Missed US Tax Returns: 2026 Deadlines for London Expats

Missed US tax returns are governed in 2026 by three dates, not one: 15 June, the automatic filing deadline for Americans whose tax home is abroad; 15 October, the extended deadline and the last practical FBAR date; and the three-year refund window, which closes silently on your oldest open year and decides which back years still carry money rather than mere compliance.

Most published deadline calendars are written for the American abroad who files every year and simply wants to know when to press submit. That is not your situation. If you have two, five or nine unfiled years sitting behind you, the 2026 calendar functions differently: some dates control what you owe, one date controls what you can still recover, and one clock — the one the IRS runs against you — has never started at all. This guide is the date map for a catch-up, written for Americans in London who need to know which years are worth filing first and which are simply housekeeping.

The three clocks running in a catch-up — and only one runs in your favour

Before any date matters, understand the structure. A delinquent US filing position is governed by three separate limitation periods, and they do not move together.

• The assessment clock (against you). Under the Internal Revenue Code, the IRS generally has three years from the date a return is filed to assess additional tax. Where no return was ever filed, that period never begins. An unfiled 2011 return is as open in 2026 as an unfiled 2025 return. Time does not cure a missing return; only filing it does.
• The refund clock (in your favour). A claim for credit or refund must generally be made within three years of filing the return, or two years of paying the tax, whichever expires later — and the amount recoverable is capped by a look-back period. For a never-filed year, the return itself is the claim, so the three-year window runs from the original due date. This is the only clock that expires, and it expires against you.
• The information-return clock. Where a required international information return — Form 5471, 3520, 3520-A, 8938 and others — was not filed, the assessment period for the entire return can remain open until three years after the missing form is finally submitted. One omitted foreign trust or controlled foreign corporation form can hold an otherwise closed year open indefinitely.

The practical consequence for a London-based American is blunt. Filing late costs you nothing in terms of exposure you have not already incurred, because the exposure is permanent until you file. But every month of delay can permanently destroy a refund you were entitled to. The asymmetry runs one way, and it argues for moving now.

The 2026 date map: every deadline that touches an American in London

The following are the federal dates that matter in calendar year 2026. Where a date falls on a weekend or a District of Columbia holiday it shifts to the next business day — a detail that occasionally rescues a refund claim by a day or two, and which should be confirmed year by year rather than assumed.

2026 date	What it governs	Why it matters in a catch-up
15 April 2026	Statutory due date for the 2025 Form 1040; date any 2025 balance must be paid; nominal FBAR due date	Interest on unpaid tax runs from here regardless of any filing extension. It is also the anchor date from which the refund look-back is measured for prior years.
15 June 2026	Automatic two-month extension for taxpayers whose tax home and abode are outside the United States	Applies by regulation, not by application — you do not file a form to claim it, you attach a statement to the return. For most Americans in London this, not 15 April, is the real 2025 filing date.
15 June 2026	Last day to file Form 4868 for the 2025 year	An expat must request the further extension by the extended June date, not by 15 April. Miss this and the October date is not available.
15 October 2026	Extended 2025 filing deadline; final FBAR date for calendar year 2025	The last date on which 2025 can be filed without a failure-to-file penalty running. Also the practical outer edge of the 2022 refund window for many filers.
15 December 2026	Discretionary further two-month extension, requested by letter	Granted at the discretion of the IRS for taxpayers abroad. Useful where a UK information source — a partnership statement, an offshore fund report — is genuinely unavailable.
Rolling	Form 2350 extension for those establishing bona fide residence	Relevant only where you need additional time to satisfy a foreign earned income exclusion qualification test. Rarely the right tool for a multi-year catch-up.

The IRS sets out the automatic extension rules for taxpayers abroad in its international taxpayer guidance and confirms the mechanics on its official Form 4868 page. It is worth reading the primary source rather than relying on a secondary calendar, because the interaction between the automatic expat extension and the further extension is exactly where self-filers go wrong.

