One Firm for US and UK Tax Returns: Why Leading Cross Border Tax Services Providers Prepare Both

Using one firm for US and UK tax returns means a single team prepares your US federal return and your UK Self Assessment return from the same facts, makes each cross-border decision once, and checks the two returns against each other before either is filed. That shared review of the gap between the returns is the real reason leading providers prepare both, rather than convenience.

Most people with a life in both countries have two filing obligations that look separate but are not. The US taxes its citizens on worldwide income; the UK taxes its residents. The same salary, the same savings interest and the same pension can appear on both returns, and the relief that stops you paying twice only works if the two returns agree. The earlier question of whether to use one firm or two is covered in accountants for US and UK taxes: one firm or two?. This guide looks at what happens inside a single engagement, how to tell a genuine one-team service from a referral arrangement, and what to ask before you sign.

What does one firm for US and UK tax returns actually do?

One firm for US and UK tax returns takes responsibility for both filings and for the links between them. In practice, the team works from one document request, one account list and one timetable, and one reviewer looks at both draft returns side by side before anything is submitted to the IRS or HMRC.

The returns themselves are the familiar ones. On the US side that is Form 1040, usually with Form 1116 to claim a credit for UK tax, plus information returns such as the FBAR. On the UK side it is the SA100 return with supplementary pages: GOV.UK describes SA106 as the pages used to record foreign income and gains and to claim Foreign Tax Credit Relief, and SA109 as the pages that record residence status, with the 2026 version titled for the new foreign income and gains regime.

What is different is the work between the forms. A single team decides the following once and applies the answer everywhere:

• Residence. UK residence decides what the UK taxes, and it also affects how the US–UK treaty applies to you.
• Where each item of income is relieved. Which country taxes first, and which country gives the credit.
• Currency. The IRS says it has no official exchange rate and accepts any posted rate used consistently, so one method has to be chosen and kept.
• The account list. The UK accounts that produce income on the UK return are the same accounts that may need to be reported on US information returns.
• Timing. Which UK tax payments are claimed against which US tax year.

Three ways providers cover both countries

Many firms say they handle US and UK tax. Behind that statement sit three quite different models, and the difference matters more than the marketing suggests. None is wrong in itself; each puts responsibility for the gap between the returns in a different place.

Question	One in-house team	Referral to a partner firm	Two independent firms
Who prepares each return?	US and UK specialists in the same firm	Your firm for one country, a partner firm for the other	Two firms you appoint separately
How many engagement letters?	One covering both returns	Usually two	Two
How many document requests?	One	Often two, sometimes shared	Two
Who decides residence and relief?	The same team, once	Depends on what the two firms agree	Whoever you name as lead, if anyone
Who checks the two returns against each other?	Built into the review	Only if a firm's scope says so	Only if you arrange it
Who do you call with a question about both?	One contact	Usually the firm you started with	You decide which firm to ask

A referral model can work well, especially where the partner firm is long-standing and the two share information routinely. The point to check is whether the cross-check between the returns is written into either firm's scope. If it is not, it falls to you. For a wider look at how to weigh these differences between firms, see how to compare US/UK cross border tax services providers fairly.

Why do the two returns depend on each other?

The US and UK returns depend on each other because each country keeps the right to tax its own people, and the relief that prevents double taxation is claimed on the returns themselves. The US Senate's report on the US–UK income tax convention summarises the treaty's starting point plainly: each country retains the right to tax its residents and citizens as if the treaty had not come into effect (Article 1), with relief from double taxation given through a credit under Article 24.

That leaves three practical links that a single team manages for you.

The credit has to match the tax actually paid

On the US return, the foreign tax credit is generally claimed on Form 1116. The IRS explains in Topic no. 856 that unused foreign tax can be carried back one year and then forward ten years. On the UK return, GOV.UK notes that how much relief you get depends on the double-taxation agreement. If the US return claims UK tax that the UK return later changes, or the UK return claims US tax that was never finalised, the two returns drift apart. Our guide to double taxation relief between the US and UK sets out how each side's relief works.

The tax years do not line up

Most individuals file their US return for the calendar year, which the IRS treats as due on 15 April. The UK tax year ends on 5 April. GOV.UK's Self Assessment deadlines for the 2025/26 tax year are 31 October 2026 for a paper return and 31 January 2027 for an online return and for paying the tax owed, with a second payment on account due on 31 July. A single UK liability can therefore be paid across two US tax years, and someone has to keep a running record of which payment was claimed where.

The account list feeds both sides

The FBAR is required where a US person's foreign financial accounts exceeded $10,000 in aggregate at any time during the calendar year. The IRS states it is due on 15 April, with an automatic extension to 15 October, and is filed through FinCEN's BSA E-Filing System rather than with the tax return. The UK adviser already sees every UK bank, savings and investment account in order to report the income, so when the same team prepares the FBAR, the account list comes from one source.

