How far back can HMRC go if I have missed UK tax returns?

It depends on the facts of each year. HMRC has four years from the end of the tax year in the ordinary case, six years where the loss of tax was brought about carelessly, twelve years where it involves an offshore matter or offshore transfer, and twenty years where behaviour was deliberate or where chargeability was never notified. All four periods run from the end of the tax year, not from any filing date. See https://www.gov.uk/hmrc-internal-manuals/self-assessment-legal-framework/salf411.

Does the twelve year offshore limit apply even if I took reasonable care?

Yes. For income tax and capital gains tax for 2015 to 2016 onwards, HMRC guidance at https://www.gov.uk/hmrc-internal-manuals/compliance-handbook/ch53510 confirms the twelve year limit applies irrespective of whether the person took reasonable care or was careless. Reasonable care shortens penalties, not the assessing window. The one restriction is that the extended limit is unavailable where HMRC already received relevant overseas information, such as Common Reporting Standard data, in time to assess within the ordinary limits.

What happens if HMRC never sent me a notice to file?

You still had an obligation. Section 7 TMA 1970 requires a chargeable person who has not been issued a return to notify HMRC on or before 5 October following the end of the tax year, per https://www.gov.uk/hmrc-internal-manuals/self-assessment-legal-framework/salf210. Crucially, HMRC states at https://www.gov.uk/hmrc-internal-manuals/compliance-handbook/ch53600 that the time limit for failure to notify is twenty years whether or not the failure was deliberate. Never registering is the highest-exposure position of all.

Is the US assessment window the same length as the UK one?

No, and they rarely expire together. The general US period is three years from the date the return was filed, extended to six for a substantial omission of gross income over 25 per cent or where over $5,000 of income attributable to a specified foreign financial asset is omitted, and unlimited where no return was filed or the return was fraudulent. See https://www.irs.gov/irm/part25/irm_25-006-001r. The UK clock instead runs from the end of the tax year.

HMRC assessed extra UK tax for an old year. Do I have to tell the IRS?

Yes. A change in your foreign tax liability is a foreign tax redetermination. IRS Publication 514 at https://www.irs.gov/publications/p514 requires you to file Form 1040-X, or another amended return, where the redetermination changes your US tax, or to attach Schedule C (Form 1116) to the original return for the year of the redetermination where it does not. Failing to notify without reasonable cause carries a penalty of 5 per cent per month, capped at 25 per cent.

Can I still claim a US credit for UK tax assessed years later?

Often yes, well beyond the ordinary refund window. The normal claim period is three years from filing or two years from payment, whichever is later, per https://www.irs.gov/taxtopics/tc308. But Publication 514 confirms a ten year period to claim a refund of US tax where you paid or accrued a larger foreign tax than you claimed a credit for, beginning the day after the regular due date, without extensions, for the year the foreign tax was actually paid or accrued.

Missed UK Tax Returns: HMRC Discovery Time Limits

If you have missed UK tax returns, the first question is not what the penalty will be but how far back HMRC can still reach, and the answer is not one number. HMRC has four years from the end of the tax year in the ordinary case, six years where the loss of tax was brought about carelessly, twelve years where the lost tax relates to an offshore matter or offshore transfer, and twenty years where the behaviour was deliberate or where a person never notified chargeability at all. Those periods sit in sections 34, 36 and 36A of the Taxes Management Act 1970 and are summarised by HMRC at https://www.gov.uk/hmrc-internal-manuals/self-assessment-legal-framework/salf411.

For a US citizen or green card holder living in the UK, that is only half the picture. The same tax year is also sitting inside a completely separate US assessment window that is a different length, starts on a different date and turns on a different behaviour test. In the returns we prepare, the failure mode we see most often is a client who has correctly worked out that one country can no longer touch a year, and has assumed the other country is in the same position. It very rarely is. This article deals with HMRC's assessing power itself, and then with what a UK discovery assessment does to a US return that has already been filed and has already claimed a credit for UK tax.

