US/UK Cross Border Tax Preparation: What the Process Looks Like Start to Finish

The US/UK cross border tax preparation process has nine stages: scoping the engagement, authorising the adviser, gathering documents from both countries, settling residence, sequencing double tax relief, preparing both returns and the US reporting, review and signature, filing and payment, and post-filing follow-up. Each stage is set out below, with what you are asked for and why.

Knowing the sequence helps in two ways. It tells you what to collect and when, and it shows why a cross-border return cannot simply be two single-country returns stapled together: decisions made on one side change the figures on the other. If you are still choosing who to work with, how to choose US/UK cross border tax specialists covers that first.

What does the US/UK cross border tax preparation process involve?

The US/UK cross border tax preparation process produces a consistent set of filings in both countries for the same period of your life. For an American living in the UK, that usually means a UK Self Assessment return, a US Form 1040 with its schedules, and US information returns such as the FBAR — each prepared so that income is taxed once in the right place and relieved in the other.

The complication is the calendar. The US tax year is the calendar year; the UK tax year runs from 6 April to 5 April. A US return for 2025 therefore draws on UK payslips and statements from two UK tax years (2024 to 2025 and 2025 to 2026), and a UK return for 2025 to 2026 draws on US figures from two calendar years. A good process collects information once and uses it for both.

The process step by step

1. Scope and engagement letter. Agree in writing which returns and reports will be filed in each country, what is excluded, and the fee.
2. Authorisation. Set up the adviser's authority with each tax authority, and for the FBAR if the adviser will file it.
3. Information gathering. Collect income, account and pension documents from both countries, plus last year's returns.
4. Residence and status. Confirm UK residence under the Statutory Residence Test, any split year, and the US filing status.
5. Sequencing relief. Decide which country taxes each item first and where relief is claimed, including the choice between the foreign tax credit and the foreign earned income exclusion.
6. Preparation. Prepare the UK return with its supplementary pages, the US return with its schedules, and the US information returns.
7. Review and signature. A reviewer checks both returns together; you review your copies and authorise filing.
8. Filing and payment. File each return by its deadline and pay, including UK payments on account.
9. After filing. Check for corrections, answer any enquiries, and roll forward for next year.

The stages below explain what happens at each one.

Stages 1 and 2: scope and authorisation

Scope is the stage clients most often skip and most often regret. The engagement letter should list the specific filings — for example the UK Self Assessment return, the US federal return, any state return, the FBAR and, where the thresholds are met, Form 8938 — and say explicitly what is not included.

Authorisation follows. For the IRS, Form 2848 authorises an eligible representative to act for you, while Form 8821 only lets someone receive your tax information. For HMRC, authorisation is given through an agent authorisation or, for certain services, a digital handshake in which you approve the agent's request from your own tax account; GOV.UK is explicit that you must not give your sign-in credentials to your agent. If the adviser will file your FBAR, FinCEN Form 114a, the Record of Authorization to Electronically File FBARs, records that permission. It is kept on file rather than submitted.

What documents do you need for a US/UK tax return?

You need income and tax documents from both countries, a full list of your non-US financial accounts, and last year's returns. Most delays in the process come from this stage, usually because one document is missing, rather than from the preparation itself.

Document	Where it comes from	Used for
P60	UK employer — by 31 May after the UK tax year ends	UK employment income and PAYE tax; UK tax paid for the US foreign tax credit
P11D benefits information	UK employer	Taxable benefits on both returns
UK bank, savings, ISA and dividend statements	UK banks and platforms	UK interest and dividends; US reporting of the same income and accounts
Pension statements	UK schemes, SIPP providers, US plans	Contributions, growth and withdrawals under each country's rules and the treaty
Forms 1099 and brokerage statements	US payers and brokers	US income; UK reporting of the same income as foreign income
Account list with highest balance in the year	You, from statements	FBAR and Form 8938
Travel calendar	You	UK residence tests and any US day-count tests
Last year's US and UK returns	You or your previous adviser	Carry-forwards, elections, payments on account

GOV.UK confirms that employers must provide the P60 by 31 May, on paper or electronically. The account list deserves the most care: the FBAR is required where a US person's foreign accounts exceeded $10,000 in aggregate at any time during the calendar year, so maximum balances matter, not year-end ones. We cover that report in the FBAR deadline guide.

Stages 4 and 5: residence, then relief

Residence and status

UK residence is confirmed first because it decides what the UK taxes. Where residence, a split year or the 4-year foreign income and gains regime is relevant, it is recorded on the Self Assessment supplementary pages SA109, titled Residence and foreign income and gains (FIG) regime etc. On the US side, a citizen or green card holder is taxed on worldwide income wherever they live, so the question is filing status, which matters most for couples where one spouse is not a US person.

