Why an HMRC Discovery Assessment Arrives Years After You Filed
An HMRC discovery assessment is the notice that reopens a tax year you reasonably believed was closed, and for US-connected taxpayers in Britain it now reaches back as far as twelve years. Furthermore, it arrives without warning, carries a thirty-day clock, and demands tax that has already accrued interest. Consequently, wealthy dual filers often discover the problem only when the payment demand lands.
Most published guidance treats an HMRC discovery assessment as a purely domestic issue. However, that framing fails anyone holding a US passport. Your investment portfolio, your brokerage account, your holding company and your rental income are all "offshore matters" from one side of the Atlantic or the other. Therefore, the extended time limits that most British taxpayers never encounter apply to you almost by default.
What an HMRC Discovery Assessment Actually Is
An HMRC discovery assessment is a statutory assessment raised under section 29 of the Taxes Management Act 1970 after the ordinary enquiry window has closed. Specifically, it lets an officer assess additional tax when they discover that income escaped assessment, that an assessment was insufficient, or that relief was excessive.
The distinction from an enquiry matters enormously. An enquiry must open within roughly twelve months of the filing date. By contrast, an HMRC discovery assessment can arrive four, six, twelve or twenty years later. Accordingly, it is the mechanism HM Revenue and Customs uses when the ordinary window has long expired.
Why US-Connected Filers Receive Them Disproportionately
International data exchange drives the volume. Additionally, HMRC receives automatic reporting on UK-resident US persons through FATCA and the Common Reporting Standard, then cross-matches it against filed returns. Any mismatch generates a risk score.
In our experience advising high-net-worth cross-border clients, three patterns trigger most HMRC discovery assessment cases. Firstly, foreign tax credit claims that do not reconcile to a US return. Secondly, unreported distributions from non-UK structures. Thirdly, remittances that were never disclosed on the residence pages.
The Legal Test HMRC Must Satisfy
HMRC cannot raise an HMRC discovery assessment simply because it has changed its mind. Rather, an officer must clear two statutory hurdles, and both are genuinely contestable. Notably, the burden of establishing a valid discovery sits with HMRC, not with you.
The Two Conditions in Section 29
The first condition is that an officer actually made a discovery. Moreover, that discovery must be a genuine change in the officer's state of mind, not a rereading of material already on file. Subsequently, HMRC must show either careless or deliberate conduct, or that the officer could not reasonably have been expected to spot the insufficiency from the information made available.
That second limb is the taxpayer's strongest ground. Specifically, if your return and its white space disclosed enough for a competent officer to identify the issue, the HMRC discovery assessment fails. HMRC's own Enquiry Manual guidance on interpretation confirms how narrowly officers are meant to read this.
Why the White Space Wins Cases
Detailed white-space disclosure is the single cheapest insurance available to a cross-border filer. For example, a short note explaining that a foreign tax credit derives from a US federal return, quantified and dated, removes the "could not reasonably have been aware" argument entirely.
Conversely, silence invites an HMRC discovery assessment years later. Therefore, we recommend that every client with treaty positions, foreign losses or unusual credits documents the reasoning on the return itself rather than in a file note nobody at HMRC will ever read.
Carelessness Is Not the Default
Carelessness means failing to take reasonable care. However, taking professional advice on a genuinely uncertain point is the opposite of careless. Consequently, an HMRC discovery assessment premised on carelessness often collapses when the adviser's contemporaneous file is produced.
Deliberate conduct sets a far higher bar still. Importantly, it requires an intention to mislead, and HMRC must prove it. Many assessments overreach precisely here.
The Time Limits: Four, Six, Twelve and Twenty Years
Time limits decide most of these disputes before the technical merits are ever reached. Furthermore, they are the first thing you should check when an HMRC discovery assessment arrives.
The Ordinary Four-Year Window
Section 34 of the Taxes Management Act 1970 sets the standard limit at four years after the end of the tax year. Accordingly, for 2021/22 that window closed on 5 April 2026. No behaviour needs to be shown for an HMRC discovery assessment inside this period.