Missed UK Tax Returns: The Four Windows HMRC Can Use

A discovery assessment is HMRC's power to assess tax for a year that is otherwise closed. HMRC's Enquiry Manual puts it plainly: the discovery provisions allow an officer to make an assessment to recover a loss of tax where the time limit to open a Self Assessment enquiry has passed, or where there is a failure to submit a return at all. That guidance is at https://www.gov.uk/hmrc-internal-manuals/enquiry-manual/em3202. The assessing time limits then cap how far back that power can go.

Four years from the end of the tax year. The ordinary limit under section 34 TMA 1970, applying where the taxpayer took reasonable care but the return was still wrong or incomplete.
Six years from the end of the tax year. The extended limit under section 36(1) where the loss of tax was brought about carelessly by the taxpayer or a person acting on their behalf.
Twelve years from the end of the tax year. The offshore limit under section 36A, for income tax and capital gains tax where the lost tax involves an offshore matter or an offshore transfer that makes the loss significantly harder for HMRC to identify.
Twenty years from the end of the tax year. The longest limit under section 36(1A), for a loss of tax brought about deliberately, and also for a failure to notify chargeability.

Note the direction of travel. Each limit runs from the end of the tax year, not from the filing date, not from the date HMRC found out, and not from the date the money was paid. A 2019 to 2020 UK tax year ended on 5 April 2020, so the ordinary four year window on that year closed on 5 April 2024, the careless window closes on 5 April 2026, the offshore window on 5 April 2032 and the deliberate window on 5 April 2040. HMRC does not need to decide which window applies in advance; it needs to be able to justify the window it eventually uses.

When Does The Enquiry Window Into A Filed Return Actually Close?

The enquiry window and the assessing time limit are two different things, and conflating them is the most common analytical error we see. An enquiry is HMRC's routine right to open up a return it has received. Where the return was delivered on or before the filing date, HMRC's Enquiry Manual states that the enquiry window runs for a full twelve months from the date the tax return is received. Where the return was delivered late, the window does not close until the quarter day next following the first anniversary of the day the return was made, the quarter days being 31 January, 30 April, 31 July and 31 October. That guidance is at https://www.gov.uk/hmrc-internal-manuals/enquiry-manual/em1506.

That late filing rule matters enormously for someone catching up. If you file five years of outstanding returns in one submission, you have just opened a fresh enquiry window on every one of them, running roughly a year from the quarter day after each was delivered. You have not reset the assessing time limits, which still run from the end of each tax year, but you have handed HMRC a clean, straightforward route into every year you filed without it needing to establish a discovery at all. That is a reason to get the returns right first time, not a reason to delay filing them.

There is a corresponding taxpayer window. GOV.UK confirms at https://www.gov.uk/self-assessment-tax-returns/corrections that you can normally change a return within twelve months of the Self Assessment deadline, and that once that has passed you must write to HMRC, with any claim for overpayment relief made within four years of the end of the tax year. In other words, your own ability to fix a year in your favour expires far sooner than HMRC's ability to assess one against you.

What Makes A Discovery Assessment Valid?

Section 29(1) TMA 1970, at https://www.legislation.gov.uk/ukpga/1970/9/section/29, lets an officer assess where they discover that an amount of income tax or capital gains tax ought to have been assessed but has not been, that an assessment has become insufficient, or that a relief given has become excessive. Where a return has actually been delivered, that power is restricted. One of two conditions must be satisfied.

The first condition, in section 29(4): the situation was brought about carelessly or deliberately by the taxpayer or a person acting on their behalf.
The second condition, in section 29(5): at the point the enquiry window closed, or the closure notice was issued, the officer could not have been reasonably expected, on the basis of the information made available before that time, to be aware of the insufficiency.

