Guide on Calculating SDLT for Variable, Uncertain, or Contingent Rent

SDLT on Lease Rent That Is Uncertain, Unascertainable or Contingent

When lease rent is not fixed at the start, SDLT still has to be calculated on the rent element using the lease’s net present value. If the rent cannot yet be measured, a reasonable estimate must be used for the first five years. If the rent depends on an event happening, the calculation generally assumes that the contingent rent will be payable during that period.

• SDLT on a new lease can apply to both any premium and the rent, with the rent worked out by calculating its net present value.
• Where rent is uncertain or unascertainable, such as after a future market review or where it depends on business performance, a reasonable estimate is required for the first five years.
• HMRC does not require a formal valuation in every case, but the taxpayer should keep clear records showing how the estimate was reached.
• Where rent is contingent on a future event, such as planning permission being granted, the SDLT calculation for the first five years assumes the extra rent will be payable.
• RPI-only rent increases are specifically ignored where the whole change in rent is solely due to that adjustment.
• The distinction between uncertain rent and contingent rent is important, and later amendment or deferment is not generally available just because the rent was not fixed at the start, although a separate adjustment rule may sometimes apply.

Scroll down for the full analysis.

SDLT on lease rent: how to deal with contingent, uncertain or unascertainable rent

This page explains how Stamp Duty Land Tax is worked out when lease rent is not fixed at the start. That matters because SDLT on leases is partly based on the net present value of the rent, and the law has special rules for rent that depends on future events, future valuation, business performance, or other amounts that cannot yet be pinned down.

What this rule is about

When a new lease is granted, SDLT may be charged on the rent as well as on any premium. The rent element is calculated by finding the net present value, often shortened to NPV, of the rent payable over the term.

If the lease states a fixed rent, that calculation is relatively straightforward. The difficulty comes where the rent can change and the amount is not known at the grant date. The legislation treats different kinds of uncertainty in different ways.

The source material deals with three related ideas:

• uncertain or unascertainable rent, where the amount cannot yet be quantified, such as a future market rent review or rent linked to future business results;
• contingent rent, where liability to pay depends on an event that may or may not happen, such as planning permission being granted; and
• variable rent more generally, for which special NPV rules apply.

The key practical question is what figure should be used in the SDLT return when the lease has not yet produced an exact rent figure.

What the official source says

The official material says that special rules apply when calculating NPV for variable rent.

Where the rent is uncertain or unascertainable, a reasonable estimate must be made of the amount payable for the first five years of the term. HMRC says this estimate does not have to be a professional valuation, but the basis for the estimate should be kept in case HMRC asks how the figure was reached.

The source gives examples of uncertain or unascertainable rent, including:

• rent reviewed to market rent at a future date;
• rent linked to business results; and
• index-linked rent.

There is a specific carve-out for some retail prices index adjustments. Where the whole change in rent is solely due to adjustments in line with RPI, those variations are ignored for this purpose.

Where the rent is contingent, the rule is stricter. For the first five years, the NPV is calculated on the basis that the contingent amount will be payable, or will not cease to be payable.

The source also makes an important negative point. Other provisions in section 51 do not apply to the rental element of leases in this context. So if the contingency later does not occur, the taxpayer cannot amend the SDLT calculation under the provision mentioned in the source merely because the rent was contingent or unascertained, and cannot defer payment of SDLT on that basis alone. However, the source notes that Schedule 17A paragraph 8 may allow an adjustment in certain circumstances.

What this means in practice

The SDLT return for a new lease cannot simply say that the rent is unknown and leave it there. A figure must still be used.

If the rent is uncertain because it depends on a future valuation or future trading figures, the return should use a reasonable estimate for the first five years. That estimate should be evidence-based. The source does not require a formal expert report in every case, but it does expect the taxpayer to be able to show how the estimate was reached.

If the rent is contingent on an event, the starting assumption for the first five years is that the contingent rent will be payable. In other words, the calculation is not done on a cautious or wait-and-see basis. It is done on the assumption that the contingency bites.

This can produce an SDLT charge based on rent that may never actually become payable. The source makes clear that this is how the legislation works, subject to the separate adjustment mechanism it refers to.

The practical distinction between uncertain rent and contingent rent therefore matters. An estimate is used for uncertainty. An assumption that the amount will be payable is used for contingency.

How to analyse it

A sensible way to analyse a lease is to ask the following questions.

• Is the rent fixed, or can it vary?
• If it can vary, is the issue that the amount cannot yet be measured, or that payment depends on a future event?
• If the amount cannot yet be measured, what is the most reasonable estimate for the first five years based on the lease terms and available evidence?
• If payment depends on an event, what amount becomes payable if the event happens, and has the NPV been calculated on the assumption that it will be payable?
• Is the only variation an RPI-only adjustment of the kind the source says should be ignored?
• What records are being kept to support the estimate or treatment adopted?
• Could the later adjustment mechanism mentioned in Schedule 17A paragraph 8 become relevant?

For conveyancers and advisers, the record-keeping point is important. HMRC says the basis of the estimate should be retained. In practice, that means keeping the workings, assumptions, lease clauses relied on, and any market or financial information used.

Example

Illustration: a tenant takes a new lease. For the first two years the rent is fixed. After that, the lease provides for rent to be reviewed to market level, and there is no way to know the exact reviewed rent on the grant date.

That future reviewed rent is uncertain or unascertainable. For SDLT, the NPV for the first five years is calculated using a reasonable estimate of what that reviewed rent will be. The taxpayer should keep evidence showing why that estimate was reasonable.

Now change the facts. Suppose instead that the lease says an extra rent becomes payable only if planning permission is granted for a change of use. That extra rent is contingent. For the first five years, the SDLT calculation is made on the basis that the contingent amount will be payable.

Why this can be difficult in practice

The hardest part is often classification. A lease may contain clauses that look both uncertain and contingent. For example, a rent clause might depend on a future event and also require valuation or calculation if that event happens. The source does not provide a full test for every mixed case, so careful reading of the lease is needed.

Another difficulty is deciding what counts as a reasonable estimate. The source says a professional valuation is not always necessary, but it does not set out a detailed formula. That means judgement is involved. An estimate should be grounded in the lease terms and objective material, not guesswork.

There can also be a mismatch between SDLT and commercial reality. A contingent rent assumption may increase SDLT even where the event never happens. The source is clear that the ordinary amendment and deferment routes mentioned there are not available simply because the rent was contingent or unascertained. Any later correction depends on the separate adjustment provision it refers to, and whether the facts fall within it.

Finally, index-linked rent needs care. The source says that where the whole change is solely due to RPI adjustments, those variations are ignored. That wording matters. If there are other moving parts in the rent formula, the position may not be the same.

Key takeaways

• If lease rent is not fixed at the start, SDLT still requires an NPV calculation using special rules.
• Uncertain or unascertainable rent is dealt with by making a reasonable estimate for the first five years and keeping evidence of how it was reached.
• Contingent rent is treated more firmly: for the first five years, the calculation assumes the contingent amount will be payable.

This page was last updated on 24 March 2026

Useful article? You may find it helpful to read the original guidance here: Guide on Calculating SDLT for Variable, Uncertain, or Contingent Rent

View all HMRC SDLT Guidance Pages Here