Missed UK Tax Returns: Overpayment Relief's 4-Year Window

When a multi-year cross-border catch-up reconciles your UK and US positions, it frequently uncovers UK tax you overpaid rather than underpaid. By then the ordinary Self Assessment amendment window has almost always closed. The route left is overpayment relief: a standalone statutory claim that must be made within four years of the end of the relevant tax year, and that fails outright if certain exclusions apply.

Anyone dealing with Missed UK Tax Returns tends to brace for what they owe. In practice, a properly reconstructed multi-year picture is symmetrical. Foreign tax credit relief that was never claimed, remittance basis positions taken on incomplete facts, employment income taxed twice across a split year, pension contributions never relieved at the higher rate, and gains reported gross of allowable costs all push the same way: the returns that were filed, or the assessments that stood in place of returns that were not, overstated the liability. At Jungle Tax the reconciliation that surfaces the arrears is usually the same reconciliation that surfaces the overpayments, and the two have to be resolved together rather than sequentially.

Why a cross-border catch-up is precisely when overpayments come to light

A single-year UK review rarely finds an overpayment, because it takes the return as its starting point. A cross-border catch-up does the opposite. It rebuilds the underlying facts from source: brokerage statements, employer payroll records for both jurisdictions, pension administrator data, trust distributions, and the US returns that were filed on a parallel and often inconsistent basis. The moment those two records are laid side by side, the discrepancies become visible in both directions.

The most common pattern we see in an IRS streamlined filing engagement is a US person resident in the UK who filed UK returns using UK-sourced payroll data alone, while the US returns captured a wider global picture. Reconciling the two reveals foreign tax credit relief in the UK return that was understated, or a source of income that was correctly taxable in the US and should have attracted UK treaty relief but did not. Neither error was deliberate. Both produced a genuine overpayment of UK tax in a year that is now, in ordinary terms, closed.

The problem is timing. These engagements typically span five to seven years of history precisely because that is the exposure period that matters for penalties and disclosure. The amendment window does not stretch that far.

What is overpayment relief, and when does it become the only route left?

Overpayment relief is the statutory mechanism that allows a person to recover overpaid income tax, capital gains tax, Class 4 National Insurance or corporation tax, or to reduce an assessment that was excessive. It was introduced by Finance Act 2009 and took effect from 1 April 2010, replacing the older error or mistake relief for individuals and partnerships and mistake relief for companies. For income tax and CGT the governing provisions sit in Schedule 1AB of the Taxes Management Act 1970; for corporation tax they sit in paragraphs 51 and 51A to 51G of Schedule 18 to the Finance Act 1998.

Critically, overpayment relief is a relief of last resort by design. HMRC will not entertain it where another correction route was, or was reasonably available to be, used in time. Understanding the hierarchy of routes is therefore the first step in any catch-up.

Route	What it corrects	Deadline	Typical availability in a catch-up
Amending your own return	Any error in a filed Self Assessment return	12 months after the Self Assessment filing deadline for that year	Almost always closed
HMRC correction of a return	Obvious errors and omissions	9 months from the date the return was filed	Closed
A standalone claim or election	Reliefs claimable outside the return	Varies by relief; often 4 years	Sometimes open, and must be checked first
Appeal against an assessment	An assessment already issued	30 days from the assessment	Closed if not appealed
Overpayment relief claim	Overpaid tax or excessive assessment where nothing else applies	4 years from the end of the relevant tax year	The usual route, if within time
Special relief	An HMRC determination raised because no return was filed	No statutory time limit	The route where determinations are in play

The sequencing matters more than most people appreciate. If a relief was available by a route with its own deadline and that deadline was missed, HMRC is entitled to refuse overpayment relief on the basis that the taxpayer should have used the other route. Choosing overpayment relief because it is administratively simpler, rather than because it is the only route left, is a common way to lose an otherwise good claim.

How is the four-year limit actually measured?

HMRC's Self Assessment Claims Manual states that claims must be made within four years after the end of the relevant tax year or accounting period. The word that catches people is relevant. It is not four years from the date the mistake was discovered, and it is not four years from the date the tax was assessed. Which period counts depends on what went wrong:

• Where the overpayment arises from a mistake in a return, the relevant period is the year or accounting period the return relates to.
• Where it arises from an assessment that was excessive, the relevant period is the year or period the assessment relates to.
• Where the amount was simply paid when it was not due, the relevant period is the year or period in which the payment was made.

For a UK tax year ending 5 April, this means the practical deadline falls on 5 April four years later. A mistake in the 2021/22 UK return therefore had to be claimed by 5 April 2026. The limit is absolute; HMRC has no general discretion to extend it, and a claim delivered a day late is simply out of time.

