US UK Tax Accountants: What They Do That a Single-Country Accountant Can't

What US UK tax accountants do is prepare your tax returns in both countries and make them agree: they decide which country taxes each item of income first, claim relief in the other, file the US information returns a UK accountant never sees, and reconcile two tax years that do not line up. A single-country accountant can do one side well. The value is in the join.

The questions below describe that work concretely — what the job is, task by task, and where a single-country engagement tends to leave a gap. If you are at the stage of comparing firms, the companion piece on how to choose US/UK cross border tax specialists covers the checks to run before you sign.

What do US UK tax accountants do that a single-country accountant can't?

US UK tax accountants own the interaction between the two systems, not just the forms in each. A UK accountant is trained to get the UK Self Assessment return right; a US preparer is trained to get the Form 1040 right. Neither is asked to check whether the two returns are consistent, whether relief has been claimed twice or not at all, or whether a UK investment that is ordinary at home creates a US reporting problem.

That gap exists because the two systems start from different questions. The UK taxes largely by residence, determined by the Statutory Residence Test. The US taxes its citizens and green card holders on worldwide income wherever they live — the IRS states plainly that US citizens and resident aliens abroad are subject to tax on worldwide income from all sources. An American living in London is therefore inside both systems at once, every year, and someone has to make the two outcomes fit together.

The job, task by task

The table sets out the recurring work and who, in practice, tends to own each piece.

Task	UK-only accountant	US-only preparer	US UK tax accountant
Residence status	Applies the Statutory Residence Test	Applies US citizenship and residency rules	Applies both, and the treaty tie-breaker where someone is resident in both
Double tax relief	Claims Foreign Tax Credit Relief on US tax where due	Claims the foreign tax credit or exclusion	Decides which country gives relief for each item so it is claimed once, in the right place
UK investments (ISAs, UK funds)	Treats as UK-efficient	May not recognise UK wrappers	Flags US treatment, including passive foreign investment company reporting
Pensions	UK rules on contributions and withdrawals	US rules on foreign pensions	Applies the treaty's pension articles on both returns consistently
Foreign account reporting	Not a UK requirement	Prepared if the client mentions the accounts	Builds it in from the account list every year
Social security	National Insurance	Social Security and self-employment tax	Applies the totalization agreement and certificate of coverage
Tax years	6 April to 5 April	Calendar year	Maps income and tax paid across both periods

None of those rows is exotic. They are the ordinary content of a cross-border file, which is why the join matters more than any single form.

Why can't the treaty just sort it out?

The US-UK income tax treaty does not remove the US tax obligation of a US citizen living in the UK. Article 1(4) of the treaty contains the saving clause, which the IRS technical explanation describes as reserving each country's right to tax its residents and citizens as if the Convention had not come into effect, except for the provisions listed in paragraph 5.

So the treaty is a set of targeted rules rather than a blanket exemption. It allocates taxing rights for particular kinds of income, decides residence for people resident in both countries, and sets the framework for relief. Applying it well means knowing which articles survive the saving clause for a US citizen and which do not — pensions and double tax relief are two areas where that distinction changes the answer. Our treaty relief service page sets out how that work is scoped.

Residence: two systems asking different questions

UK residence is decided each tax year by the Statutory Residence Test. GOV.UK lists the automatic tests — including spending 183 or more days in the UK in the tax year — and a sufficient ties test for people who fall between them. In the year someone arrives or leaves, the UK tax year is usually split into a non-resident part and a resident part.

Since 6 April 2025, the remittance basis has been replaced by the 4-year foreign income and gains (FIG) regime. GOV.UK says it is available to someone within their first 4 years of UK tax residence after at least a 10-year period as a non-UK tax resident, claimed on the Self Assessment return, and that claiming it means losing the Income Tax and Capital Gains Tax tax-free allowances.

This is where the join shows. For a new UK resident who is also American, removing foreign income from UK tax does not remove it from US tax, and the US calculation may then have less UK tax to credit. Whether the FIG claim helps depends on both returns together, which is exactly the decision a single-country accountant is not positioned to make. We cover the regime in more detail in the checklist for Americans living in the UK.

Credit or exclusion: the first decision on an American's return

Most Americans in the UK avoid double tax on earnings in one of two ways. The foreign tax credit, claimed on Form 1116, credits foreign income tax paid against US tax on the same income. The foreign earned income exclusion, claimed on Form 2555, removes qualifying foreign earned income from US tax for someone who meets the bona fide residence test or the physical presence test. The IRS limit on the exclusion is adjusted annually; check the current figure on the IRS foreign earned income exclusion page.

The two interact. The IRS is explicit that if you exclude foreign earned income, you cannot take a foreign tax credit for taxes on the income you exclude. Choosing between them affects future years too, so the decision is modelled rather than defaulted.

