Overage and Clawback Agreements: The CGT and SDLT Tax Treatment for Developers and Landowners

A landowner selling a field at agricultural value, with planning permission a live possibility, faces an obvious problem: sell now and miss the uplift, or wait and risk the sale falling through. An overage agreement is the usual answer — but it creates two separate tax questions that are frequently got wrong: when is the seller actually taxed, and when does the buyer owe more SDLT.

What an overage agreement actually does

An overage (also called clawback, or an uplift agreement) is a contractual right, usually secured by a restrictive covenant or a legal charge on the land, entitling the seller to a further payment if a defined trigger happens within an agreed period — commonly 15 to 30 years. Typical triggers include planning permission being granted, permission being implemented, or the land being sold on or developed. The payment is normally a percentage of the increase in value the trigger creates, net of costs.

It lets a seller price the immediate sale realistically — often close to current-use value — while keeping a claim on the upside if the buyer (or a later purchaser) unlocks it. For developers, it is often the only way to secure a site from a landowner who would otherwise hold out indefinitely for planning gain.

The seller's CGT problem: ascertainable vs unascertainable

The Capital Gains Tax treatment turns entirely on one distinction, and it is the point most non-specialist advisers miss.

• Ascertainable consideration — where the overage is a fixed sum, or calculable by a formula with no real contingency (for example, a fixed percentage of a value already determined), HMRC treats it as part of the original sale proceeds. The whole amount is brought into the CGT computation for the tax year of the original disposal, even though the cash has not been received and may not be for years.
• Unascertainable consideration — where the right depends on a genuine future event that may or may not happen (planning permission being granted is the classic example), the right itself is treated as a separate chargeable asset under the deferred consideration rules. It is valued at the date of the original sale — broadly, what a buyer would pay today for that contingent right — and that valuation is taxed as part of the disposal. When the overage is later triggered and paid, the receipt is treated as disposal proceeds for that separate asset, triggering a second CGT computation, with the earlier valuation used as the base cost.

The practical effect: a landowner can be taxed twice on the same underlying uplift — once (notionally) on the estimated value of the right at the time of sale, and again when the real cash arrives. Relief exists to adjust the first computation if the actual amount received differs materially from what was assessed, but getting that adjustment right depends on how carefully the original disposal was reported, not something to reconstruct years later from memory.

Why the ascertainable/unascertainable line matters so much

Getting the classification wrong at the point of the original sale has consequences years later. If a landowner (or their adviser) treats a genuinely contingent overage as ascertainable and reports the full potential value upfront, they may pay CGT on an uplift that never actually materialises — with the practical difficulty of correcting a return long after the enquiry window has closed. Get it the other way round, and HMRC can challenge an artificially low valuation of the deferred right as an attempt to defer tax that should have been paid at the point of sale.

This is a valuation question as much as a tax question — what would an independent buyer actually pay today for the right to receive this payment, given the real probability and timing of the trigger — and it needs to be addressed properly at the point of sale, not after the overage has already been triggered.

The buyer's SDLT problem

Overage does not only affect the seller. For the buyer — usually the developer — the overage payment is additional consideration for the land, which means additional SDLT, and it becomes payable on a delay that does not fit neatly with the standard 14-day filing window.

Where consideration is contingent or uncertain at completion, the buyer has a choice: pay SDLT upfront based on a reasonable estimate of the maximum likely consideration, or apply to HMRC to defer the SDLT attributable to the uncertain element. When the overage trigger later occurs and the amount is fixed, a further SDLT return is required and the additional tax becomes due — broadly within 30 days of the trigger event, or the end of any agreed deferral period. Missing this second filing is a common and entirely avoidable compliance failure, because by the time the trigger event happens, the original purchase can be years in the past and easy to forget was ever contingent.

Developers running multiple sites with overage arrangements need a system for tracking these trigger dates independently of the original conveyancing file — the liability does not go away because nobody was watching for it.

Drafting points that shape the tax outcome

The tax treatment is set largely by how the agreement itself is drafted, which is exactly why this needs to be modelled before terms are agreed, not after:

• How the trigger is defined — a fixed percentage of an already-known figure behaves very differently from a percentage of a value only determinable once planning permission is granted.
• The time limit — a longer overage period increases the contingent right's uncertainty (and therefore, often, its taxable value at the point of sale is lower), but ties the land up for longer and can affect its marketability to a future buyer.
• How the uplift is calculated — net of development costs, professional fees and finance, or gross — changes both the commercial value of the right and how straightforward the eventual valuation dispute with HMRC is likely to be.
• Security — whether the overage is secured by a restrictive covenant, a legal charge, or merely a personal contractual obligation affects both its practical enforceability and, potentially, its valuation.

This sits alongside the wider question of whether a land sale is even taxed as a capital gain in the first place — see our guide on property trading vs investment for when HMRC treats a disposal as trading income instead, which changes the whole computation.

Common questions

What is an overage or clawback agreement?

An overage (or clawback) agreement is a contractual right for a landowner to receive further payment after selling land, if a specified trigger happens later — typically planning permission being granted, implemented, or the land being sold on or developed within an agreed period. It lets a seller share in a value uplift they would otherwise miss by selling at current-use value.

Is an overage payment subject to Capital Gains Tax?

Yes, but the timing depends on whether the right is ascertainable or unascertainable. An ascertainable amount is taxed as part of the original disposal proceeds even though paid later. An unascertainable (contingent) right is treated as a separate asset, valued and taxed at the date of the original sale, with a further CGT calculation when the overage is actually received.

Does SDLT apply to overage payments?

Yes. Where consideration for a land purchase is contingent or uncertain, the buyer can apply to defer the SDLT attributable to it. When the overage trigger occurs and the payment crystallises, the buyer must submit a further return and pay the additional SDLT, normally within 30 days of the event or the end of any deferral period.

Can double taxation arise on an overage right?

It can in principle, because the original sale taxes the value of the right itself and the later receipt is a disposal of that right. Relief exists to adjust the position where the amount actually received differs from the value originally assessed, but claiming it correctly depends on how the agreement was structured and reported from the outset.

This article is general information, not personal tax advice, and tax rules change. Your own position depends on facts we cannot see from here — please take advice before acting on anything above.

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