease before this one, and how long you kept it, is.

Two points that trip people up:

A break only the landlord can exercise never shortens the term (paragraph 20.38(c)). It is the landlord's option, not yours.

The date in a break clause is often the day the break opens, not the day the lease ends. A rolling break "at any time after the third anniversary on six months' notice" does not end the lease at year three. Put that date in as an end date and the liability is understated from day one.

The judgement is made at commencement and revisited only on a significant event or change within your own control (paragraph 20.43): a decision to relocate, a fit-out approved, a sale agreed. A move in market rents on its own is not a trigger.

Lease102 records each break and renewal option with its dates, who holds it and whether the date is an end date or the day the break opens; a landlord-only break is held on the record but never shortens the term; and changing the certainty tick on a lease already measured is challenged before it is saved, because that is a remeasurement, not a correction.