Missed UK Tax Returns: The 12-Year Window's Carve-Out

Missed UK tax returns involving an offshore matter sit inside HMRC's 12-year assessment window under TMA 1970 s.36A, regardless of whether you were careless or took reasonable care. But the statute contains a carve-out: where HMRC already held relevant overseas information in time to assess, the 12-year limit falls away.

That carve-out is the subject of this guide. It is not a technicality, and for US-connected UK residents it is frequently the single most valuable argument available — because the very data exchange that alerted HMRC to your foreign accounts is capable of destroying HMRC's right to reach back twelve years. Almost nobody runs it. We think that is a mistake.

Jungle Tax already publishes separate guides on the general question of how far back HMRC can go, what the late-filing penalties look like, and how to sequence a UK disclosure alongside a US one. This guide deliberately does not restate that ground. It is about one narrow, powerful provision and how to use it.

What is the 12-year offshore window, and why is it your default?

Finance Act 2019 sections 80 and 81 inserted TMA 1970 s.36A (for income tax and capital gains tax) and IHTA 1984 s.240B (for inheritance tax). HMRC's Compliance Handbook at CH53510 sets out the architecture. The essential points:

• The 12-year limit applies only to income tax, capital gains tax and inheritance tax, and only where the lost tax involves an offshore matter or an offshore transfer.
• For income tax and CGT, the tax years 2013-14 and 2014-15 are caught only where the taxpayer was careless.
• For 2015-16 onwards, an assessment can be made within 12 years of the end of the relevant tax period irrespective of whether the person took reasonable care or was careless.
• For inheritance tax, transfers between 1 April 2013 and 1 April 2015 require carelessness; transfers from 1 April 2015 onwards do not.

Read that third bullet again. It is the provision that makes the 12-year window the norm rather than the exception for our client base. The familiar four-year and six-year limits are behaviour-driven: four years if you took reasonable care, six if you were careless. Section 36A severs the link between behaviour and time. Take flawless care, engage a reputable adviser, make an honest interpretive error — and HMRC still has twelve years, provided the matter is offshore.

What counts as an "offshore matter"?

CH53520 defines it broadly. Lost tax is charged on or by reference to an offshore matter where it relates to 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, activities carried on wholly or mainly outside the UK, or anything having effect as if it were such income, assets or activities.

For a US citizen or green card holder resident in the UK, that definition swallows almost the entire fact pattern. Your Fidelity or Schwab brokerage account is an asset held outside the UK. Dividends on your US equities are income arising from a source outside the UK. An RSU vest from a US parent, a distribution from a Delaware LLC, interest on a US money market fund, a gain on the sale of a former US home, your Roth IRA, your 401(k) — all offshore matters from HMRC's perspective. The irony is not lost on our clients: HMRC treats the United States as "offshore", and the IRS treats the United Kingdom as "foreign". You are offshore to both.

There is one filter. CH53540 requires, for offshore transfers, that the arrangement made the loss of tax significantly harder for HMRC to identify — meaning HMRC was significantly less likely to become aware of it, or likely to become aware only significantly later, than in a comparable domestic case. HMRC reads "significantly" as noteworthy, important or consequential.

Where is the carve-out, and what exactly does it say?

The carve-out lives at CH53550, headed "Relevant overseas information". The 12-year time limit does not apply where both of the following are true:

• HMRC received information from an overseas authority which reasonably enabled HMRC to become aware of and assess the lost tax before the normal time limits expired; and
• It was reasonable to expect HMRC to assess the lost tax before the normal time limits expired.

"Relevant overseas information" means any information received by HMRC from an authority in a territory outside the United Kingdom, under EU tax law provisions or under a bilateral arrangement between the UK and that territory. HMRC's own manual gives Common Reporting Standard data and tax treaty exchanges as examples.

Two structural points that practitioners routinely miss. First, the test is not whether HMRC actually noticed. It is whether the information reasonably enabled HMRC to become aware, and whether it was reasonable to expect HMRC to act. HMRC's internal inertia is not a defence to the carve-out; it is the reason the carve-out exists. Second, the carve-out does not extinguish the assessment. It collapses the window back to the ordinary limits — four years where reasonable care was taken, six where the behaviour was careless. Those years remain assessable. What disappears is the tail.

The argument nobody makes: CRS and FATCA cut both ways

Here is the position most disclosures walk straight past. In a typical case the client comes to us because HMRC has written to them. HMRC wrote because it received automatic exchange data. That letter is then treated as an unanswerable demonstration of exposure, and the disclosure is prepared on a twelve-year basis.

But the letter is also evidence. If HMRC's knowledge came from an overseas authority under a bilateral arrangement, and it arrived before the four or six year window shut, the taxpayer has the raw material for the s.36A carve-out. The information that created the problem is the information that limits it. Establishing what HMRC received and when it received it is therefore the first analytical step in any offshore catch-up, not an afterthought.

