US Tax Preparation for American Expats: London Decision Guide
When does US tax preparation for American expats in London outgrow software? A decision framework for PFICs, share awards and treaty claims. Talk to us.
When software stops being enough
Most Americans in London can file a simple US return with software. You cannot, once your affairs touch UK pooled funds, closely held UK company shares, employer equity, UK rental property or a treaty position. Those five triggers move US tax preparation for American expats out of consumer packages and into specialist preparation, because the required forms and elections simply are not produced by them.
This guide, written by the cross-border team at Jungle Tax, is a decision framework rather than a sales pitch. It is aimed at the American in London earning well into six figures, holding a portfolio, sitting on employer equity, perhaps owning a slice of a UK company, and quietly wondering whether the return they filed last April was actually correct. In our experience of reviewing prior-year returns for incoming clients, a meaningful proportion were not.
The short answer: three questions that settle it
Before the detail, run these three questions. If you answer yes to any one of them, your return is no longer a software return.
- Does anything you own pool other people's money? A UK unit trust, OEIC, investment trust, UK or Irish-domiciled ETF, or a stocks and shares ISA holding any of those. If yes, you are in the passive foreign investment company (PFIC) regime.
- Do you own 10% or more of a company that is not American? Including a UK personal service company, a family trading company, a property SPV, or a stake in a private business. If yes, you are in the controlled foreign corporation regime.
- Are you relying on the US-UK treaty for anything? Pension growth, a resourcing position, a residence tie-breaker, a social security exemption. If yes, you may need a disclosed treaty position, and consumer software will not draft one.
Everything that follows explains why each trigger is genuinely disqualifying, what the correct treatment looks like, and what it costs to get it wrong.
Why is a London American's US return different from a domestic 1040?
Two structural mismatches sit underneath everything. Neither is a matter of opinion, and neither is handled by software designed for domestic filers.
The tax years do not line up
The US tax year ends 31 December. The UK tax year ends 5 April. Every foreign tax credit claim therefore requires you to take a UK liability computed over one twelve-month window and apportion it to a different twelve-month window, then defend that apportionment. Do it crudely and you either lose credit you were entitled to or claim credit you cannot support. The problem compounds when income is lumpy: a March bonus, an April share vest, a completion payment. UK residence itself is determined by the Statutory Residence Test, which HMRC summarises in its residence guidance, and that determination drives which UK tax there is to credit in the first place.
The two systems disagree about what income even is
The UK and the US do not merely tax at different rates. They recognise income at different moments, characterise it differently, and shelter it differently. An ISA is invisible to HMRC and fully visible to the IRS. A UK pension enjoys tax-free growth under UK law and may or may not enjoy it under the treaty. A growth share may be taxed at acquisition in the UK and at a different point in the US. Software assumes one definition of income. Cross-border reality has two.
Trigger one: do you hold UK funds, investment trusts or a stocks and shares ISA?
This is the single most common reason a London American's return is wrong, and it is almost always invisible to the filer because the UK side looks so clean.
Nearly every pooled UK investment vehicle is a PFIC for US purposes: unit trusts, OEICs, investment trusts, UK and Irish-domiciled ETFs, and the funds held inside a stocks and shares ISA. The PFIC regime is punitive by design. Absent a valid election, gains and "excess distributions" are thrown back across your holding period, taxed at the highest ordinary rate applicable in each of those years, and then charged an interest factor on the deferred tax. The effective rate can exceed what you would have paid had you simply held a US-listed fund, sometimes substantially.
The reporting obligation sits on Form 8621, filed per fund, per year. An ISA holding six funds can generate six separate Forms 8621. Two elections can rescue the position, but both must be made properly and, critically, in time: a qualified electing fund (QEF) election, which requires the fund to provide a PFIC annual information statement that most UK managers do not produce, or a mark-to-market election, available only where the shares are treated as marketable. Consumer software does not produce Form 8621 at all, and no software will tell you that your election window has closed.
The mirror-image trap runs the other way. UK tax treats offshore funds that lack HMRC reporting fund status as producing "offshore income gains" taxed as income rather than capital. HMRC publishes the list of approved reporting funds, updated monthly. So an American in London holding a US-listed ETF to sidestep PFIC status may be walking into an offshore income gain on the UK side. The only portfolio that is clean in both systems is one built deliberately for both systems, which is why we look at holdings before we look at forms.
Trigger two: do you own shares in a closely held UK company?
If you are a US person holding 10% or more of a UK limited company, you almost certainly have a Form 5471 obligation. That includes the founder with a trading company, the consultant operating through a personal service company, the investor in a friend's business, and in some cases the American who is merely an officer or director when other US persons hold the shares.