Why 15 June 2026 is not really your deadline if you are behind

If you have unfiled years, the 2025 return is not a standalone obligation — it is the top layer of a stack. Filing 2025 in isolation, cleanly and on time, while 2019 through 2024 remain missing creates a specific and avoidable problem: it puts a compliant, signed return in front of the IRS carrying a UK address and foreign financial account boxes ticked, with no explanation of the years behind it. That is the classic quiet-disclosure posture, and it forfeits the penalty protection a properly structured catch-up would have provided.

The correct sequencing is almost always to determine the shape of the whole catch-up first — how many years, which forms, whether the conduct was non-wilful — and then to file the current year inside that structure rather than ahead of it. Where a formal programme applies, the current year and the back years are prepared together and submitted as a coherent package. Our streamlined filing specialists build the year map before a single return is drafted, precisely because the order of filing changes the penalty outcome.

What is the three-year refund window on missed US tax returns?

This is the date nobody puts on a calendar, and it is the one that decides whether a back year is worth money. Two rules operate together.

Rule one: the claim deadline

A claim for credit or refund must generally be filed within three years from the date the return was filed, or two years from the date the tax was paid, whichever is later. For a year you never filed there is no filing date to run from — the delinquent return is itself the refund claim, so the operative period runs from the original due date of that return.

Rule two: the look-back cap

Even a timely claim recovers only tax paid within a defined look-back period: three years immediately preceding the claim, plus the period of any extension of time to file. This matters enormously for expats, because US tax withheld at source and estimated payments are treated as paid on the original due date of the return — not on the date you actually parted with the money. So withholding on a 2022 US-source distribution is deemed paid in April 2023, and the look-back must reach back that far for you to recover it.

Which year is at the cliff edge right now?

Work it through for a reader in August 2026. Tax year 2022 was due in April 2023. Three years from that date has already passed. Whether anything remains recoverable turns on whether the look-back can be extended by the period of any extension — the two-month automatic expat extension, and the further six months where a Form 4868 was actually filed. For an American in London who did file a Form 4868 for 2022 and then never filed the return, the outer edge falls around mid-October 2026. For one who relied only on the automatic expat extension it falls considerably earlier. For one who did neither, 2022 is very likely already closed.

The planning point is unambiguous. If 2022 shows a refund position — over-withheld US-source dividends, a refundable child credit, excess estimated payments — that year requires analysis this month, not this quarter. Tax year 2023 sits one year behind on the same conveyor and expires through 2027. Every year older than 2022 is, in refund terms, almost certainly gone; those returns are filed for compliance and to start the assessment clock, not for cash.

The exception nobody mentions: the ten-year foreign tax credit window

Here is where generalist US expat pages consistently under-serve a London reader. Where a refund claim is attributable to foreign taxes paid or accrued — which, for an American paying HMRC on UK employment, self-employment, rental or investment income, is very often the whole substance of the claim — a substantially longer special limitation period applies, running approximately ten years rather than three from the due date of the return for the year the foreign taxes related to.

The practical effect is that two apparently identical back years can have completely different economics. A 2019 year whose refund arises from over-withholding is dead. A 2019 year whose position turns on a foreign tax credit for UK income tax paid may still be live. This distinction is routinely missed, and it is the single most valuable piece of analysis in a UK-based catch-up. It also interacts with the choice between the foreign earned income exclusion and the foreign tax credit — a choice which, made correctly on a delinquent return, can convert a compliance exercise into a recovery. Our cross-border team models both elections across every open year before filing, because the election is difficult to unwind once made.

Which three years does a streamlined submission cover in 2026?

The Streamlined Foreign Offshore Procedures remain the principal route for a non-wilful American abroad with missed US tax returns. The submission covers the three most recent years for which the US return due date — or properly extended due date — has passed, together with six years of FBARs and a signed certification of non-wilfulness on Form 14653. For a qualifying taxpayer resident outside the United States, the miscellaneous offshore penalty is nil.

The date sensitivity is real and largely unremarked. Because the covered years are defined by reference to due dates that have passed, the composition of a 2026 submission changes during the year:

• A package assembled early in 2026, before the 2025 due date passed, would generally cover 2022, 2023 and 2024.
• A package assembled after the automatic June 2026 date has passed generally covers 2023, 2024 and 2025.
• Where a valid Form 4868 extension is in place for 2025, the extended October date can keep 2022 in the frame longer — which may be desirable or undesirable depending on where the tax sits.