For anyone running a catch-up in 2026, the arithmetic is stark. A seven-year reconstruction will produce overpayments in years that are already irrecoverable, alongside underpayments in the same years that remain fully assessable — HMRC can assess up to four years for innocent error, six for carelessness, and twenty where the loss of tax is deliberate or involves an offshore matter that was not disclosed. The asymmetry is not a drafting accident; it is the settled position, and it is the single strongest argument for starting a catch-up early rather than waiting for a nudge letter.

Does an open enquiry or a discovery assessment change the four years?

No. An HMRC enquiry into a later year does not reopen an earlier year for the purpose of an overpayment relief claim, and a discovery assessment issued against you in year one does not extend your own claim window in year two. Where a discovery assessment is issued, the correct response is an appeal within 30 days, not an overpayment relief claim; and if you did not appeal when you knew or should have known the grounds, one of the statutory exclusions will usually bar the later claim.

What must a valid overpayment relief claim contain?

Overpayment relief claims are unusually formal. The requirements derive from paragraph 1(4) of Schedule 1AB TMA 1970 and paragraph 51(4) of Schedule 18 FA 1998, and HMRC applies them strictly. A claim that is missing an element is not a weak claim; it is not a claim at all, which matters enormously if the four-year deadline passes while the defect is being corrected.

A valid claim must, per HMRC's guidance at SACM12150:

• State clearly and expressly that the person is making a claim for overpayment relief. Implying it is not enough.
• Identify the tax year or accounting period to which the claim relates. A single letter covering several years should treat each year as a separate claim.
• State the grounds on which the person considers the tax was overpaid or the assessment excessive — the actual reasoning, not a bare assertion.
• State the amount claimed, computed and supported.
• State whether any appeal has previously been made in connection with the payment or the assessment.
• Include documentary proof of tax deducted or suffered, where a repayment is sought.
• Carry a signed declaration that the particulars given are correct and complete to the best of the claimant's knowledge and belief.

The declaration is the trap. It must be signed by the claimant personally — or, for a company, by a proper officer of that company. A tax agent cannot sign it on the client's behalf, however comprehensive the authority held. For internationally mobile clients this is a genuine logistical constraint, and it is worth solving weeks before the deadline rather than days.

Claims are made in writing outside the return, and general claims rules in Schedule 1A TMA 1970 apply — which means HMRC may enquire into the claim before giving effect to it, and must issue a closure notice if it concludes the claim is wrong. Unless HMRC is enquiring into it, it must give effect to a valid claim.

When will HMRC refuse? The eight exclusion Cases

Schedule 1AB sets out eight Cases in which HMRC is not liable to give effect to a claim. Reading them before drafting saves a great deal of wasted work, because in a cross-border catch-up at least one Case is usually in play.

• Case A — the amount arises from a mistake in a claim, election or notice, or certain mistakes relating to capital allowances. Errors in the making of a claim are corrected through that claim's own rules, not through overpayment relief.
• Case B — the person can correct the overpayment or over-assessment by other means. If the amendment window is still open, use it.
• Case C — the person could have obtained relief by other means when they first knew, or ought reasonably to have known, that the relief was available, but did not do so within the time allowed.
• Case D — a court or tribunal has already considered the grounds, or HMRC considered them and the appeal was settled by agreement.
• Case E — the person knew, or should reasonably have known, the grounds at a time when they could have put them to a court or tribunal on appeal.
• Case F — HMRC has taken proceedings to enforce payment of the amount in question.
• Case G — the amount was calculated in accordance with the practice generally prevailing at the time (other than PAYE income).
• Case H — for PAYE income, the amount was calculated in accordance with the practice generally prevailing 12 months after the end of the tax year.

Case C is the exclusion that defeats most catch-up claims

Cases G and H matter for test-case litigation. Cases D, E and F matter where there has been a prior dispute. But in a voluntary cross-border catch-up, the exclusion that actually bites is Case C.

The logic is uncomfortable. Suppose a US person in London realises in 2023 that foreign tax credit relief was understated on the 2020/21 UK return. At that point the amendment window for 2020/21 was still open. If the amendment is not made, and the point is raised for the first time in a 2026 overpayment relief claim, HMRC can argue that relief could have been obtained by other means at a time when the claimant knew or ought reasonably to have known it was available. The four-year clock has not expired, and yet the claim can still fail.

This is why the moment of knowledge in a catch-up is documented so carefully. Where the underlying facts genuinely could not have been known earlier — a foreign administrator's restatement, a late corrected information return, a US amended return that changed the credit position — Case C should not apply, and the claim should say so explicitly and evidence it. Where the client did know, the honest answer is that the claim is weak, and the engagement is better served by concentrating on years where it is not.

