Cross Border Tax Specialists for US and UK Dual Citizens: What They Should Handle

Cross border tax specialists for dual citizens of the US and UK prepare both countries' returns as one position, and also handle the issues that only dual citizens face for life: the treaty's saving clause, investments that are penalised in both directions, children's US status, estate planning under two regimes and, if it comes up, the cost of renouncing. The annual returns are the visible part of the work; the cross-border decisions behind them are what you are paying for.

Most cross-border clients have a temporary problem: an American posted to London for five years, or a Briton who moves to New York and may move back. Dual citizens are different. A US/UK dual citizen remains a US taxpayer for as long as they hold US citizenship, wherever they live, and may be a UK resident for decades. Every pension, fund, property and gift has to work under two tax codes at once. This guide explains what a specialist adviser should cover, where the work differs from ordinary expat filing, and the questions that show whether a firm can do it. For the mechanics of the annual cycle, see our separate guide to dual citizen tax filing in both countries.

Why do dual citizens need cross border tax specialists?

Dual citizens need cross border tax specialists because the US and UK tax them on different grounds that never switch off. The IRS states that for a US citizen, worldwide income is subject to U.S. income tax, regardless of where you live. The UK, by contrast, taxes by residence: GOV.UK explains that residents normally pay UK tax on all their income, while non-residents pay UK tax only on UK income. Nationality plays no part in the UK test.

The result is that a US/UK dual citizen living in the UK is fully taxable in both countries every year, while one living in the US is fully taxable in the US and taxable in the UK only on UK income. A specialist's job is to make the two sets of rules produce one consistent answer, so that the same income is relieved once and reported correctly on each side.

The treaty does not end US filing

Many dual citizens assume the US–UK tax treaty settles which country taxes them. For US citizens it largely does not. Article 1(4) of the US–UK income tax convention lets each country tax its residents, and the US tax its citizens, "as if this Convention had not come into effect". Article 1(5) then lists the benefits that survive for citizens, including parts of Article 17 (pensions and social security), Article 18(1) (pension schemes), Article 24 (relief from double taxation), Article 25 (non-discrimination) and Article 26 (mutual agreement). A specialist knows which of those survive and claims exactly those. Our article on the saving clause in the US–UK tax treaty goes through each exception.

What should cross border tax specialists for dual citizens actually handle?

Cross border tax specialists for dual citizens should handle six areas: the two annual returns, US information reporting, investments, pensions, family and estate planning, and decisions about citizenship itself. A firm that covers only the first is an expat return preparer, which may be enough for a simple case but leaves the rest to you.

Area	What a specialist should do	Why it matters for a dual citizen
Annual returns	Prepare the US Form 1040 and, where needed, the UK Self Assessment return from one set of facts	The same income appears on both returns and relief must match
US information reporting	FBAR, Form 8938, Form 8621 for funds, Form 3520 for large foreign gifts	Penalties for missing forms can apply even when no tax is due
Investments	Choose holdings that are not PFICs for the US and are reporting funds for HMRC	Many funds are penalised by one country or the other
Pensions	Apply the treaty pension articles that survive the saving clause	Contributions, growth and lump sums are treated differently by each country
Family and estates	Check children's US status and plan gifts and estates under both regimes	US estate tax follows citizenship; UK Inheritance Tax follows long-term residence
Citizenship decisions	Model the exit tax and the compliance history before any renunciation	The cost of renouncing depends on decisions made years earlier

US information reporting

For a dual citizen living in the UK, the US information returns usually carry more risk than the tax itself, because a UK resident often owes little US tax once the foreign tax credit is claimed. The FBAR is required where foreign financial accounts exceed $10,000 in aggregate at any time in the calendar year and is filed through FinCEN's BSA E-Filing System. Form 8938 is filed with the tax return; for an unmarried taxpayer living abroad, the IRS threshold is more than $200,000 on the last day of the year or more than $300,000 at any time (double for married couples filing jointly). Gifts or bequests from a non-resident alien individual or foreign estate are reported on Form 3520 where they exceed $100,000 in the tax year, which matters when a British parent passes on money.

Investments that work under both tax codes

Investments are where dual citizens are most often penalised twice. On the US side, a UK-domiciled fund is usually a passive foreign investment company, and a US person holding one files Form 8621. On the UK side, HMRC's manual explains that investors in a non-reporting offshore fund are, on disposal, taxed on any gain as if it were income rather than as a capital gain. Many US-domiciled mutual funds are offshore funds for HMRC. The workable middle ground is usually a set of holdings that are taxed normally by the IRS and appear on HMRC's list of reporting funds, which HMRC updates monthly. Our guide to HMRC reporting funds for US citizens explains how to check a fund.

Living in the UK or the US: how the work differs

Where a dual citizen lives changes which return carries the weight. The table shows the usual pattern; your own facts, including where your income arises, decide the details.

Question	Dual citizen living in the UK	Dual citizen living in the US
US federal return	Yes, on worldwide income, while US citizenship is held	Yes, on worldwide income
UK Self Assessment return	Usually, where there is income not fully taxed through PAYE	Only for UK income, or if HMRC asks for one
Main double-tax relief	US foreign tax credit for UK tax paid	UK relief or treaty exemption for UK-source income, US credit where UK tax is paid
FBAR	Almost always, for UK accounts	Where any UK accounts are kept above the threshold
US filing date	Automatic extension to 15 June for citizens living abroad	15 April
UK Inheritance Tax on worldwide assets	Once a long-term UK resident	Can continue for several years after leaving the UK

The IRS allows US citizens whose home and main place of business are outside the US an automatic extension to 15 June for calendar-year returns, but interest still runs on any tax not paid by 15 April. Unused foreign tax can be carried back one year and forward ten years, according to IRS Topic no. 856, so a specialist keeps a running record of UK tax claimed in each US year.