Three things make this dangerous. First, the penalty for non-filing starts at US$10,000 per company per year and applies whether or not any tax is due. Second, a missing Form 5471 can leave the statute of limitations on your entire return open indefinitely, so a return you thought was closed never closes. Third, a UK company owned by US persons is typically a controlled foreign corporation, which pulls in the anti-deferral regimes: Subpart F and the global intangible low-taxed income rules, redesignated as net CFC tested income (NCTI) by 2025 US legislation. Those regimes can tax you personally on profits you have not distributed and cannot access.
The planning that mitigates this, whether a section 962 election, a high-tax exception, an entity classification election, or simply timing distributions against UK dividend rates, has to be modelled across both systems in the same year. No consumer package attempts any of it. Founders and shareholders in this position should read our work on cross-border tax planning before the next accounting period closes.
Trigger three: employer share awards, RSUs, options and growth shares
Equity compensation is where high earners lose the most money to poor preparation, because the two systems tax the same award on different dates and by reference to different amounts.
The UK generally taxes RSUs as employment income at vest, with income tax and National Insurance collected through PAYE. The US also taxes at vest, but sources the income by reference to where the services were performed over the vesting period. If you were granted awards in New York and vested them in London, only part of that income is foreign-source, and only the foreign-source part supports a foreign tax credit. Get the sourcing fraction wrong and you either strand UK tax you paid or claim credit against US-source income, which the IRS will disallow.
Options are harder still. The UK and US differ on the taxable moment for unapproved options, EMI options carry UK reliefs the US does not recognise, and growth shares or hurdle shares may be taxed at acquisition in the UK under the employment-related securities rules while the US waits. Section 83(b) elections, where relevant, must be filed within 30 days of transfer and cannot be made late. Software has no concept of a UK section 431 election, and will not ask whether you signed one.
Trigger four: UK rental property
UK property looks simple and is not. The two systems compute the same rental profit differently in at least four ways, and depreciation is the one that reaches furthest into the future.
| Item | US treatment | UK treatment |
|---|---|---|
| Depreciation on the building | Required; foreign residential property depreciated over a longer recovery period under the alternative depreciation system | No depreciation relief available |
| Mortgage interest on a let residential property | Deductible against rental income | Restricted to a basic-rate tax reducer for individual landlords |
| Losses | Passive activity loss rules may suspend them | Carried forward against future UK property profits |
| Sale of the property | Gain computed after depreciation recapture; exchange movement on the mortgage can create a separate taxable gain | Capital gains tax, with private residence relief where applicable |
| Currency | Everything translated into US dollars, transaction by transaction | Sterling throughout |
The depreciation point deserves emphasis. It is not optional. If you did not claim it, the IRS still treats it as allowable on sale, so you face recapture on deductions you never took. Correcting several years of missed depreciation is generally an accounting method change rather than an amended return, a distinction no software will surface. The foreign currency gain on repaying or remortgaging a sterling loan is a second, separate item that regularly surprises clients who sold a London flat at what they believed was a modest gain.
Trigger five: pensions, treaty positions and Form 8833
Treaty analysis is the clearest dividing line between preparation and specialist preparation, because a treaty claim is a legal position you are asserting, not a checkbox you are ticking.
The US-UK treaty governs whether growth inside a UK registered pension is deferred for US purposes, how employer and employee contributions are treated, how lump sums are taxed, and how each category of income is resourced for credit purposes. Certain positions must be disclosed on Form 8833, and the disclosure has to state the article relied on, the facts, and the analysis. That is a drafting exercise. It also has to be consistent with what you filed last year and what you intend to file next year, because an inconsistent treaty position across years is precisely what draws examination.
Adjacent to this sits the totalisation agreement governing National Insurance and US social security, which decides which system you contribute to and prevents double social charges for the self-employed. Americans running a UK business through self-employment rather than a company sometimes pay into both, unnecessarily, for years.
Foreign earned income exclusion or foreign tax credit: the choice that compounds
Software will happily apply the foreign earned income exclusion because it produces a clean, low number on screen. For a well-paid London executive that is frequently the wrong answer.
The exclusion is capped, at roughly US$130,000 of earned income for the 2025 tax year, and it shelters only earned income, leaving dividends, gains, rent and much equity income exposed. More importantly, excluding income also removes the UK tax paid on that income from the credit calculation, so it generates no foreign tax credit. Because UK effective rates on high earners generally exceed US rates, the credit method typically eliminates US tax outright and banks surplus credits that carry forward for up to ten years. Those carryforwards are what shelter a future bonus, a share vest, or a property sale.
Revoking the exclusion once claimed generally locks you out for five years absent IRS consent. This is why a decision made casually in year one, by a package that asked a single question, can cost a six-figure earner materially more across a decade. The IRS overview of the exclusion sits in its international taxpayers guidance, but that guidance describes the rule; it does not model your position.