On the UK side, where US tax is due on income the UK also taxes, relief is claimed as Foreign Tax Credit Relief on the Self Assessment return, using the SA106 foreign pages. GOV.UK notes that how much relief you get depends on the double-taxation agreement with the other country. The US and UK claims have to be consistent: the same income should be relieved in one country, not in both and not in neither. Our guide to double taxation relief between the US and UK walks through the mechanics on each return.

The reporting a UK accountant never sees

The US asks for information returns that have no UK equivalent, and they are separate from the income tax calculation.

• The FBAR. FinCEN Form 114 is required where a US person's foreign financial accounts exceed $10,000 in aggregate at any time during the calendar year. The IRS says it is filed electronically through FinCEN's BSA E-Filing System and not with your federal tax return.
• Form 8938. Filed with the income tax return where specified foreign financial assets exceed the thresholds for your filing status and where you live. The difference between the two is set out in FBAR vs Form 8938.
• Form 8621. The information return by a shareholder of a passive foreign investment company or qualified electing fund. Many UK-domiciled investment funds, including funds held inside an ISA, raise this question for an American investor — see why an ISA can be a PFIC problem.

A UK accountant has no reason to prepare any of these, and a US preparer only prepares them for accounts and funds the client thinks to mention. A cross-border accountant starts from the full list of UK accounts and holdings each year and works out what each one triggers.

Social security, currency and the other joins

Social security

The US and UK have a Social Security agreement, often called a totalization agreement. Under it, an employee normally pays into the system of the country where they work. The Social Security Administration explains that if an employer sends someone to the other country for five years or fewer, they normally stay covered by their home country and are exempt in the other. That exemption is evidenced by a certificate of coverage: the SSA's UK agreement page says employers and self-employed workers must request one to establish an exemption from US Social Security contributions, with UK certificates handled by HM Revenue & Customs.

Currency

Every figure on a US return is in dollars; every figure on a UK return is in pounds. The IRS says amounts must be expressed in US dollars and that, in general, you use the rate prevailing when you receive, pay or accrue the item. It also says it has no official exchange rate and generally accepts any posted rate applied consistently. Consistency across both returns is part of the job, especially for capital gains, where currency movement alone can create a US gain on a UK asset.

Illustrative example: an American employee living in London has a UK salary taxed through PAYE, a stocks and shares ISA holding UK funds, and a US brokerage account paying dividends. A UK accountant would treat the ISA as tax-free and report the US dividends on the Self Assessment return with credit for any US tax withheld. A US preparer would report the salary and dividends and claim a credit for UK tax. What neither is asked to do is look at the ISA's funds for US reporting, include the UK accounts in foreign account reporting, and check that the dividend relief on each return is not claimed twice. Those three items are the cross-border work.

A year in the life of a US/UK engagement

Because the tax years differ, the work runs across the whole calendar. Using the published dates:

1. January. UK Self Assessment for 2025 to 2026: the online return and payment are due by 11:59pm on 31 January 2027.
2. 5 April. The UK tax year ends.
3. 15 April. The US return and the FBAR are due for the previous calendar year. Filing can be extended, but the IRS charges interest on any tax not paid by this regular due date.
4. 15 June. The automatic two-month US filing extension for taxpayers whose tax home is abroad.
5. 5 October. Deadline to tell HMRC you need to file Self Assessment, if you are new to it — 5 October 2026 for the 2025 to 2026 tax year.
6. 15 October. The extended US deadline where Form 4868 was filed, and the automatic FBAR extension.
7. 31 October. Paper UK returns — 31 October 2026 for the 2025 to 2026 tax year.

A good engagement collects information once and uses it for both returns. The US return for a calendar year draws on UK payslips and statements that straddle two UK tax years, and the UK return draws on US figures that may not be final until the US return is prepared.

What people get wrong about cross-border accountants

Assuming a UK firm with American clients files US returns. Many prepare the UK side only, quite properly. Ask which returns the firm itself files.

Treating the treaty as a one-country answer. For a US citizen, the saving clause means the US return is still required every year.

Leaving information returns to memory. The FBAR and Form 8621 depend on a complete list of accounts and funds. If nobody asks, nobody files.

Deciding UK reliefs in isolation. A UK claim such as the FIG regime can change the US outcome. It should be modelled on both returns before it is made.

Is it worth hiring US UK tax accountants?

It is worth it wherever someone is inside both tax systems at once: Americans and green card holders living in the UK, dual citizens, and Brits who have moved to the US but keep UK income, property or pensions. For a single, simple item a single-country accountant may be enough. Once there are UK investments, a pension, a business, a property or a move between the two countries, the join is where the risk is.

US/UK Cross Border Tax — US CPAs and UK tax advisers working as one team; London, Manchester, New York, San Francisco. You can see the full range of work our US UK tax accountants handle, read how we approach UK pensions under the treaty, or ask for a fee quote for both returns together.