Where the lost tax was brought about carelessly, section 36 extends the period to six years. HMRC's published time-limit guidance at EM3214 sets out both limits in full.
The Twelve-Year Offshore Rule Almost Nobody Explains
Here is the provision that transforms the risk for American clients. Section 36A of the Taxes Management Act 1970 permits an assessment up to twelve years after the end of the tax year where the lost income tax or capital gains tax involves an offshore matter or an offshore transfer.
Critically, from 2015/16 onwards no careless behaviour is required. An innocent, well-advised mistake carries the same twelve-year exposure as a sloppy one. Moreover, "offshore matter" is defined expansively: income from a non-UK source, assets situated outside the UK, income received outside the UK, or activities carried on largely abroad.
For a US citizen in London, that definition captures almost the entire balance sheet. Therefore, an HMRC discovery assessment touching your American brokerage account, your US company shares or your dollar deposits sits inside the twelve-year regime by default, not by exception.
Twenty Years for Deliberate Behaviour
Deliberate loss of tax, or a failure to notify chargeability at all, extends the HMRC discovery assessment limit to twenty years. Additionally, that twenty-year period survives independently of the offshore rules. Consequently, a taxpayer who never registered for Self Assessment faces the longest exposure of all.
The FATCA Defence Most Advisers Overlook
This is the section competitors omit entirely, and it is the most valuable point in this article. Notably, the twelve-year offshore window is not unconditional.
How Section 36A(7) Closes the Twelve-Year Window
Subsection (7) of section 36A disapplies the extended limit in defined circumstances. Specifically, where HMRC received relevant overseas information from another tax authority before the ordinary time limit expired, and could reasonably have been expected to raise the assessment within that ordinary limit, the twelve-year period is unavailable.
The commercial logic is straightforward. HMRC should not benefit from a longer window when the data was already sitting in its systems. Therefore, if your UK bank or your US institution reported the account under FATCA or the Common Reporting Standard years ago, an HMRC discovery assessment raised in year eleven may be statute-barred.
Proving What HMRC Already Held
You defeat an HMRC discovery assessment on this ground with evidence, not assertion. Firstly, request the FATCA and CRS data HMRC holds about you. Secondly, obtain the reporting confirmations your banks issued. Thirdly, date the point at which HMRC could reasonably have matched that data to your record.
In practice, few taxpayers ever run this argument, because few advisers connect the FATCA and FBAR reporting chain to the UK statutory time limits. Nevertheless, for a US person the exchange trail is unusually rich, and that richness works in your favour.
What a UK Discovery Assessment Does to Your US Return
A reopened UK year is never only a UK problem. Furthermore, ignoring the American consequences turns a manageable assessment into a double-tax outcome.
Section 905(c) Makes Notification Mandatory
When your UK liability changes, the foreign tax you originally accrued no longer matches the tax finally paid. That mismatch is a foreign tax redetermination under section 905(c) of the Internal Revenue Code. Consequently, notifying the IRS is a requirement, not an option.
The mechanics sit in Treasury Regulation 1.905-4. Broadly, you file an amended return carrying a revised Form 1116 for each affected year. Accordingly, an HMRC discovery assessment covering six UK years can generate six amended US returns.
The Ten-Year Refund Window Is Your Friend
Most taxpayers assume the ordinary three-year US refund limit blocks recovery on old years. However, section 6511(d)(3) of the Internal Revenue Code provides a special ten-year period for refunds attributable to an increased foreign tax credit.
That distinction is worth real money. Specifically, extra UK tax assessed today can unlock US refunds stretching back a decade. Therefore, the correct response to a UK assessment is frequently to claim, not merely to pay. Careful tax treaty and foreign tax credit work usually recovers a substantial share of the UK cost.
Sequencing Matters More Than Speed
Order the work around the HMRC discovery assessment deliberately. Firstly, settle or challenge the UK figures. Secondly, quantify the final UK tax by year. Thirdly, amend the affected US tax returns for expats once the UK position is fixed. Otherwise, you will amend twice and pay for the privilege.