HMRC's guidance at https://www.gov.uk/hmrc-internal-manuals/enquiry-manual/em3232 confirms that only one of the two conditions needs to be met, though HMRC considers its position stronger where both are. The second condition is tested against a hypothetical officer of general competence, knowledge and skill. Information made available includes the return and its accompanying documents, relevant claims and their supporting documents, anything supplied during an enquiry, and matters whose existence could reasonably be inferred from those sources, with relevant returns covering the year of assessment and the two preceding years. HMRC sets that out at https://www.gov.uk/hmrc-internal-manuals/enquiry-manual/em3233. The practical standard is high: the disclosure has to make the officer aware of the insufficiency itself, not merely curious enough to ask a question.

Where no return was ever delivered, section 29(5) has nothing to bite on. There is no enquiry window to have closed and no information to have been made available. That is why unfiled years are structurally weaker ground for a taxpayer than badly filed years.

How The Twelve Year Offshore Limit Works For A US Person In The UK

The offshore limit is the one that catches internationally mobile people, because almost everything a US person in the UK owns is, from HMRC's perspective, offshore. HMRC's Compliance Handbook at https://www.gov.uk/hmrc-internal-manuals/compliance-handbook/ch53520 describes an offshore matter as including income arising from a source in a territory outside the UK, assets situated or held outside the UK, income or assets received outside the UK, and activities carried on wholly or mainly outside the UK. A US brokerage account, a US rental property, US employment income earned on assignment and shares in a US company all sit squarely inside that description.

The critical point, set out at https://www.gov.uk/hmrc-internal-manuals/compliance-handbook/ch53510, is that for income tax and capital gains tax for 2015 to 2016 onwards the twelve year limit applies irrespective of whether the person took reasonable care or was careless. Reasonable care is not a defence to the length of the window. It overrides the four and six year limits, and it is itself superseded by the twenty year limit where behaviour was deliberate. So the honest, careful, professionally advised US citizen in London who simply did not appreciate that a US source item was reportable in the UK is looking at a twelve year window on that item, not a four year one.

There is one meaningful restriction. HMRC guidance at https://www.gov.uk/hmrc-internal-manuals/compliance-handbook/ch53550 confirms the twelve year limit does not apply where HMRC received relevant overseas information, including data received under the Common Reporting Standard or under exchange of information articles in tax treaties, that reasonably enabled it to become aware of and assess the lost tax before the ordinary time limits expired. If HMRC was handed the account data in time and did nothing, it cannot use the extended window for that matter. In practice this is worth checking rather than assuming, because a great deal of offshore account data does reach HMRC automatically.

Does A Discovery Go Stale If HMRC Sits On It?

For several years taxpayers argued that a discovery could become stale, so that an officer who discovered an insufficiency and then delayed for years lost the ability to assess. HMRC now maintains a dedicated page on the point at https://www.gov.uk/hmrc-internal-manuals/enquiry-manual/em3260, following the Supreme Court's decision in HMRC v Tooth. The position is that a discovery does not cease to be a discovery simply through the passage of time. Provided a qualifying discovery is made, the other conditions are satisfied and the assessment is issued inside the statutory time limit, delay by HMRC is not by itself an answer. Waiting does not improve your position; it only shortens the time available to prepare a proper response.

No Return Was Ever Issued: The Duty To Notify Chargeability

Many people who arrive in the UK are never issued a notice to file, conclude that no UK return is required, and stop there. That reasoning is backwards. Section 7 TMA 1970 imposes a free-standing obligation: a person who has not received a notice to file but who is chargeable to income tax or capital gains tax must notify HMRC, and HMRC guidance at https://www.gov.uk/hmrc-internal-manuals/self-assessment-legal-framework/salf210 confirms that notification must be given on or before 5 October following the end of the tax year. GOV.UK repeats the same 5 October registration date, alongside the 31 October paper filing date and the 31 January online filing and payment date, at https://www.gov.uk/self-assessment-tax-returns/deadlines.

The consequence of missing that notification is the single most important fact in this article. HMRC states at https://www.gov.uk/hmrc-internal-manuals/compliance-handbook/ch53600 that the time limit for failure to notify is twenty years whether or not the failure was deliberate. There is no carelessness test to argue about and no offshore analysis to run. A person who was chargeable, was never issued a return, and never notified is exposed for two decades on that year regardless of how innocent the omission was. This is why we treat never-registered clients differently from late-filing clients: the underlying legal exposure is a different order of magnitude.

