Tax specialists for US and UK returns: what they handle that a generalist does not.

The US taxes citizens on worldwide income wherever they live. The UK taxes residents. Those two rules are individually simple and jointly awkward, because a person can satisfy both at the same time and neither system was designed around the other.

What follows is a set of specific mismatches rather than a general fog. A UK product that is tax-free in the UK may be fully taxable in the US. A UK pension may be protected by the treaty in one respect and not in another. A gain may be taxed on different bases, in different currencies, in different tax years. Each of those is a discrete, learnable thing, and each is invisible to someone trained in one system alone.

That is why the honest description of the specialism is narrow. A cross-border specialist is not better at arithmetic. They know which of roughly twenty interactions apply to you, and in what order to resolve them.

The distinction is not seniority and it is not diligence. A very good UK accountant will prepare a flawless Self Assessment and will not ask whether the fund inside your ISA is a passive foreign investment company, because for a UK-only client the question has no meaning.

The table below is the concrete version. The left column is ordinary domestic work; the right is the cross-border overlay on the same fact pattern.

It is worth stating the negative case plainly, because no commercial page does. Having a filing obligation is not the same as needing help with it. Plenty of Americans in the UK have a genuinely simple position, and telling them otherwise would be selling rather than advising.

The list below is the trigger list. One item is enough; they compound quickly.

The US side attaches to status: citizenship, or lawful permanent residence, or meeting a residence test. Citizenship is the one that surprises people, because it does not lapse through absence and it does not depend on ever having lived in the US. Someone born in the US to visiting parents and raised entirely in Britain is a US citizen for tax purposes — see accidental Americans.

The UK side attaches to residence, decided by the statutory residence test, and it can change year to year on facts as ordinary as how many nights you spent in the country.

The direction people forget is inbound. A British person working in the US, or spending substantial time there, can become a US tax resident under the substantial presence test without ever holding a green card. That population is barely served by the market — Brits in the US sets out the position from that side.

The weighting is what catches people. Someone spending four months a year in the US never approaches 183 days in a single year but can cross the weighted threshold on a rolling basis without any year looking unusual.

The transit exclusion is the part no competitor page mentions, and stating it backwards — claiming that partial days always count in full — pushes frequent connectors over the line for no reason. If you change planes in the US on your way between two non-US places and are in the country for less than 24 hours, that day does not count. For a London-based traveller routing through a US hub, this is the difference between a correct count and a fabricated residence.

Partial days are otherwise counted, and there are further exclusions for certain visa categories and for a medical condition arising while present. The substantial presence test calculator does the weighted arithmetic, and the glossary entry sets out the exclusions. The IRS explains the test on its own page.

Where the test is met but the closer connection is elsewhere, Form 8840 may be relevant. That is a claim with conditions, not a formality.

The order matters. If an automatic overseas test is met you are non-resident and the rest does not apply. Only if no automatic test resolves the position do the ties come into play, and the number of days that makes you resident falls as your number of ties rises.

Because the ties are factual, they are evidenced or not. Nights in the country, where your family lives, whether accommodation is available to you, days worked here — all of it is the kind of thing that is trivial to record at the time and painful to reconstruct two years later under enquiry.

HMRC publishes guidance on the statutory residence test, and the UK residence test tool works through the steps. Note that UK residence is decided independently of your US status: being a US citizen neither creates nor prevents UK residence.

Neither is a default. Both are claims that depend on facts — arrival and departure dates, work patterns, where a home was available — and both are far easier to evidence contemporaneously than to reconstruct. Missing the claim generally means both countries tax the full year and the treaty is left to untangle what paperwork could have prevented.

The year of the move is also when one-time elections are made, and several of them echo for years. Revoking the Foreign Earned Income Exclusion locks you out of it for the following 5 years. Accounts opened and funds bought innocently in the first month can create reporting that persists indefinitely.

The practical advice is unglamorous: record dates from day one, and settle what the first pair of returns will claim before the first pay packet arrives. Your first year abroad works through the sequence.