Your Thirty Days: Appealing and Postponing
The response window is short and unforgiving. Moreover, two separate applications are required, and taxpayers routinely make only one.
The Clock Runs From Issue, Not Receipt
You have thirty days to appeal an HMRC discovery assessment, and that period runs from the date on the assessment rather than the date it reached you. Consequently, post sitting in a redirected overseas mailbox erodes the window before you have read it. HMRC's appeals guidance sets out the process.
Late appeals are possible but discretionary. Therefore, treat the deadline as absolute.
Postponement Is a Separate Application
Appealing does not suspend payment. Rather, you must also apply to postpone the tax under section 55 of the Taxes Management Act 1970. Without it, interest accrues throughout the dispute at HMRC's published late-payment rate, currently 7.75%.
Subsequently, if the internal review does not resolve matters, the First-tier Tribunal hears the appeal. Alternatively, where the underlying position is genuinely wrong, the Worldwide Disclosure Facility may deliver a better outcome than litigation.
A Worked Case Study With Real Numbers
Consider a client we will call Marcus, an American investment banker who has lived in London since 2012. He filed UK returns throughout and claimed foreign tax credits for his US federal liability each year.
Marcus held a US brokerage account generating roughly £48,000 of dividends and £160,000 of realised gains between 2016/17 and 2019/20. He assumed his US reporting covered it. However, he never entered the income on his UK returns, and he made no white-space disclosure. In March 2026 an HMRC discovery assessment reopened all four years, assessing £71,400 of tax plus £19,800 of interest.
The twelve-year offshore rule applied, so time limits offered no immediate escape. Nevertheless, two arguments reduced the exposure materially. Firstly, HMRC had received CRS data on the account from 2018 onwards, which supported a section 36A(7) challenge to the two earliest years. Secondly, careful recomputation of the UK gains using correct base costs cut the assessed profit by £26,000.
The settled UK liability came to £41,900. Meanwhile, amended US returns claiming the additional UK tax as a credit under the ten-year rule recovered $34,200 of previously paid US federal tax. Ultimately, Marcus's net cost fell from a headline £91,200 to roughly £16,000 once the American recovery landed.
How TaxYork Can Help
TaxYork prepares and defends both sides of the cross-border position. Furthermore, we handle the UK assessment and the corresponding US amendments as one engagement, because splitting them across two firms is where value leaks away.
Our work on an HMRC discovery assessment begins with the statutory analysis. Specifically, we test whether a valid discovery occurred, whether the correct time limit was applied, and whether section 36A(7) defeats the extended window. Additionally, we reconstruct the underlying figures from source records rather than accepting HMRC's estimates.
We then rebuild the American position. Accordingly, we quantify the foreign tax credit uplift, file the required redetermination notices, and pursue refunds within the ten-year window. Comprehensive cross-border tax planning then prevents the same exposure recurring.
Conclusion
An HMRC discovery assessment is a demand, not a verdict. Moreover, the twelve-year offshore window that makes it so alarming for US-connected taxpayers carries a statutory exception that few advisers ever invoke. Therefore, the first step is always analysis rather than payment.
Equally important, the American consequences run in your favour. Extra UK tax generally converts into US foreign tax credits, and the ten-year refund period reaches years you assumed were shut. Ultimately, taxpayers who address both jurisdictions together settle for materially less than those who treat the assessment as a British problem alone.
Contact Us
If an assessment has landed, act within the thirty days. Please contact us to review your position and protect your appeal rights. Alternatively, book a consultation with our cross-border team at hello@taxyork.com or 020 3488 8606.
Disclaimer
This article provides general information about UK and US tax compliance and does not constitute tax, legal or financial advice. Tax rules change frequently, and their application depends entirely on your individual circumstances. Accordingly, you should obtain professional guidance before acting on anything set out above. TaxYork accepts no liability for any loss arising from reliance on this material. Readers who are catching up on missed US tax returns should take specialist advice on route selection before making any submission.