What if HMRC raised a determination because no return was filed?

Where returns were never filed at all, HMRC may have issued determinations of tax due. A determination is not an assessment you can simply displace with an overpayment relief claim in the ordinary way, and the usual answer is to file the outstanding return within the statutory period, which supersedes the determination.

Where that period has passed, special relief may be available. It is a form of overpayment relief for amounts charged in a determination, available since 1 April 2011, and it carries no statutory time limit — which makes it materially different from ordinary overpayment relief. In exchange, it is conditional: it requires that it would be unconscionable for HMRC to seek to recover the amount, that the claimant's filing and payment affairs are otherwise up to date or that reasonable steps have been taken to bring them up to date, and that the claimant has not previously had a special relief or equitable liability claim allowed. Cases C and F do not apply to special relief in the same way.

In practice, special relief is the correct instrument for the older, more serious end of a private client catch-up: the years where determinations are outstanding, the tax charged bears no relationship to reality, and the ordinary four-year window has long gone.

What happens on the US side when HMRC actually repays you?

This is the part generalist UK guidance omits entirely, and it is the part that causes the most trouble for US persons. A UK repayment is not free money. If the UK tax that is being refunded was previously claimed as a foreign tax credit on a US return, the refund is a foreign tax redetermination under section 905(c) of the Internal Revenue Code, and it carries an affirmative reporting obligation.

The mechanics are these. Foreign tax credits are claimed on Form 1116. When foreign tax that was credited is later refunded, the credit was, retrospectively, too large. The taxpayer must notify the IRS. Where the redetermination changes US tax for a prior year, an amended return for the affected year is generally required. Where it does not change US tax for any year, notification can instead be made by attaching a completed Schedule C (Form 1116), Foreign Tax Redeterminations, to the original return for the year in which the redetermination occurs. The IRS instructions for Schedule C (Form 1116) set out which route applies, and the Form 1116 instructions cover the credit itself. We deal with the mechanics in detail in our guide to the section 905(c) foreign tax redetermination.

Failure to notify is not a technicality. It can extend the assessment period for the affected year and expose the taxpayer to penalties, and it sits particularly badly alongside a streamlined submission in which non-wilfulness has just been certified. A UK repayment cheque that arrives quietly, is banked, and is never reflected on the US side is exactly the kind of loose end that undermines an otherwise clean disclosure.

Does the US refund window help where the UK one has closed?

Sometimes, and in the opposite direction to what people expect. The ordinary US refund limitation period is the later of three years from filing or two years from payment. But where the overpayment is attributable to foreign taxes for which a credit is allowed under section 901, section 6511(d)(3)(A) extends the period to ten years from the date prescribed for filing the return for the year in which those foreign taxes were paid or accrued. The extended period has been read narrowly by the courts and applies to credits, not to foreign taxes taken as a deduction.

The practical consequence in a catch-up is a striking asymmetry: a UK error in 2018/19 may be irrecoverable from HMRC while the corresponding US credit position for the same economic year remains open. Any recovery may therefore have to be engineered on the US side rather than the UK side, which is a very different piece of work.

Issue	UK / HMRC	US / IRS
Ordinary window to amend	12 months after the filing deadline for that year	Generally 3 years from filing or 2 years from payment
Standalone recovery route	Overpayment relief, Schedule 1AB TMA 1970	Amended return or refund claim
Long-stop for the recovery claim	4 years from the end of the relevant tax year	10 years where attributable to creditable foreign taxes under section 6511(d)(3)(A)
Form of the claim	Written claim, prescribed contents, signed personally	Amended return, or Schedule C (Form 1116) notification
Agent may sign	No — claimant or company officer only	Yes, under an appropriate authorisation
Effect of a foreign refund	Duty to give notice where an adjustment makes credit excessive	Foreign tax redetermination under section 905(c)
Route where no return was ever filed	File the return, or claim special relief against a determination	File the delinquent return; streamlined procedures if eligible

The UK mirror: what if the IRS repays you instead?

The obligation runs both ways, and this is regularly missed. Where a UK return claimed credit for US tax and the US tax is subsequently adjusted downward — because a US amended return, an IRS examination or a streamlined submission changed the position — the UK credit given was excessive. TIOPA 2010 contains both a provision on time limits for action where a tax adjustment makes credit excessive or insufficient, and a duty to give notice that an adjustment has rendered credit excessive. In other words, a US refund can trigger a UK notification obligation just as a UK refund triggers a US one.

