SDLT and Assignment of Contract: The Substantial Performance Trap for Developers

A developer exchanges contracts on a site, then before completion finds a buyer willing to pay more and assigns the benefit of the contract across rather than completing the purchase themselves. It's a routine move in site assembly and off-plan resale, and the tax mechanism that makes it work — sub-sale relief, more properly the transfer of rights rules — is designed so SDLT gets charged once, on what the final buyer actually pays, rather than twice. Get the sequencing wrong, though, and that single charge becomes two: one on the original purchase, and another on the transfer that follows it.

How transfer of rights relief is meant to work

Under FA 2003 s.45, where a buyer (B) contracts to purchase land from a seller (A), then assigns the benefit of that contract, or agrees to sell the property on, to a third buyer (C) before completion, the rules treat the transaction as if C were buying directly from A under a "secondary contract". B's original contract with A is disregarded entirely for SDLT purposes, so B doesn't pay duty on the deal at all, and only one SDLT charge arises — on the secondary contract, based on whatever C actually pays A or B under the arrangement. If C is paying more than B originally agreed with A, that uplift isn't taxed twice; it simply becomes part of the one consideration figure the final SDLT charge is based on.

This is a genuinely useful mechanism, not a loophole. It lets a chain of commercial arrangements — an option holder assigning to a developer, a developer assigning part of a site to a JV partner, an off-plan buyer selling their position on before legal completion — go through without SDLT stacking up at every link. The whole point is that only the final, real transfer of the property gets taxed.

The trap: substantial performance before the assignment

Relief only survives if B's original contract stays just that: a contract, never itself completed or treated as completed. The problem is that SDLT doesn't only trigger on legal completion. Under s.44, a contract is treated as substantially performed — and so taxed as if it had completed — once the purchaser takes possession of the property, or pays or becomes liable to pay the whole or substantially the whole of the consideration, whichever happens first. Once either of those has happened, B is treated as having already completed a land transaction of their own, with its own SDLT liability, regardless of what happens next. Assigning the deal to C afterwards doesn't undo that; it creates a second, entirely separate transaction on top of it.

That's exactly the situation a developer can walk into without meaning to. Exchange contracts to buy a site, get the seller's agreement to start demolition or groundworks ahead of formal completion because the build programme can't wait, and that early possession can be substantial performance in its own right — even though nothing has legally completed and no funds beyond a deposit may have changed hands. If a buyer for the site then turns up before completion and the developer tries to assign the contract across rather than complete themselves, the SDLT position has already been set: the developer owes duty on their own purchase, and the assignee owes duty again on theirs.

What counts as substantial performance in practice

Beyond taking possession, the other trigger is paying, or becoming contractually liable to pay, substantially the whole of the price — HMRC's practice treats 90% or more as the benchmark, though the test looks at the substance of what's actually happened rather than a single mechanical figure. For a developer, the possession limb is usually the more live risk: gaining access to start site works, storing materials on site, or being handed keys ahead of completion, all on the strength of an exchanged contract, can each be enough on their own. None of this requires any intention to avoid tax. It's simply a function of the build programme moving faster than the legal completion, and the SDLT consequence follows regardless of why it happened.

The 2016 tightening: relief denied where avoidance is the point

Sub-sale relief was historically exploited in SDLT avoidance schemes that deliberately engineered a chain of contracts to strip out duty that should have been paid. Finance Act 2016 added s.45(3A), which denies transfer of rights relief specifically where the secondary contract has been substantially performed without being completed, the original purchaser remains in possession, and a main purpose of the arrangements was obtaining an SDLT advantage. That provision is aimed at engineered avoidance rather than an ordinary commercial assignment, but it sits on top of the substantial performance trap rather than replacing it — a genuine developer flip can still lose relief on the general substantial performance point even where there's no avoidance motive anywhere in sight.

Structuring a clean assignment

The practical fix is sequencing. Where a site or unit might get assigned on before completion, the safest position is to avoid taking possession or making payment beyond a normal deposit until it's clear whether the deal is completing in the developer's own name or being assigned across, and to have the assignment and the ultimate completion happen as close to simultaneously as the conveyancing allows. Where early access for site works is commercially unavoidable — a tight build programme that can't wait for legal completion — that's the point at which the SDLT position needs checking before access is granted, not after, because once possession has been taken the transfer of rights relief is already off the table for that contract.

Common mistakes

• Taking early possession or starting site works ahead of completion without checking whether that constitutes substantial performance first
• Assuming a planned assignment automatically keeps SDLT to a single charge, regardless of what's happened to the original contract in the meantime
• Treating the 90% payment threshold as the only trigger, and overlooking that taking possession alone is enough on its own
• Leaving a long gap between exchange and either completion or assignment, during which site access or staged payments quietly tip into substantial performance
• Not identifying, before contracts are exchanged, whether the deal is genuinely being bought to completion or is likely to be assigned on, and structuring the paperwork accordingly from the outset

What this means for developers

Assignment of contract remains a legitimate and useful way to move a deal on before completion without SDLT being charged twice, and the relief that makes it work hasn't been withdrawn from ordinary commercial arrangements. What's changed the calculation is how easily a busy build programme can trigger substantial performance without anyone treating it as a tax event at the time — a site access date agreed to keep a programme moving is, in SDLT terms, indistinguishable from taking possession as a purchaser. Any deal that might get assigned on, rather than completed in the developer's own name, is worth flagging before site access or early payment happens, not once the assignment is already being drafted.

Common questions

If I assign my purchase contract to another buyer before completion, do I pay SDLT on my own purchase as well as the buyer paying on theirs?

Not if the assignment is a clean one. Under the transfer of rights rules in FA 2003 s.45, your original contract with the seller is disregarded for SDLT and the final buyer is treated as purchasing directly from the seller, so only one SDLT charge arises, based on what the final buyer actually pays. The relief is lost, and a second charge can arise on your own purchase, if you've substantially performed the original contract first, typically by taking possession or paying most of the price before assigning it on.

Does taking early possession of a site under an exchanged contract affect the SDLT position if I plan to assign the deal on?

Yes. Taking possession, or starting works such as demolition or groundworks on the strength of an exchanged contract, is one of the main ways substantial performance is triggered, even without legal completion having happened. Once that's happened, your original contract is treated as a completed land transaction in its own right, and assigning the deal to someone else afterwards creates a second, separate SDLT charge rather than a single charge on the final buyer.

Can I still use sub-sale relief to avoid a double SDLT charge on a genuine commercial assignment?

Yes, for a straightforward assignment that completes as a single transaction with no substantial performance of the original contract beforehand, transfer of rights relief still works exactly as intended and only one SDLT charge arises. What changed from 2016 is that relief is specifically denied where the secondary contract has been substantially performed without completing, the original purchaser is still in possession, and a main purpose of the arrangement was obtaining an SDLT advantage, which targets engineered avoidance rather than an ordinary commercial flip.

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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