That is not a technicality. It determines which year’s income, which year’s UK tax and which year’s foreign tax credit position lands inside the protected package. A year containing a large one-off UK event — a share option exercise, a property disposal, a bonus — is materially better inside the streamlined three than outside it. The IRS publishes the eligibility framework on its streamlined filing compliance procedures page, including the non-residency test and the bar on participating once an examination has begun.

Two eligibility points deserve emphasis for London residents. First, the non-residency test requires that in at least one of the three covered years you had no abode in the United States and were physically outside the country for at least 330 full days. Frequent transatlantic travellers holding a retained US property should have this tested, not assumed. Second, eligibility is lost the moment the IRS initiates a civil examination or criminal investigation — which is precisely why the arrival of an IRS letter, or a FATCA notification from a UK bank, converts a merely urgent matter into an immediate one.

The FBAR position: six years, 15 October, and the delinquent route

FBAR is a separate filing to FinCEN, not part of the Form 1040, and it follows its own calendar. The 2025 report is nominally due 15 April 2026 with an automatic extension to 15 October 2026 — an extension granted without application. The reporting threshold is an aggregate exceeding $10,000 across all foreign accounts at any point in the year, tested on the highest balance rather than the year-end balance.

For a London-based American that threshold is met with almost trivial ease: a current account, an ISA, an arrangement over which you hold signature authority, a joint account with a UK spouse and a Premium Bonds holding will frequently aggregate past $10,000 on a single day. Aggregation, not individual account size, is the test — a point on which even sophisticated clients are routinely caught out. The IRS sets out the requirement and the automatic extension on its FBAR guidance page.

Where FBARs are missing, the route depends on the underlying facts. If the associated income was properly reported and tax paid, the delinquent FBAR submission procedures may allow the reports to be filed late with a reason statement. If income was not reported, the FBARs belong inside a streamlined package covering six years. Choosing the wrong route is expensive: FBAR penalties are assessed per report and, in wilful cases, can reach a substantial proportion of account balances. Modelling the downside before choosing a route is essential, and our FBAR penalty calculator gives a first-order indication of the exposure at stake.

How the UK calendar collides with the US one

An American in London runs two tax systems on non-aligned years, and a catch-up has to reconcile them. The UK tax year ends 5 April; the US year ends 31 December. Every foreign tax credit claim therefore requires apportionment of UK tax across two US years — the single most common source of error in self-prepared delinquent returns.

Feature	United States (IRS)	United Kingdom (HMRC)
Tax year	1 January – 31 December	6 April – 5 April
Basis of taxation	Citizenship — worldwide income, wherever you live	Residence, under the residence-based regime that replaced the domicile and remittance basis from April 2025
Filing deadline, current cycle	15 April 2026, automatically extended to 15 June 2026 for those abroad, then 15 October 2026 on request	31 October 2026 for paper; 31 January 2027 for online filing of the 2025/26 return
Payment deadline	15 April 2026 — interest runs from this date irrespective of extensions	31 January 2027, with a second payment on account due 31 July
Late-filing penalty structure	A monthly failure-to-file charge on unpaid tax up to a capped maximum, plus a separate failure-to-pay charge; a minimum charge applies where a return is more than 60 days late	Fixed £100 immediately, daily charges after three months, then tax-geared penalties at six and twelve months
Refund window	Approximately three years from the original due date, extended for foreign-tax-credit claims	Overpayment relief claims generally within four years of the end of the tax year
Assessment window where no return filed	Open indefinitely	Discovery assessment windows extending to twenty years in cases of deliberate behaviour

HMRC publishes the Self Assessment dates on gov.uk. Note the asymmetry in the bottom two rows: the US refund window is short and the US assessment window is infinite, whereas the UK positions are the reverse. A dual catch-up therefore carries different urgency on each side. On the US side you are racing a refund deadline; on the UK side you are managing a penalty and a behaviour classification. Where UK returns are also outstanding, the Digital Disclosure Service is usually the correct vehicle, and the behaviour classification adopted there must be consistent with the non-wilfulness certification signed for the IRS. Inconsistent characterisations across two disclosures are indefensible if either authority looks closely. Our UK tax team handles the HMRC side of dual catch-ups in step with the US filing.

