Specialist US UK Tax Services: What Your Accountant Missed

If your UK Self Assessment has been filed on time for years but nobody has ever asked you for your bank balances, your ISA fund holdings or your shareholding in a UK limited company, you almost certainly have an unfiled US position behind a clean UK one. This guide sets out the specific triggers, the forms usually missed, and the route back.

That situation is not a failure of your UK accountant so much as a failure of scope. A UK practice is engaged to file a UK return. It does that competently. What it is not engaged to do — and in most cases is not licensed, insured or trained to do — is prepare the parallel US filings that a US citizen or green card holder owes every year regardless of where they live. Specialist US UK tax services exist precisely for that gap, and at Jungle Tax it is the single most common reason a new client arrives with a decade of immaculate UK paperwork and nothing at all on the US side.

Why does a faultless UK Self Assessment record tell you nothing about your US position?

The United States taxes on citizenship. The United Kingdom taxes on residence. Those are not two versions of the same rule; they are two entirely independent systems that happen to apply to the same person at the same time. A UK accountant discharging their engagement letter perfectly will produce a correct SA100, claim the right reliefs, and never once touch the reporting layer that sits on top of it for an American.

The reporting layer is the problem. Most of the US filings that go missing are information returns, not tax returns. They frequently produce no tax at all. They still carry penalties measured in tens of thousands of dollars per form per year, and — the point almost nobody appreciates — several of them hold the assessment window open indefinitely until they are filed. A year that would otherwise have closed after three years never closes.

The UK side compounds it. HMRC receives account data on you from UK financial institutions and exchanges it with the IRS under the FATCA intergovernmental agreement. Your UK bank has almost certainly already asked you to confirm your US status on a self-certification form. If you ticked the box honestly, the IRS has a data feed on accounts you have never reported. The information asymmetry that protected people fifteen years ago no longer exists.

The triggers: how to tell a UK-only practice has left a US catch-up behind it

These are the practical signals we see repeatedly in first consultations. Any one of them is enough to warrant a review; three or more and you should assume a catch-up is required.

• Nobody has ever asked for your maximum account balances. An adviser preparing your FBAR needs the highest balance in every foreign account during the year, converted at the Treasury year-end rate. If that question has never been asked, no FBAR has been filed.
• You hold a stocks and shares ISA and nobody has mentioned Form 8621. This is the clearest single tell. A UK adviser sees a tax-free wrapper; a US adviser sees a stack of passive foreign investment companies.
• You are a director and shareholder of a UK limited company and have never signed a Form 5471. Your accountant filed the CT600 and the Companies House accounts. Neither of those is a US filing.
• Your engagement letter says “preparation of UK Self Assessment tax return” and nothing else. Read it. Scope is usually stated explicitly, and US filings are usually excluded explicitly.
• You have a workplace pension or SIPP and nobody has discussed treaty positions with you. UK pensions are reportable and the treaty treatment needs to be claimed, not assumed.
• Your UK bank or platform sent you a FATCA self-certification and your adviser did not react to it. That letter is a notification that your data is being reported to the IRS via HMRC.
• You were born in the US, left as a child, and have never filed anything. The accidental American profile. UK-only advisers rarely spot it because your passport and your accent are British.
• You received UK employer share awards, RSUs or an EMI option exercise. These require sourcing and apportionment across both systems, and the UK payroll treatment does not carry over.
• Your fee has been modest and unchanged for years. Genuine dual filing is materially more work than a UK return. A flat, low fee is a strong indicator that only one country is being served.

US versus UK: what each system actually requires of you

Issue	United Kingdom / HMRC	United States / IRS
Basis of taxation	Residence (and, since April 2025, a four-year foreign income and gains regime for new arrivals)	Citizenship and lawful permanent residence, worldwide, wherever you live
Tax year	6 April to 5 April	1 January to 31 December
Filing deadline	31 January following the tax year, for online returns	15 April, with an automatic extension to 15 June for those abroad and to 15 October on request
Stocks and shares ISA	Income and gains free of UK tax	No recognition; underlying funds are usually PFICs requiring Form 8621
UK pension / SIPP	Relief on contributions, 25 per cent tax-free lump sum	Reportable; growth and lump sums require treaty analysis, not assumption
Personal company	CT600 and Companies House filings	Form 5471, controlled foreign corporation rules, and the successor to the GILTI regime
Bank accounts	No standalone reporting by the individual	FBAR (FinCEN Form 114) and, above higher thresholds, Form 8938
Main residence gain	Private residence relief, commonly full exemption	Limited exclusion only; sterling mortgage movements can create phantom gain
Penalty for a nil-tax omission	Generally none if no tax is due	Fixed penalties per form per year, irrespective of tax due

Form by form: what typically went unfiled

FBAR — FinCEN Form 114

Required if the aggregate maximum value of your foreign financial accounts exceeded 10,000 US dollars at any point in the calendar year. Aggregate is the trap: five current accounts, a cash ISA, a joint account with your spouse and a dormant savings account will clear the threshold easily even though no single account does. Signature authority over an employer or family account counts too. Detail on scope is published by the IRS on its FBAR guidance page.