How one team runs the year: a typical timeline

The calendar below shows how a single engagement usually runs for a US citizen living in the UK. The dates come from the IRS and GOV.UK; the order of work is how a one-team engagement typically plans it, and your own timetable may differ.

1. January–February: one document request covering both countries — UK P60 and payslips, UK and US account statements, pension statements, and a note of days spent in each country.
2. By 15 April: the regular US due date. The IRS allows an automatic 2-month extension to 15 June for US citizens and resident aliens whose home and main place of business are outside the US, but interest runs on any tax unpaid after the regular due date, so the team estimates any US balance in time to pay it.
3. April–June: the US return is finalised, using UK tax figures the team has already reconciled, and the FBAR is prepared from the same account list.
4. After 5 April: the UK tax year closes. The team starts the UK return using the US figures it has just finalised, converted with the same exchange-rate method.
5. Before filing either return: one reviewer compares the two drafts: income by type, tax paid, credits claimed and the residence position.
6. By 31 January: the UK online return is filed and the balance paid. Any change to UK tax is fed back to the US side, in case a credit needs adjusting.

Illustrative example: an American living in Manchester has a UK salary, interest from a UK savings account, and dividends from a US brokerage account she kept after moving. With two unconnected firms, the UK adviser reports the US dividends on the foreign pages and claims credit for US tax withheld, while the US preparer reports the UK salary and claims credit for UK PAYE. Each return is reasonable on its own. But the US preparer uses the UK figures from a P60 that was later corrected, and the UK adviser converts the dividends at a different rate. When one firm prepares both returns, the corrected P60, the dividend figures and the exchange rate are fixed once, and the cross-check catches any figure that does not match before either return goes in.

How can you tell if a provider really prepares both returns?

The simplest way to tell whether a provider genuinely offers one firm for US and UK tax returns is to ask who reviews the two returns against each other, and to ask for that answer in the engagement letter. Firms that work as one team answer this quickly. Firms that refer one side out usually describe a handover instead.

These questions bring out the model behind the marketing:

1. Who prepares each return? Ask whether the US and UK preparers are employed by the same firm, or whether one side is referred to a partner.
2. How many engagement letters will I sign? One letter covering both returns usually means one firm takes responsibility for both.
3. Who makes the residence decision? It should be one named role, not "both firms".
4. Which information returns are included? The FBAR and any other US reporting should be listed by name.
5. Who represents me if either return is questioned? On the US side, the IRS states that enrolled agents, certified public accountants and attorneys have unlimited representation rights, and authority is given on Form 2848. On the UK side, ask who will be authorised as your agent with HMRC.
6. What happens when a figure changes after filing? Listen for a process that updates the other return, not just the one that changed.

These questions sit alongside the general checks for any adviser. For the full list, see how to choose US/UK cross border tax specialists.

When is one firm for US and UK tax returns not the right choice?

One firm is the simpler default for most individuals with income, accounts or pensions in both countries, but it is not the only sensible answer. Two firms can be the better choice where:

• A UK firm already runs your company's accounts, payroll and corporation tax, and you only need a personal US return alongside it.
• You have a long-standing adviser in one country who knows your history, and you would rather add a specialist than move everything.
• You are in the middle of an enquiry or examination in one country and changing representation would cause delay.

In each case the same rule applies: name one adviser as lead for the cross-border positions and put the cross-check in someone's written scope. If you later decide to bring both returns together, switching US UK tax accountants mid-year explains how to move without missing a deadline.

What people get wrong about using one firm

Assuming "we do US and UK" means one team. It may mean a referral. Ask the questions above.

Thinking one firm removes their own responsibility. You remain responsible for your own returns in both countries, whoever prepares them. A single firm reduces the chance of a gap; it does not transfer the liability.

Leaving out the "boring" accounts. A dormant UK savings account or a child's account in a parent's name can still matter for US reporting. Give the team every account, not just the ones that produce income.

Expecting the UK return to be done first. The US return is usually due earlier in the calendar, so the order of work is planned around both deadlines rather than the UK tax year alone.

Treating the first year as typical. The first year with a new firm involves gathering history: prior returns from both countries, the residence position and any elections already made. Later years are usually quicker.

Choosing among the best US/UK cross border tax services providers

When you compare the best US/UK cross border tax services providers, the quality of each return matters, but so does who owns the space between them. Ask each firm the same six questions, compare written scopes rather than headline fees, and look for one name against the cross-check.

US/UK Cross Border Tax — US CPAs and UK tax advisers working as one team; London, Manchester, New York, San Francisco. We prepare US individual tax returns and UK Self Assessment returns in one engagement, together with FBAR and foreign income reporting. If you are an American living in the UK, that is the situation we see most often. To see how the work runs from start to finish, read the US/UK cross border tax preparation process, or ask us for a scoped quote covering both returns.