What it actually costs to be late

The headline US penalties are a failure-to-file charge accruing monthly on unpaid tax up to a maximum, a smaller failure-to-pay charge accruing over a longer period, and interest running from the original due date. The decisive fact for most Americans in London is that these charges are calculated on unpaid tax — and after the foreign earned income exclusion and a foreign tax credit for UK income tax paid at UK rates, a great many delinquent years carry no US tax at all.

The real exposure in a London catch-up is rarely income tax. It is the information-return penalties: fixed charges per unfiled Form 5471 for a UK company you control, per unfiled Form 3520 for a distribution from a UK trust or certain UK pension arrangements, per unfiled Form 8938, and FBAR penalties per report. These are not proportionate to tax owed. They apply whether or not any tax was due, which is why a client who owes nothing can nonetheless face a substantial theoretical exposure — and why the penalty protection inside a properly filed streamlined package is the substantive prize, not the return preparation itself.

One further point specific to delinquent filers: the foreign earned income exclusion is elected on the return. Where a return is filed late the election is generally still available, provided it is made before the IRS discovers the failure to file, or where no US tax is due after applying the exclusion. Filing before contact from the IRS preserves the election. Waiting until a letter arrives can cost it — and losing the exclusion on a high-earning London year is a materially expensive outcome.

The sequence that works

• Build the year map first. List every year from the point of first filing obligation. For each, record US filing status, UK income, UK tax paid, foreign accounts and highest balances, and any entity or trust interest. This map, not the calendar, drives everything.
• Test each year for refund life. Separate the years into three buckets — refund still recoverable on the ordinary three-year rule, refund potentially recoverable under the extended foreign-tax-credit period, and compliance-only. Prioritise the first bucket immediately.
• Fix the streamlined three. Determine, by reference to the 2026 due dates, exactly which three years the package will cover, and confirm the non-residency test is satisfied in at least one of them.
• Reconstruct the UK data. Obtain P60s, P11Ds, HMRC Self Assessment statements and account histories across the six FBAR years. UK banks retain limited history, so start requests early — this is almost always the critical path.
• Model the elections. Run exclusion versus credit across every open year before filing anything. The result differs year to year and the choice is sticky.
• File as one coherent package. Returns, FBARs, information returns and certification together — not the current year first, and never a silent amendment.

The mistakes that cost the most

• Filing the current year alone to show good faith. It shows the opposite once the back years surface, and it undermines the framing of any later streamlined submission.
• Assuming no tax means no obligation. The filing obligation is independent of liability. A year with zero US tax and an unfiled Form 5471 is a serious exposure.
• Treating an ISA as tax-free. A UK ISA is a UK wrapper with no US recognition. The underlying funds are frequently passive foreign investment companies requiring separate reporting, and the growth is US-taxable.
• Letting the oldest refund year lapse while gathering documents. A protective filing on the cliff-edge year can preserve the claim while the rest of the package is built.
• Waiting for a UK bank’s FATCA letter. UK financial institutions report US-indicia accounts automatically. Arrival of that letter narrows your options; it does not start the clock.

Speak to us before the next date passes

Jungle Tax prepares US and UK returns for founders, executives, investors and private clients across London and the United States. We do not sell structures or investment advice; we prepare and file the returns, correctly and discreetly, and we build the year map that tells you which of your missed years still carry money and which are simply compliance. If any part of your position is unfiled — a return, an FBAR, a Form 8938, a 5471 — the analysis is far better done before 15 October 2026 than after it. Contact our cross-border team for a confidential conversation about your position. Nothing is filed and no disclosure is made until you have seen the full picture and decided how to proceed. You may also wish to review our wider cross-border guides and the way we work with high-net-worth clients.