Which return comes first?

Neither return can be finished in isolation, because relief on one depends on tax on the other. The working order is to establish, item by item, which country has the first right to tax, calculate that country's tax, and claim relief in the other. On the US side, the foreign tax credit on Form 1116 credits UK tax against US tax on the same income, and the IRS does not allow a credit for tax on income excluded under the foreign earned income exclusion. On the UK side, relief for US tax is claimed as Foreign Tax Credit Relief on the SA106 foreign pages, and GOV.UK notes that how much relief you get depends on the double-taxation agreement. Our guide to double taxation relief between the US and UK covers the mechanics.

Illustrative example: an American living in Manchester earns a UK salary and receives dividends from a US brokerage account. The UK taxes the salary, so UK tax on it is credited on the US return. The US dividends are taxed in the UK as foreign income, and any UK relief for US tax on them is limited to what the treaty allows. The adviser works out each item in order and records the same income and tax figures on both returns. Getting the order wrong does not usually produce an error message, only an overpayment or a relief claimed twice.

Stage 6: preparing both returns and the US reporting

With residence and relief settled, the returns are prepared in parallel.

• UK: the SA100 main return with the supplementary pages that apply — commonly SA102 for employment, SA106 for foreign income and relief, SA109 for residence, and property or capital gains pages where relevant.
• US: Form 1040 with its schedules, Form 1116 or Form 2555, and information returns such as Form 8938 and Form 8621 for UK investment funds where they apply.
• FBAR: FinCEN Form 114, filed electronically through FinCEN's BSA E-Filing System and not with your federal tax return.

Currency is applied consistently across both: US figures in dollars, UK figures in pounds, with one conversion method used throughout. The broader range of work involved is described in what US UK tax accountants do.

Stages 7 and 8: review, signature, filing and payment

Review is where a cross-border process earns its fee. The reviewer checks that income appears on both returns, that relief is claimed once and in the right place, and that every account and fund on the list has been considered for US reporting. You then receive copies of both returns to check.

For an e-filed US return, you authorise filing by signing Form 8879, IRS e-file Signature Authorization, the declaration and signature authority for a return e-filed by an electronic return originator. The UK return is submitted online by the agent once you have approved it.

Filing or payment	Date
US return (calendar year)	15 April; automatic extension to 15 June where your tax home is abroad; to 15 October on Form 4868
FBAR	15 April, with an automatic extension to 15 October
Tell HMRC you need to file (2025 to 2026 tax year)	5 October 2026
UK paper return (2025 to 2026)	31 October 2026
UK online return and balancing payment (2025 to 2026)	31 January 2027
UK payments on account	31 January and 31 July

Two payment points catch people out. The US extensions are extensions to file, not to pay: the IRS charges interest on any tax not paid by the regular due date. And in the UK, GOV.UK says each payment on account is half of the tax you owed last year, due by 31 January and 31 July, unless last year's bill was under £1,000 or more than 80% of it was collected at source. A first Self Assessment year can therefore bring a larger January payment than expected. The UK dates are the ones on GOV.UK for the 2025 to 2026 tax year; if you are new to Self Assessment, see registering by 5 October.

Stage 9: what happens after the returns are filed?

After filing, the process checks whether either return needs correcting, deals with any enquiries, and records what carries forward. A change on one return can change relief on the other, so a correction in one country is always checked against the other.

In the UK you can correct a return within 12 months of the Self Assessment deadline — GOV.UK's example is that a 2024 to 2025 return will usually need to be changed by 31 January 2027 — and an online return cannot be updated until 72 hours after it was filed. In the US, a filed return is corrected on Form 1040-X, the amended individual income tax return. Finally, carry-forwards — unused foreign tax credits, elections made on either return, and next year's payments on account — are recorded so next year starts where this one ended.

What people get wrong about the process

Sending documents in batches. A partial set means provisional figures that have to be reworked on both returns. One complete delivery is quicker for everyone.

Using year-end balances for the account list. The FBAR test looks at the aggregate value at any time in the year, so it needs the highest balances.

Treating the US extension as more time to pay. It extends the filing date only; interest runs from the regular April date.

Forgetting the July payment. The second UK payment on account arrives outside the usual filing season and is easy to miss.

Getting started

The simplest way to start is with last year's returns from both countries and a list of your accounts. From those, an adviser can confirm the scope, send the authorisations and give you a document list for the year.

US/UK Cross Border Tax — US CPAs and UK tax advisers working as one team; London, Manchester, New York, San Francisco. Our US/UK cross border tax preparation service covers both returns and the US reporting together, UK Self Assessment explains the UK side, and Americans in the UK covers the most common position. To start, ask for a fee quote.