The tie-breaker is a sequence, not a balancing exercise: each test is applied only if the one before it fails to resolve the position. It is also a treaty concept rather than a domestic one, so it decides which country treats you as resident for the treaty's purposes without changing your domestic status.

The saving clause is the catch, and it is the single most misunderstood provision in this treaty. It preserves each country's right to tax its own citizens as though parts of the treaty did not exist. So a US citizen cannot generally use the tie-breaker to stop being taxed by the US, whereas a UK national without US citizenship often can achieve rather more with it. The saving clause entry sets out which articles survive it.

That asymmetry is why advice that works for a UK national in the US frequently fails for a dual citizen, and why the two directions genuinely need separating rather than treating as one topic.

The honest framing is that relief is designed to prevent double taxation of the same income, not to guarantee you pay only the lower rate. Where UK rates exceed US rates on a given item, the practical outcome for a US citizen is often that the UK bill sets the floor and the US credit absorbs the rest.

The foreign tax credit gives relief for foreign tax paid or accrued, and unused credit carries back 1 year and forward 10. That carryforward is an asset, and it is one of the things most often lost when someone changes adviser without collecting their schedules.

Relief is not automatic in either direction. It is claimed, on a form, with figures that have to be converted at the right rate for the right purpose — and the rate for income is not the rate for account balances. The foreign tax credit calculator and exchange rates cover both. The IRS sets out how to claim treaty benefits, and HMRC's Double Taxation Relief Manual is the UK-side reference.

This is one of the questions where the shape of a confident answer is itself the warning sign. The variables are your income level, the UK tax actually paid, whether you have children who bring refundable credits into play, whether you contribute to US retirement accounts, and what you expect the next few years to look like.

The asymmetry that matters: the exclusion removes income from the US calculation, which also removes the foreign tax paid on it from the credit computation. So choosing the exclusion can waste UK tax that would otherwise have become carryforward. And revoking the exclusion later locks you out for 5 years, which makes this a decision rather than an annual preference.

Model it once, properly, in the first year. The FEIE calculator and the comparison of the two are the starting point, and the IRS describes the exclusion on its own page.

The married-filing-separately figure is the one that matters most for this audience and it appears on almost no competitor page. A US citizen married to a non-US spouse usually files separately, because including a non-US spouse on a joint return means electing to bring their worldwide income into the US system. So the threshold that applies is $5, which in practice means almost everyone in that position files.

Note also that the threshold test is gross income, before the exclusion or credits are applied. Claiming the exclusion does not remove the obligation to file — it is claimed on the return you were required to file anyway. People frequently reason in the wrong order here and conclude they need not file because they would owe nothing.

Disclosure forms have their own thresholds, entirely separate from the income ones, and can require filing even in a year with no tax due. The filing obligation checker works through the whole set, and Publication 501 is the IRS source for the income thresholds.

The FBAR test is the one most often misread. It is not per account and it is not a year-end balance. Add the highest balance each account reached at any moment in the year, convert at the Treasury year-end rate, and if the total passes $10,000, every account gets reported. Moving the same money between two of your own accounts puts it through the test twice, which is how a modest salary crosses the line by February.

Form 8938 is filed with your return, covers a wider class of assets than bank accounts, and for taxpayers abroad starts at $200,000 at year end or $300,000 at any time during it, doubling for joint filers. Its penalty structure is its own: $10,000 with continuation penalties to a maximum of $60,000.

Neither form changes the tax you owe. Both carry penalties for silence, which is what makes them disproportionately dangerous relative to how boring they are. FBAR and FATCA reporting and the thresholds guide work through what counts, and the IRS publishes a comparison of the two regimes.

This is the most common unpleasant surprise for Americans in Britain, precisely because the UK marketing is unambiguous and correct. It is tax-free — in the UK. The US simply does not have a rule that recognises the wrapper, so it looks through it.

Whether that is a mild annoyance or a serious problem depends on the contents. A cash ISA generates interest, which is reportable but straightforward. A stocks and shares ISA holding UK funds is a different matter, because those funds are very often passive foreign investment companies, which brings its own regime.

The accounts are also reportable in their own right: an ISA counts toward the FBAR aggregate and may appear on Form 8938. That is separate from, and in addition to, taxing what it earns.