The practical rule for anyone running a genuine two-jurisdiction catch-up is that no repayment in either country is final until it has been tested against the other country's return for the same economic year. Treating the UK and US files as separate workstreams is the root cause of most of the second-order problems we are asked to fix, and it is why cross-border sequencing is done as one exercise rather than two.

A worked sequence for a five-year cross-border catch-up

The order of operations decides whether the money is recoverable. A defensible sequence looks like this:

• Reconstruct both sides first. Build the UK and US positions for every open year before filing or claiming anything. Claims filed on a partial picture get amended, and amended claims invite enquiry.
• Map the windows. For each year, identify whether the UK amendment window is open, whether the four-year overpayment relief window is open, whether determinations exist, and what the US position is. Diary the earliest hard date.
• Use the open amendment windows immediately. Anything correctable by amendment must be amended, both because it is faster and because failing to do so creates Case B and Case C problems later.
• Draft the overpayment relief claims year by year. One claim per year, each with its own grounds, computation, appeal history and signed declaration. Address the exclusion Cases head-on in the claim itself rather than waiting to be asked.
• Model the US consequence before submitting. Quantify what each UK repayment does to the foreign tax credit position for the corresponding US year, and decide in advance whether an amended return or Schedule C notification is required.
• Sequence the disclosures consistently. Where the UK arrears require a disclosure and the US arrears require a streamlined submission, the facts certified in each must match. HMRC and the IRS exchange data automatically, and inconsistency is far more damaging than the underlying arrears.
• Reconcile the cash. Repayments, repayment supplement, interest charged on arrears and US credit adjustments should be tracked to a single schedule, so the net position is known rather than assumed.

Which cross-border errors actually produce recoverable UK overpayments?

Not every discrepancy is worth a claim. In our experience the errors that recur and that genuinely support overpayment relief are these:

• Foreign tax credit relief understated because US federal tax was allocated to UK-taxable income on a crude basis, or state tax was ignored where treaty relief was in fact available on the underlying source.
• Employment income taxed in full in the UK for a year in which split-year treatment, or a treaty article on dependent personal services, restricted the UK's taxing right.
• Higher and additional rate relief on personal pension contributions never claimed, because the contributions were made through a mechanism the UK return did not capture.
• Capital gains reported without allowable acquisition costs or incidental costs of disposal, which is common where the asset was acquired while the client was US-resident and the records sat with a US broker.
• Dividend and interest income reported gross where withholding tax had already been suffered and no credit was taken.
• Amounts reported on the arising basis in a year in which a remittance basis claim was made, or vice versa, where the two returns were prepared by different advisers.

Each of these is a mistake in a return, which is squarely what overpayment relief is designed for. What it is not designed for is a change of mind about a claim or election — that falls into Case A, and the answer, if there is one, lies in the rules for that particular claim.

Interest, repayment supplement and what you actually receive

A successful claim does not simply return the headline figure. Where HMRC repays tax, repayment supplement may be added, calculated by reference to the period the money was held. Where the same catch-up produces arrears in other years, late payment interest runs on those arrears from the original due dates, and interest rates on underpayments have historically exceeded those on repayments. A catch-up that produces a gross recovery in three years and arrears in two can still be net negative in cash terms.

For high-net-worth clients the more important number is usually not the refund but the closure: a documented, consistent, defensible position across both jurisdictions, with the exposure period running out rather than extending. The refund is a welcome by-product of doing the reconciliation properly, not the reason to do it.

The mistakes that forfeit an otherwise valid claim

• Writing one letter for six years. Each year is a separate claim with separate grounds. A composite letter risks the whole thing being treated as defective.
• Having the agent sign the declaration. It invalidates the claim, and the discovery usually comes after the deadline.
• Omitting the appeal history. It is a prescribed element, and its absence gives HMRC a clean procedural refusal.
• Claiming instead of amending. If the amendment window is open, the claim falls into Case B.
• Sitting on knowledge. Once you know relief is available, the clock on the alternative route matters, and Case C follows.
• Banking a UK repayment with no US follow-through. A section 905(c) redetermination does not go away because it was not noticed.
• Leaving determinations unaddressed. Where a determination stands, the correct instrument is a return in time or special relief, not an ordinary overpayment relief claim.

Speak to a specialist before the window closes

Overpayment relief is a narrow, formal, unforgiving remedy, and in a cross-border catch-up it is only ever half the picture — the other half is what a UK repayment does to your US foreign tax credit position and your disclosure narrative. If a reconciliation of your UK and US years is underway, or you suspect that returns filed on incomplete facts overstated your UK liability, the four-year window on the oldest year is the constraint that should set your timetable. To review your position in confidence, contact our cross-border team for a discreet, senior-led assessment of which years remain recoverable, which exclusions are in play, and how the two jurisdictions should be sequenced.