Form 8938 — statement of specified foreign financial assets

The FATCA individual return. Thresholds are higher for those living abroad than for US residents, and the asset class is broader than the FBAR: it reaches unlisted shares, certain pension interests and interests in foreign entities that never appear on an FBAR. The two overlap heavily but neither substitutes for the other, and both are generally required.

Form 8621 — PFIC reporting on ISA and unit trust holdings

The most under-diagnosed filing in the UK market. Almost every non-US pooled fund — OEICs, unit trusts, investment trusts, most ETFs listed in London — is a passive foreign investment company for US purposes. A single stocks and shares ISA holding a diversified multi-asset portfolio can generate a dozen separate Forms 8621. The IRS explains who must file on its Form 8621 page.

Form 5471 — your UK limited company

If you are a US person owning or controlling a UK company, you are almost certainly in one of the Form 5471 filer categories. The form requires the company's accounts to be restated on a US basis, with earnings and profits tracked in dollars. Where the company is a controlled foreign corporation, undistributed profits may be taxed to you personally in the year they arise, before any dividend is ever paid. Recent US legislation has renamed and recalibrated that regime with effect from 2026, which makes an accurate historic reconstruction more important, not less.

Forms 8858 and 3520 — the ones nobody expects

Form 8858 can be triggered by a UK sole trade, a foreign branch or a member interest in a UK LLP. Forms 3520 and 3520-A can be triggered by certain non-US trusts and by large gifts or inheritances from non-US persons. A gift from a British parent creates no UK tax for you and may still create a US reporting obligation. UK advisers almost never flag it because, from where they sit, nothing taxable happened.

Why is the ISA the single most expensive blind spot?

Because HMRC-approved and IRS-approved are unrelated concepts. A fund can carry HMRC reporting fund status and still be a punitive PFIC to the IRS. Under the default excess distribution regime, gains are thrown back across your holding period, taxed at the highest ordinary rate rather than long-term capital gains rates, and an interest charge is added for each year of deferral. The tax-free wrapper your UK adviser correctly recommended can, on the US side, produce an effective rate materially above what an ordinary taxable US brokerage account would have suffered.

Two elections — qualified electing fund and mark-to-market — can improve the outcome, but the QEF election generally requires the fund to supply a PFIC annual information statement, which most UK retail funds do not produce, and elections are far more effective made prospectively than reconstructed a decade late. This is why the cost of delay here is real rather than theoretical, and why we treat ISA and general investment account holdings as the first item in any cross-border compliance review.

Why the statute of limitations may never have started running

Most taxpayers assume that old years eventually go quiet. For a US person with unfiled international information returns, they may not. Where a required information return has not been filed, the assessment period for the return can remain open until the information is supplied, and where the failure was not due to reasonable cause the extension can reach the entire return rather than only the offending item. A 2014 year with an unfiled Form 8621 attached to it is not a closed year. It is an open one that has been open for over a decade.

This is the reason a “quiet disclosure” — simply posting several years of late returns and hoping — is a poor strategy for a high-net-worth filer. It forfeits penalty protection, it is visibly a late-filed cluster, and it leaves every one of those years open.

How does the IRS Streamlined Foreign Offshore Procedure fix this?

For non-willful failures — which describes the overwhelming majority of people whose UK accountant simply never mentioned the US side — the Streamlined Foreign Offshore Procedure is the intended route. The IRS sets out eligibility on its streamlined filing compliance procedures page. In outline:

• Three years of delinquent or amended federal income tax returns, for the most recent years whose due dates have passed.
• Six years of FBARs, filed electronically with the streamlined reference.
• Form 14653, a signed certification of non-willfulness that must set out, in narrative, the specific facts of why the filings were missed.
• Full payment of any tax and statutory interest due on the three years.
• No miscellaneous offshore penalty for those who meet the foreign residency test — the equivalent domestic programme carries a percentage-based penalty on the asset base, which is why residency evidence matters.

Do you meet the non-residency test?

For a US citizen or green card holder, the test is broadly that in at least one of the three most recent years you had no US abode and were physically outside the United States for at least 330 full days. An American living in London year-round meets it comfortably; someone who spends five months a year in New York may not, and the difference between the two programmes is the difference between a nil penalty and a percentage of your worldwide financial assets. We work this out before anything is filed, and our streamlined filing team treats it as the gating question.