The information returns most software silently omits
Beyond the 1040 itself, an American in London with real assets typically owes a stack of information returns. These carry penalties that are not proportionate to tax due. They are flat, per form, and often per year.
- FBAR (FinCEN Form 114) — required where aggregate foreign account balances exceed US$10,000 at any point in the year, counting accounts you merely have signature authority over, such as a company account or a parent's account. Non-wilful and wilful penalties differ enormously; our FBAR penalty calculator illustrates the range.
- Form 8938 — FATCA reporting, with higher thresholds for taxpayers living abroad, commonly US$200,000 at year end or US$300,000 at any time for a single filer, and double those amounts for joint filers.
- Form 8621 — per PFIC, per year.
- Form 5471 — per foreign corporation, per year.
- Forms 3520 and 3520-A — foreign trusts and large foreign gifts. Some UK arrangements, including certain employee benefit structures and family settlements, are foreign trusts for US purposes even though nobody in the UK calls them that.
- Form 8833 — disclosed treaty-based return positions.
A decision framework you can apply this week
Score your own position honestly. This is the same triage we run on a first call.
| Your situation | Software | Generalist preparer | Cross-border specialist |
|---|---|---|---|
| PAYE salary only, one UK current account, no investments | Workable | Fine | Not required |
| Salary plus cash savings above the FBAR threshold | Marginal | Fine | Not required |
| Any UK fund, OEIC, investment trust or stocks and shares ISA | No | Risky | Required |
| RSUs, options or growth shares vesting across a US-UK move | No | Risky | Required |
| 10% or more of a UK or other non-US company | No | No | Required |
| UK rental property, especially held for several years | No | Risky | Required |
| Any disclosed treaty position or UK pension lump sum | No | No | Required |
| Missed years, missed FBARs, or an unfiled Form 5471 | No | No | Required |
A note on the middle column. A competent London chartered accountant who prepares your self assessment is not thereby qualified to prepare your 1040, and a good US CPA who has never seen a P60 is not qualified to co-ordinate the credit position. The failure mode we see most often is two competent advisers, each correct in isolation, producing an incoherent combined result: UK tax claimed in the wrong US year, income sourced inconsistently, a treaty position asserted on one side and ignored on the other. Coherence across both returns is the actual deliverable, which is why we operate as integrated US-UK tax accountants rather than as two separate desks.
What if previous years were prepared badly, or not at all?
This is the most common reason wealthy Americans in London contact us, and the position is usually more recoverable than they fear.
Where the failure to file or report was non-wilful, the IRS Streamlined Foreign Offshore Procedures generally allow you to file three years of returns and six years of FBARs with a signed non-wilfulness certification, with the miscellaneous offshore penalty waived for taxpayers who meet the non-residency requirement. The IRS sets out the eligibility conditions in its streamlined filing compliance procedures guidance. Two points matter more than any other. The programme is only available before the IRS contacts you, so the option has a shelf life. And the non-wilfulness narrative is the document the whole submission turns on: it is a characterisation of your conduct, and it should never be drafted casually. We cover the mechanics in detail as IRS streamlined filing experts.
Where the omission was a mishandled PFIC or an unclaimed treaty position rather than a missing return, the route is usually amended returns, late elections under available relief procedures, or an accounting method change. Choosing the wrong remediation route is itself a costly error, because some doors close once you have walked through another.
What a specialist engagement actually looks like
Clients are entitled to know what they are buying. A properly run cross-border preparation engagement is not simply a return typed by someone more expensive.
- Position review before preparation. Holdings, entity interests, equity awards and pension arrangements are classified for both systems before any form is opened, because classification drives everything downstream.
- A single credit model. One computation that reconciles the UK liability across the 6 April year to the US 31 December year, with the apportionment methodology documented so that it survives review and is repeatable next year.
- Elections tracked as a calendar. QEF, mark-to-market, section 962, section 83(b) and treaty positions all carry deadlines that are unforgiving and frequently invisible until they have passed.
- Both returns prepared with sight of each other. The self assessment and the 1040 are drafted by people reading the same file, in the same sequence, each year.
- A carryforward register. Excess foreign tax credits, suspended passive losses, PFIC basis and UK losses tracked year to year, because their value is realised on exit, sale or a bonus year, sometimes a decade later.
For clients with substantial portfolios, multiple entities or family structures, this sits within a broader high net worth engagement, where return preparation is the annual output of a position that is managed continuously rather than reconstructed each spring.
Speak to us in confidence
If you read the decision table above and landed in the "required" column, the sensible next step is a review of your last filed return rather than a leap into a new engagement. We will tell you plainly whether it was prepared correctly, what exposure exists, and whether remediation is warranted. If your affairs genuinely are simple, we will tell you that too. To arrange a confidential consultation, contact our cross-border team. The conversation is entirely without obligation.