Either test is enough, and an ordinary UK unit trust, OEIC or investment trust will normally meet at least one. That is not a quirk of aggressive products; it is what a diversified fund is. So a perfectly sensible UK portfolio can be a stack of PFICs without containing anything exotic.

There is a de minimis exception that is worth knowing because it is genuinely useful and almost never mentioned. Where the aggregate value of PFIC stock on the last day of the year is at most $25,000 — $50,000 for joint filers — and you received no excess distribution and recognised no gain, Part I of Form 8621 need not be filed. Indirect holdings use a separate $5,000 figure for the proportionate share.

Two things are worth correcting because the opposite is widely written. Form 8621 carries no monetary penalty for being missed, unlike Forms 8938 and 3520 — though an unfiled 8621 can keep the return's limitation period open, which is its own exposure. And it is not true that individual shares are never PFICs: a foreign company that is itself mostly passive can be one, so a direct holding is not automatically safe.

The elections available — qualified electing fund, or mark-to-market — can improve the outcome substantially, but they generally have to be made in time and depend on the fund providing information it may not produce. The PFIC checker and PFIC reporting go through it.

Bundling those two paragraphs is the most common technical error in this area, and it is consequential in both directions. Told that the treaty protects UK pensions, an American assumes their salary sacrifice is also protected — and it generally is not, for a citizen. Told that the treaty protects nothing, they assume tax on annual growth inside the scheme, which is usually wrong.

The mechanism is the saving clause, which preserves each country's right to tax its own citizens as though parts of the treaty did not apply, with a list of exceptions. Article 18(1) is among the provisions preserved for everybody. Article 18(2) is in the group preserved only for people who are not citizens or permanent residents of the taxing state. So the same treaty produces different answers for a dual citizen and for a UK national on assignment.

Distributions are a separate question again, with their own paragraph and their own interaction with domestic rules in each country. Where a specific lump sum treatment is at stake, that is a question to work through against the current HMRC and IRS material on the facts rather than a rule to generalise from — the guidance in this area has moved recently enough that a confident general statement would be the wrong output.

Practically, the pension questions worth getting advice on before acting are transfers, consolidation, and how much to contribute. Pensions and retirement accounts covers the scope, and the treaty text and HMRC's manual are linked in the sources below.

Rental income is the more routine half, but the profit figures rarely match. The two systems differ on what is deductible, on how mortgage interest is treated, and on depreciation — the US requires it on rental property, which has consequences on eventual sale that people do not anticipate.

Sales are where the divergence is sharpest. The UK principal residence relief and the US principal residence exclusion are different reliefs with different conditions, so a sale can be entirely relieved in one country and partly taxable in the other. And because the US measures gain in dollars, exchange rate movement between purchase and sale creates or destroys gain that does not exist in sterling at all.

The 60-day UK reporting and payment window for residential property disposals is a hard deadline that is easy to miss when the buyer is arranging things around you. Property and capital gains works through both sides, and GOV.UK covers reporting and paying CGT on UK property.

The National Insurance point disappoints people reliably. It is a substantial UK cost that does not behave like the income tax sitting next to it, because the credit mechanism is for income taxes and NI is a social security contribution.

That is precisely what the totalization agreement addresses, from the other direction: without it, a self-employed American in the UK would face both UK National Insurance and US self-employment tax on the same earnings. The agreement assigns coverage to one system, and a certificate evidences it.

Employment and self-employment differ here, and so do short assignments and long ones. The IRS explains totalization agreements and self-employment tax for those abroad; HMRC's NI38 booklet covers the UK position. Contractors sets out the common patterns.

Nobody in this market covers these forms, and the reason they matter is that the underlying events feel entirely innocent. Receiving money from your parents is not a taxable event. Reporting it may nonetheless be required, and the penalty for not doing so is calculated against the amount rather than capped at a fixed figure.

Foreign trust is also a broader category in US terms than people expect, and can catch arrangements set up for ordinary UK reasons without anyone thinking of them as trusts. That is a question of the arrangement's terms, not its label.