UK And US Assessing Windows For The Same Year, Side By Side

Now the comparison that almost no guidance makes. Take a single year of income for a US citizen resident in the UK. That income sits inside two assessment regimes that share nothing except the income itself.

Start date. The UK clock runs from the end of the tax year, 5 April. The US clock generally runs from the date the return was filed or received, per the IRS at https://www.irs.gov/irm/part25/irm_25-006-001r, so a late-filed US return pushes the US expiry date later while the UK date does not move.
Ordinary length. Four years in the UK. Three years in the US.
The middle tier. Six years in the UK for careless behaviour, a behaviour test. Six years in the US for a substantial omission of gross income exceeding 25 per cent, or where more than $5,000 of income attributable to a specified foreign financial asset is omitted, both of which are arithmetic tests with no behaviour element.
The offshore tier. Twelve years in the UK for offshore matters regardless of reasonable care. The US has no direct equivalent tier, but under IRC 6501(c)(8) the assessment period does not start to run at all until a required international information return such as Form 8938 or Form 5471 is filed, per the IRS practice unit at https://www.irs.gov/pub/fatca/int_practice_units/int_c_115r.pdf.
The longest tier. Twenty years in the UK for deliberate behaviour or failure to notify. In the US the period is unlimited where no return was filed or the return was false or fraudulent.

Read that list again and the asymmetry is obvious. The US ordinary window is shorter but conditional on having filed; the UK ordinary window is longer but runs automatically from a fixed date. The UK punishes offshore facts with time; the US punishes missing information returns with time. A year can therefore be closed in one country and wide open in the other, and the two can be closed in either order.

Which Clock Closes First, And What That Means For Sequencing

For a UK-resident US citizen who has actually been filing complete US returns, including every required international information return, the US clock normally closes first. Three years from the US filing date will usually expire before four years from the following 5 April, and well before six or twelve. The practical implication is that US years quietly go out of reach while the UK years remain live, which is the opposite of what most clients assume when they come to us worried primarily about the IRS.

For the more common catch-up profile, where UK returns were filed and US returns or Forms 8938 and 5471 were not, the ordering flips completely. The unfiled or incomplete US years never started their clock, so they remain open indefinitely, while the UK years march steadily towards their four, six or twelve year expiry. That is the case for dealing with the US side first, because it is the side that will not close on its own. Where a taxpayer qualifies, non-wilful US catch-up is normally handled through the Streamlined Filing Compliance Procedures, and our approach to that is set out at us-uktax.com/streamlined-foreign-offshore-procedures.

A Worked Scenario: One Tax Year, Two Open Windows

The following is an illustration, not a client file, and the figures are assumed. A US citizen moved to London and became UK resident in 2018. She filed UK Self Assessment returns on time each year but omitted dividends and a capital gain from a US brokerage account, believing that income taxed in the US did not need reporting in the UK. She filed her US returns on time, reported the brokerage income there, and claimed a foreign tax credit for the UK tax she had actually paid.

In 2026 HMRC opens a check on the 2019 to 2020 year. The enquiry window on that return closed long ago, so HMRC must proceed by discovery. The omission relates to income arising from a source outside the UK, so it is an offshore matter, and the twelve year limit under section 36A is available on those facts irrespective of whether she took reasonable care. HMRC raises a discovery assessment for additional UK tax on the omitted dividends and gain. Meanwhile her US return for the corresponding US tax year was filed on time and complete, so the ordinary three year US assessment window closed years ago. The IRS cannot assess more US tax for that year. But the UK tax for that year has just gone up, and that has US consequences running the other way.

When A UK Discovery Assessment Reopens Your Already-Filed US Return

A change in your foreign tax liability is a foreign tax redetermination for US purposes. IRS Publication 514, at https://www.irs.gov/publications/p514, defines it as a change in your foreign tax liability, and certain other changes that may affect your US income tax liability. A HMRC discovery assessment that increases UK tax for an earlier year is precisely that. So is a UK refund that reduces it, which is the direction that creates a liability rather than a claim.