The certification is the document that matters

Form 14653 is not a tick-box. It is a sworn narrative, and the IRS reads it. It must explain who advised you, what you were told, what you did and did not know, and why. “My accountant never mentioned it” is a true and often persuasive fact pattern — but only if it is documented with engagement letters, correspondence and a coherent timeline. A weak certification is the most common reason an otherwise clean submission draws follow-up.

What if you did file US returns, but they were wrong?

A meaningful minority of clients have filed something — a bare Form 1040 prepared from a UK payslip, with a foreign earned income exclusion claimed and nothing else. No FBAR, no 8938, no 8621, no 5471. Streamlined is still available, because the procedures accommodate amended returns as well as delinquent ones. Where income was correctly reported and only information returns are missing, the delinquent international information return route may be more proportionate. Choosing between them is a judgement call about risk, cost and the strength of the reasonable-cause story, and it should be made before a single form is submitted.

The UK side is not static either

Two UK changes have altered the cross-border arithmetic for exactly this client group. From April 2025 the remittance basis for non-domiciled individuals was replaced by a residence-based four-year foreign income and gains regime, and inheritance tax exposure moved to a long-term residence test rather than domicile. For an American in London the practical consequence is that more foreign income now falls into the UK net sooner, which changes the foreign tax credit position on the US return — sometimes favourably, because more UK tax is available to credit, and sometimes not, because the timing of UK and US taxation of the same item diverges across the 6 April to 31 December offset. HMRC's overview of the rules for taxing foreign income sits on GOV.UK. Reconstructing old years correctly means applying the rules that were in force then, not the rules in force now — a distinction generic catch-up providers routinely get wrong.

Sequencing: the order this work has to be done in

1. Establish status and exposure. Citizenship or green card, years at risk, residency test, and a full asset inventory including accounts you consider trivial.
2. Reconstruct the data. Six years of maximum balances, fund-level ISA and GIA holdings, company accounts, pension statements, share award histories.
3. Decide the programme. Streamlined foreign, streamlined domestic, delinquent information returns, or, in genuinely willful cases, a different route entirely with legal privilege in place from the outset.
4. Model the tax before you file. Foreign tax credits by basket, PFIC elections, treaty positions on pensions. Most clients owe far less than they fear; some owe more than they expect because of PFIC interest charges.
5. Draft the certification. Written alongside the returns, not after them, so the narrative and the numbers agree.
6. Align the UK filings. Amend Self Assessment where the reconstruction changes the UK position, and set up a going-forward calendar that runs both cycles in one workflow.

A worked illustration

An American executive, resident in London for eleven years, UK Self Assessment filed on time every year by a respected West End practice. Assets: two current accounts, a cash ISA, a stocks and shares ISA of roughly £180,000 across nine funds, a SIPP, a workplace pension, and 100 per cent of a consultancy limited company. UK position: entirely clean. US position: eleven years of unfiled FBARs, eleven years of Form 8938, nine PFICs per year on Form 8621, eleven years of Form 5471, and eleven unfiled Forms 1040.

Under streamlined, the filing set collapses to three years of returns and six years of FBARs. The actual US tax, after foreign tax credits for UK tax already paid, was modest — the material cost sat in the PFIC calculation on the ISA and in reconstructing the company's earnings and profits. Total exposure resolved at a fraction of the penalty position that would have applied had the IRS made contact first, which is the point: eligibility for the programme ends the moment the IRS opens an examination. You can use our FBAR penalty calculator to see what the alternative arithmetic looks like.

What a specialist engagement should look like

Ask any prospective adviser three questions. Who signs the US return, and are they an enrolled agent or CPA? Do you prepare the UK return and the US return in the same workflow, or do you subcontract one of them? And how many Forms 8621 and 5471 did your practice file last year? The answers separate genuine dual-capability firms from UK practices with a US referral partner and US practices with no feel for a SIPP.

A proper engagement covers both returns, the full information-return set, the treaty analysis, and a forward calendar reconciling a 5 April year-end to a 31 December one. For clients with substantial portfolios, corporate interests or trust exposure, it should also sit alongside specialist private client support rather than being treated as a compliance afterthought.

Act before the correspondence arrives

Streamlined relief is voluntary-disclosure relief. It is available while the initiative is still yours and it disappears once the IRS contacts you — and with FATCA data flowing from UK institutions through HMRC, the interval between the two is shortening. If you recognise three or more of the triggers above, the sensible next step is a scoped review rather than a decade of assumptions. Contact our cross-border team for a confidential, without-obligation assessment of your US position. We will tell you plainly whether there is a gap, how large it is, and what it costs to close — before anything is filed and before anyone else is told.