Family, estates and the next generation

Dual citizenship often passes down a family, and with it the US filing obligation. A child born in the UK to a US citizen parent may be a US citizen from birth, depending on the parent's time in the US, and a US citizen child follows the same filing rules as any other citizen once income or account balances pass the thresholds. Specialists should confirm the child's status early, because a Social Security number is generally needed to claim US child credits. Our guide to an American baby born in the UK covers the first-year paperwork.

Estate planning is harder still. The US taxes estates of citizens on worldwide assets. The UK changed its own test on 6 April 2025: GOV.UK states that the domicile and deemed domicile rules were replaced by new long-term UK resident rules, and a person is a long-term UK resident if they have been UK tax resident for 10 consecutive years or for at least 10 of the previous 20 tax years. A dual citizen who has lived in the UK for a decade can therefore face both regimes on the same estate, with relief depending on the separate US–UK estate tax treaty. Read more in UK Inheritance Tax for Americans.

Illustrative example: a dual citizen born in Ohio has lived in Bristol for 15 years, holds a UK workplace pension, a Stocks and Shares ISA invested in UK index funds, and expects an inheritance from a British parent. A general preparer files a US return each year that reports the salary and claims the foreign tax credit. A specialist also flags that the ISA funds are PFICs needing Form 8621, that the inheritance may need Form 3520 if it exceeds $100,000, that the pension should be reviewed against the treaty articles the saving clause preserves, and that UK Inheritance Tax now applies to the client's worldwide estate as a long-term UK resident. None of these points shows up on a basic Form 1040, and each costs more to fix later than to plan now.

Renunciation and catching up: two decisions specialists should model

Some dual citizens eventually ask whether to give up US citizenship. A specialist should not push either way but should show the numbers. The IRS treats a person as a covered expatriate, subject to the exit tax, if their net worth is $2 million or more, their average annual net income tax for the previous five years exceeds an inflation-adjusted threshold ($206,000 for 2025), or they fail to certify five years of US tax compliance on Form 8854. The last test catches people who are otherwise well below the money thresholds, which is why compliance comes before any renunciation. See renouncing US citizenship as a UK resident and our expatriation service.

Others discover late that they should have been filing all along. For dual citizens whose failure was non-wilful, the IRS's Streamlined Foreign Offshore Procedures require returns for the most recent 3 years, FBARs for the most recent 6 years and a signed Form 14653 certification. Unpaid tax also has a passport consequence: the IRS can certify a seriously delinquent tax debt to the State Department, and its threshold is $66,000 for 2026, adjusted yearly. For a dual citizen who travels to the US, that is not an abstract risk. Our guide to the Streamlined Foreign Offshore Procedures walks through the filing.

How to choose cross border tax specialists for dual citizens

The way to choose cross border tax specialists for dual citizens is to test them on items that are treated differently in each country, not on general expat questions. These steps help you compare firms on substance:

1. Ask how they would treat a UK fund on both returns. A good answer mentions PFIC rules and Form 8621 for the US and reporting fund status for HMRC in the same breath.
2. Ask which treaty articles survive the saving clause for you. The answer should refer to Article 1(5) and your actual pensions, not to "the treaty" in general.
3. Ask who prepares each return. Check whether US and UK specialists sit in the same firm or one side is referred out, and who checks the two returns against each other.
4. Ask what they need about your family. A specialist will ask about children's citizenship, gifts from parents and where your estate would be taxed.
5. Ask how they handle years you did not file. They should explain the Streamlined procedures and when they do not apply, without promising an outcome.
6. Ask for a written scope. The engagement letter should list every form, both countries' returns and any planning advice included.

For a broader checklist that applies to any adviser, see how to choose US/UK cross border tax specialists.

What dual citizens get wrong about specialist advice

Treating the US return as a formality. A UK resident often owes no US tax, but the information returns still apply, and missed forms carry their own penalties.

Assuming a British passport changes the US position. US tax follows US citizenship. Holding a second passport does not reduce it.

Buying UK investment products without a US check. An ISA is tax-free in the UK, but the IRS does not recognise it, and the funds inside it are usually PFICs.

Leaving estate planning to one country's adviser. A UK will drafted without US input, or a US trust set up without UK input, can produce tax in both countries.

Deciding to renounce before getting compliant. The five-year certification on Form 8854 is one of the covered expatriate tests, so missing years can turn a modest estate into an exit-tax case.

Working with cross border tax specialists for US and UK dual citizens

A dual citizen's tax affairs do not end when a posting ends, because there is no posting. The right adviser treats the US and UK positions as one permanent plan, revisited each year and whenever life changes: a new job, a property purchase, a child, an inheritance or a move between the two countries. Our page for cross border tax specialists for US and UK dual citizens sets out how we work with clients who hold both passports.

US/UK Cross Border Tax — US CPAs and UK tax advisers working as one team; London, Manchester, New York, San Francisco. We prepare both countries' returns together, including PFIC reporting for UK funds, and advise on the decisions that follow dual citizenship through a lifetime. Contact us for a scoped quote covering your US return, your UK return and the planning between them.