Understanding Contingent, Uncertain, and Unascertained Consideration in FA03

SDLT treatment of contingent and uncertain consideration

For SDLT, payments linked to future events are not usually ignored just because the final position is not known on completion. The rules distinguish between contingent consideration, where payment depends on whether an event happens, and uncertain consideration, where some payment is due but the amount depends on future events. In many contingent cases, SDLT works on the basis that the payment is treated as payable from the outset.

• Contingent consideration covers payments due only if an uncertain future event happens, or payments that cease to be due if such an event happens.
• Uncertain consideration is different: payment is expected, but the amount or value cannot yet be fixed because it depends on future events.
• If a contract says an amount is payable if a future event occurs, SDLT assumes that the event does occur and the amount is paid.
• If a contract says an amount will stop being payable if a future event occurs, SDLT assumes that the stopping event does not occur and the amount remains payable.
• When reviewing a contract, focus on the legal effect of the clause rather than labels such as overage, deferred payment, retention or earn-out.
• Common difficulties include telling apart true contingencies, variable price formulas and clauses that look conditional but are really valuation or price-adjustment mechanisms.

Scroll down for the full analysis.

SDLT and contingent or uncertain consideration: what counts and how it is treated

This page explains how Stamp Duty Land Tax deals with consideration that depends on future events. The issue matters because SDLT is charged on the consideration for a land transaction, and sometimes the amount to be paid is not fixed on completion. The legislation has special rules for consideration that is contingent, uncertain, or not yet fully known.

What this rule is about

In some property transactions, the buyer does not simply pay a fixed sum on completion. Instead, part of the price may depend on something that happens later. For example, an extra amount may become payable only if planning permission is obtained, or a payment may stop being due if a particular event happens.

The rule in the source material is about how to classify that kind of consideration. It distinguishes between:

• contingent consideration, where payment depends on whether a future event happens at all, and
• uncertain consideration, where some payment will be due but the amount or value depends on future events.

This matters because SDLT cannot wait indefinitely for future events to unfold. The rules therefore require assumptions to be made about certain future events when working out the chargeable consideration.

What the official source says

The source says that, for SDLT purposes:

• consideration is contingent if it is to be paid or provided only if an uncertain future event occurs, or if it will stop being paid or provided if an uncertain future event occurs;
• consideration is uncertain if the amount or value depends on uncertain future events.

The source also sets out two important assumptions:

• if an amount would be paid if a future event happens, you assume that the event will happen and that the amount will be paid;
• if an amount will stop being payable if a future event happens, you assume that the event will not happen and that the amount will be paid.

In other words, where future events affect whether consideration is payable, the starting point is to treat the amount as payable.

What this means in practice

The practical effect is that SDLT generally does not ignore a payment just because it depends on a future event. If the contract says an amount becomes payable if something happens later, the legislation requires that event to be assumed to happen when working out the consideration.

Likewise, if an amount is payable unless some future event happens and brings the payment to an end, the legislation requires you to assume that stopping event does not happen.

So in both cases, the analysis points towards including the amount in the chargeable consideration at the outset, rather than leaving it out because the future is uncertain.

The separate idea of uncertain consideration is slightly different. There, the issue is not whether any payment is due at all, but that the amount or value cannot yet be fixed because it depends on future events. The source material defines the term, but does not on its own set out the wider computational consequences. Even so, the key point is that SDLT recognises this as a distinct category from a purely contingent payment.

How to analyse it

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

• Is there an amount that is only payable if a future event happens? If yes, that points to contingent consideration.
• Is there an amount that is payable now or under the contract, but will stop being payable if a future event happens? That is also treated as contingent.
• Is the real issue not whether payment is due, but how much is due because the amount depends on future events? That points to uncertain consideration.
• Does the contract identify a specific sum that becomes payable on a trigger event? If so, the source says to assume the trigger event happens.
• Does the contract provide for a payment to fall away on a trigger event? If so, the source says to assume that trigger event does not happen.

When reading the contract, focus on the legal effect of the drafting. The label used by the parties is less important than the actual mechanism. A clause described commercially as an overage, deferred payment, retention, adjustment, or earn-out may still fall within these rules if payment depends on future events.

Example

Illustration: A buyer acquires land for a fixed sum plus an extra £200,000 if planning permission is granted within three years. That extra amount is contingent on an uncertain future event. Under the rule in the source, the planning permission event is assumed to happen, so the £200,000 is treated as payable for this purpose.

Illustration: A contract provides that £150,000 is payable in two years, but the payment will be cancelled if the seller fails to obtain vacant possession by then. Under the rule in the source, the cancelling event is assumed not to happen, so the £150,000 is treated as payable for this purpose.

Why this can be difficult in practice

The main difficulty is telling apart three different ideas:

• a payment that is contingent on whether an event happens;
• a payment that is certain in principle but variable in amount;
• a payment mechanism that looks conditional commercially but is legally part of a wider price-adjustment formula.

Drafting can also be deceptive. A clause may say that a payment is “deferred”, but in substance it may only be due if a future event occurs. Equally, a clause may look contingent, but the true uncertainty may be about valuation rather than liability to pay.

Another practical difficulty is that this page gives definitions and assumptions, not the full set of consequences that may follow elsewhere in the SDLT rules. So while it clearly tells you how to treat future-event conditions at the definitional stage, the wider filing and adjustment position may require looking at the surrounding legislation as well.

Key takeaways

• A payment can still count for SDLT even if it depends on a future event.
• If an amount is payable only if an event happens, the rule assumes that the event happens and the amount is paid.
• If an amount stops being payable if an event happens, the rule assumes that the event does not happen and the amount is paid.

This page was last updated on 24 March 2026

Useful article? You may find it helpful to read the original guidance here: Understanding Contingent, Uncertain, and Unascertained Consideration in FA03

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