Why the US side behaves differently from everywhere else

This is where a generalist UK tax page will lead you badly astray, and where the cross-border analysis genuinely diverges. The United States is not a participating jurisdiction in the Common Reporting Standard. It never adopted it. So the reflexive assumption — "HMRC gets everything under CRS" — is simply false for the United States.

What exists instead is the UK-US intergovernmental agreement implementing FATCA, which is reciprocal but asymmetrically so. UK financial institutions report comprehensive data to HMRC for onward transmission to the IRS. The US undertakes to provide HMRC with a materially narrower dataset in return. That asymmetry is the whole ballgame for the carve-out, because it determines, income stream by income stream, whether HMRC was reasonably enabled to identify your lost tax.

Your US-side item	Typically visible to HMRC via IRS reciprocal exchange?	Carve-out prospects
US-source dividends on a US brokerage account	Generally yes — US-source dividends paid to a UK-resident account holder are within the reciprocal reporting	Strongest. HMRC arguably had the income figure and the account holder's identity in time
Interest on a US depository (bank) account	Generally yes — gross interest on depository accounts is reported	Strong, on the same basis
Gross proceeds from selling US securities (CGT)	Generally no — sale proceeds are not within the US reciprocal dataset	Weak. HMRC could not compute a gain it never saw
Account balance or value	Generally no — unlike CRS, balances are not part of the US reciprocal feed	Weak as a standalone route
RSU or option vest from a US employer	No — this is employer payroll data, not financial-institution reporting	Very weak unless UK payroll or a Form 42/ERS return carried it
Distribution from a US LLC or partnership	No — K-1 income is not exchanged automatically	Very weak
Non-US accounts (EU, Channel Islands, Switzerland, Singapore)	Yes — full CRS dataset including balances	Strongest of all. CRS is far richer than the US feed

The practical conclusion is uncomfortable but useful: the carve-out is selective. A UK resident American with a Schwab account may well defeat the 12-year window on the dividend and interest element while remaining exposed on the capital gains element of the very same account. A disclosure that treats the account as a single monolithic exposure gives away the argument for free. A disclosure that segments the account by income stream and matches each stream against what HMRC could actually see does not.

Timing matters equally. Exchange under these arrangements happens on an annual cycle, typically by 30 September following the end of the reporting year. So for a given tax year you can usually place HMRC's receipt of the data within a defined window, and then ask whether four or six years remained on the clock afterwards. In most 2015-16 to 2019-20 cases involving pre-existing accounts, they did.

How long does each authority actually have?

Clients consistently assume the two systems mirror each other. They do not, and the mismatch drives the sequencing of any dual catch-up.

Position	UK / HMRC	US / IRS
Return filed, reasonable care taken	4 years from end of tax year	3 years from filing under IRC s.6501
Careless / substantial omission	6 years	6 years where gross income omitted exceeds 25%, or where omitted foreign income exceeds the statutory threshold
Offshore matter, any behaviour (2015-16 onwards)	12 years under TMA 1970 s.36A	No direct equivalent — the US uses information-return triggers instead
Deliberate conduct	20 years	Unlimited where the return is fraudulent
No return filed at all	20 years for failure to notify	Unlimited — the period never starts
Missing information return (8938, 5471, 3520)	Not applicable as a separate trigger	Under IRC s.6501(c)(8) the period for the whole return can stay open until three years after the missing form is filed
Statutory route out of arrears	Worldwide Disclosure Facility	Streamlined Filing Compliance Procedures

Note the asymmetry in the bottom rows. On the US side an unfiled Form 8938 or 5471 can hold an entire year open indefinitely — there is no carve-out equivalent to s.36A's. That is why the UK argument, where it succeeds, rarely shortens the US work. The two exercises need to be run in parallel by people who understand both, which is what our US-UK cross-border accountants do.

How do you prove HMRC had the information in time?

An argument you cannot evidence is a conversation, not a defence. Three practical steps.

1. Make a subject access request to HMRC

Under UK data protection law you are entitled to the personal data HMRC holds about you, which includes automatic exchange of information records received from overseas authorities. A properly framed request — specifying the AEOI, CRS and FATCA reciprocal data held, and the dates of receipt — is the single most productive document-gathering step in a s.36A case. It converts an assertion into a dated record.

2. Reconstruct the timeline against the ordinary limits

For each tax year in scope, build a simple chronology: end of tax year; date HMRC received the relevant overseas data; expiry of the four-year limit; expiry of the six-year limit. The carve-out bites where receipt sits comfortably before the applicable ordinary expiry. Where receipt is close to the line, expect argument about whether it was reasonable to expect HMRC to assess in the time remaining.