Because the events are one-off and unprompted, the reporting is typically discovered late. Trusts and estates covers the scope; Forms 3520 and 3520-A are linked in the sources.

The two do not align on almost anything: what is in the estate, who is liable, what reliefs apply and what a spouse transfer achieves. The spousal position is a particular trap, because a transfer that is entirely relieved in one system may not be in the other where the spouse is not a citizen of the taxing country.

The UK basis has also changed, with long-term residence now central to exposure rather than the older domicile concept. That makes the number of years someone has been UK resident a planning variable rather than a background fact.

This is an area where the value is almost entirely in acting before the event, and where general statements are least useful. Trusts and estates sets out the scope, and the primary material is linked below.

The distinction is between using a mechanism the Code itself provides and asserting that the treaty changes the Code's answer. The first is ordinary return preparation. The second is a position the IRS wants told about explicitly.

The practical significance is that disclosure is what makes a defensible position visible as a position. A claim that should have been disclosed and was not looks materially worse under examination than the same claim properly flagged.

Whether a given claim needs an 8833 is a judgement on the specific article and the specific facts, and it is exactly the sort of call a generalist has no reason to have encountered. Treaty relief covers the ground.

The route is decided principally by where you have been living — the Foreign route requires meeting a non-residency test, which for a US citizen turns on being outside the US for at least 330 days in one of the last three years. Certification is on Form 14653 for the Foreign route and Form 14654 for the Domestic one.

This is the distinction competitor pages collapse, and collapsing it is not a nuance. A reader told the programme is "penalty-free" who is in fact on the Domestic route is deciding on a false basis, because 5% of a peak balance is a real number. Never accept the phrase without the route named.

Both routes are also only available before the IRS makes contact about the years concerned. There are separate routes where the only failure was an unfiled FBAR, and the right choice among them depends on what was missed and why.

Non-wilfulness is the eligibility condition and it is a statement you sign. It is not a formality, and where conduct was not plainly non-wilful, Streamlined is the wrong route and a different disclosure programme is the conversation. Streamlined filing and the glossary entry set out both routes.

This is the obligation people are most likely not to know exists, because the intuition is that leaving the US ends US state tax. The treaty is a federal instrument and does not bind the states, so treaty relief on the federal return does nothing for a state that still claims you.

What ends state residence is a factual severing — where your home is, where you are registered to vote, where your licence and vehicles are, where your professional and family ties sit. The states with the most demanding tests will look at intent to return as well as physical presence.

It is worth resolving deliberately in the year of the move rather than discovering it later. Cutting state tax ties sets out the mechanics, and the specific state's own rules govern.

The mismatch is structural rather than inconvenient. A single UK tax year overlaps parts of two US ones, so there is no year in which the two simply line up, and the extensions available on the US side are what make it possible to compute a credit from final UK figures rather than estimates.

The FBAR has its own deadline and its own automatic extension, which is separate from the return's. Payments on account in the UK arrive on dates that have nothing to do with either. The deadline calendar keeps both countries in one view, and the underlying dates are on GOV.UK and the IRS.

One thing that does not move: an extension to file is not an extension to pay. Interest runs from the original date in both systems, which is why an extension is a tool for accuracy rather than for delay.

The quality of the first conversation is the most reliable early signal you will get. Someone who asks for your income and quotes a fee has not yet established what the work is. Someone who asks what is inside your ISA has.

These are the questions worth hearing. If none of them comes up, the scope has not been established, and a fee quoted against an unestablished scope will change.

The work this page describes is the work. One specialist stays on a file from first call to submission, so a decision taken in year one — an election, a treaty position, a PFIC treatment — is remembered by the person who took it when it matters again in year three.

Every figure on this page comes from a single internal register with its source recorded, and every rule was checked against the Code, the form instructions, the treaty text or the relevant HMRC manual rather than against memory. Where something could not be substantiated to that standard it was left out, which is why one paragraph above declines to state a general position on pension lump sums.

If you want to know what applies to you before speaking to anyone, the free tools will get you most of the way. How we work sets out the process, and our team describes how the two halves of a file are staffed.