Publication 514 sets out two routes. Where the redetermination changes your US tax liability you must file Form 1040-X, or another amended return, to notify the IRS so that your US tax for the affected year or years can be redetermined. Where the redetermination does not change your US tax liability, you may instead notify the IRS by attaching a completed Schedule C (Form 1116) for each applicable separate category of income to the original return for the tax year in which the redetermination occurs. The Form 1116 instructions at https://www.irs.gov/instructions/i1116 confirm the same split, and Schedule C is filed for the year the redetermination occurs, by separate category.

This is a notification obligation, not an option. Publication 514 states that if you fail to notify the IRS of a foreign tax redetermination and cannot show reasonable cause, you may face a penalty of 5 per cent of the tax due resulting from the redetermination for each month or part of a month the failure continues, capped at 25 per cent. Clients frequently settle a UK discovery assessment, pay the additional UK tax, and never mention it on the US side. That is an unreported redetermination.

The good news is that the US side stays open in the taxpayer's favour far longer than most people expect. The ordinary US refund window, per the IRS at https://www.irs.gov/taxtopics/tc308, is three years from filing the original return or two years from paying the tax, whichever is later. But Publication 514 confirms a special rule: you have ten years to file a claim for refund of US tax if you find that you paid or accrued a larger foreign tax than you claimed a credit for, and that ten year period begins the day after the regular due date, without extensions, for filing the return for the year in which the taxes were actually paid or accrued. So the extra UK tax generated by a discovery assessment on an old year can still produce a recoverable US credit long after the ordinary refund window has shut.

Returning to the illustration above: the additional UK tax assessed by HMRC is creditable against the US tax on the same income for the affected year, and the claim is made under the ten year rule rather than the three year rule. Handled properly, a UK discovery assessment on a doubly-taxed item is often substantially recovered on the US side. Handled by ignoring it, the client pays the UK tax twice over in economic terms and carries an unreported redetermination as well.

How We Sequence A Two-Country Catch-Up

Establish, year by year, whether a UK return was filed, whether a notice to file was ever issued, and whether chargeability was notified by the 5 October deadline. The never-notified years carry the twenty year exposure and are triaged first.
Map each UK year against the correct assessing limit on its own facts, distinguishing purely domestic items on the ordinary or careless limit from offshore items on the twelve year limit, and check whether HMRC already held Common Reporting Standard or treaty data that removes the extended window.
Map the corresponding US years: filed or unfiled, complete or missing an international information return, and therefore closed, open for three or six years, or never started under IRC 6501(c)(8).
Fix the side that will not close on its own first, which for most catch-up clients is the US side, then bring the UK years current so the numbers reported in each country reconcile to the same underlying income.
Recompute the foreign tax credit position for every year touched, and file the redetermination notifications, using the ten year claim window where additional UK tax has been assessed for an earlier year.
Keep the evidence pack: bank and broker statements, remittance records, HMRC correspondence and payment dates, since the date UK tax was paid or accrued drives the US credit year.

The Practical Cost Of Waiting

Time limits are not a strategy. After Tooth, HMRC delay is not a defence, the twenty year failure to notify window does not depend on anyone proving bad faith, and the twelve year offshore window applies even to a taxpayer who took reasonable care. On the US side, an unfiled Form 8938 or Form 5471 means the assessment period on that return has not begun. Meanwhile the taxpayer's own remedies do expire: twelve months to amend a UK return, four years for overpayment relief, and, for foreign tax credit claims, a finite ten years.

The clients who come out of this well are the ones who map both clocks for every open year before they touch a single form, and then remediate in the right order. If you are catching up on either side of the Atlantic, our UK compliance work is described at us-uktax.com/uk-tax-services, the US return preparation side at us-uktax.com/us-tax-services, and the combined two-country position at us-uktax.com/cross-border-tax-planning. You can start a confidential review at us-uktax.com/contact.