3. Establish that the data was sufficient, not merely present

The statutory test is that the information reasonably enabled HMRC to become aware of and assess the lost tax. A record showing an account exists is weaker than a record showing identified income of a stated amount attributed to a named UK-resident taxpayer with a UK identifier. This is precisely why the income-stream segmentation in the table above matters, and why the argument is stronger on dividends than on gains.

Who bears the burden is itself contested. HMRC must establish that a discovery assessment is validly made and that the conditions for an extended time limit are satisfied. In practice, once a taxpayer raises the carve-out with a credible evidential foundation, HMRC is required to engage with it rather than simply assert twelve years.

What the carve-out will not do for you

We are candid with clients about the limits, because an overstated argument damages a disclosure.

• It does not cancel the tax. Four or six years of liability, interest and penalties remain.
• It does not apply to deliberate behaviour. Where HMRC establishes deliberate conduct, the 20-year limit applies and s.36A is irrelevant. The carve-out is a shield against the offshore extension, not against dishonesty findings.
• It does not help where no return was filed at all in the ordinary case, because failure to notify carries its own 20-year exposure. The carve-out is most potent where returns were filed but foreign income was omitted or understated.
• It does not travel to corporation tax or VAT. The 12-year limit, and therefore its carve-out, is confined to income tax, CGT and IHT.
• It sits alongside, not instead of, other regimes. Separate correction obligations for older years have their own architecture and their own penalty consequences, and the offshore time limit legislation did not displace them.

Running the UK carve-out and a US disclosure together

Nearly every client who needs this argument also has a US problem. The interaction requires care, and getting it wrong is expensive in both directions.

The behavioural vocabularies do not map. HMRC asks whether you were careless or deliberate. The IRS asks whether you were non-wilful. A UK disclosure that concedes carelessness to secure a lower penalty band can be quoted back at you in a Streamlined Filing non-wilfulness certification, and a US certification drafted without regard to HMRC's language can undercut a UK reasonable-care position that would have delivered a four-year rather than six-year outcome under the carve-out. These documents must be drafted as a single set.

Foreign tax credit relief also moves. If the carve-out reduces the UK years from twelve to four, the UK tax available to credit against US liability in the closed years changes, and any US amended returns claiming credit for UK tax on those years need to reflect the final agreed UK position. Filing the US side first and the UK side second, without modelling the carve-out, routinely produces credits that later have to be unwound.

A practical sequence we run

• Weeks 1-2. Full asset and income mapping across both jurisdictions. Identify which items are offshore matters for s.36A and which income streams within each account are visible to HMRC.
• Weeks 2-4. Subject access request to HMRC for AEOI records. Simultaneously assemble US account statements and any correspondence already received.
• Weeks 4-8. Build the year-by-year chronology, quantify the exposure on both a 12-year and a carve-out basis, and model the delta. This number decides strategy.
• Weeks 8-12. Draft the UK disclosure with the carve-out argued explicitly and evidenced, and prepare the US filings in parallel with consistent factual narratives.
• Ongoing. Manage HMRC's response to the carve-out. Expect resistance on the first exchange; expect movement where the chronology is documented.

Fact patterns where this argument is worth real money

The delta between a twelve-year and a four-year settlement is frequently six figures for our clients. The cases where we see the carve-out do most work:

• A long-term UK-resident American with a legacy US brokerage account generating substantial dividend income, who filed UK returns but omitted the US portfolio in the belief that US tax had settled the matter.
• An accidental American or dual national with European and Channel Islands accounts fully reported under CRS — where the data HMRC holds is comprehensive, the carve-out is at its strongest.
• A UK-resident founder with a US bank account holding sale proceeds, where interest was reported to HMRC years before the enquiry opened.
• Executives who moved from New York to London and left a 401(k), IRA and taxable account behind, and whose UK returns understated the foreign investment income.

Each of these is a case where a competent generalist would prepare twelve years of computations without ever asking what HMRC already knew. Our private client team asks that question first. You can find our wider library of cross-border compliance material in our guides.

The point

Section 36A was justified to Parliament on the basis that offshore non-compliance takes longer to detect. The carve-out is the statutory acknowledgement that where detection was not in fact difficult — because a foreign authority handed HMRC the information — the extension is not warranted. In an era of near-universal automatic exchange, that acknowledgement has far more purchase than the drafting probably anticipated. It is, in effect, a sunset clause that most taxpayers never invoke.

If you have missed UK tax returns with a US or other offshore dimension, the analysis should begin with what HMRC already holds, not with twelve years of spreadsheets. Every case is different, the evidential threshold is real, and the argument must be built properly to be taken seriously. Contact our cross-border team for a confidential, privileged consultation. We will tell you honestly whether the carve-out is available to you, what it is worth, and how to present it — before you file anything that forecloses it.