Accountants for US and UK Taxes: One Firm or Two?

Choosing accountants for US and UK taxes comes down to one question: who owns the seam between the two returns? One firm covering both sides owns it by default. Two single-country firms can work well, but only if one is named as lead, they are allowed to speak to each other, and every cross-border decision has an owner in writing.

This is not a question about competence. A good UK accountant and a good US preparer will each do their own work correctly. The risk sits in the space between them, where nobody has been asked to look — and that space is where double tax relief, information reporting and residence decisions live.

Should you use one firm or two accountants for US and UK taxes?

For most individuals with income, accounts or pensions in both countries, one firm is simpler and the decisions get made once. Two firms are worth keeping where each brings depth you actually need — a UK firm that handles your company's accounts and payroll, say, alongside a US preparer for your personal return — and where someone is willing to coordinate them.

Your position	Usually works best	Why
Employment income in one country, ordinary savings in the other	One firm	Relief and reporting decisions are routine but must match across both returns
UK investments, an ISA, or UK-domiciled funds	One firm	US reporting of those holdings only surfaces if someone looks at the UK portfolio
Pensions being drawn or transferred	One firm	Treaty positions have to be applied consistently on both returns
A UK business plus a personal US return	Either	The company work can sit with a UK firm if the personal seam is owned by one of them
An existing long-standing adviser in one country	Two, with a lead	Keeps institutional knowledge, provided the coordination rules below are set
An enquiry or examination in one country	Depends on representation	Only certain credentials can represent you before the IRS; HMRC authorisation is separate

Where a two-firm arrangement actually breaks

In practice, problems cluster in five places. None of them is exotic, and all of them are avoidable if the split is agreed in advance.

Double tax relief and timing

The UK tax year ends on 5 April and the US tax year on 31 December, so the two firms are looking at different periods. The IRS requires that a foreign tax has been paid or accrued and meets the other tests before it is creditable, so the firms have to agree which UK tax belongs against which US year. It matters beyond the current return: unused foreign taxes can be carried back one year and carried forward ten. On the UK side, relief for US tax is claimed as Foreign Tax Credit Relief, 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 sets out each side.

Information returns

US information returns have no UK equivalent, so a UK firm has no reason to prepare them and a US firm only prepares them for accounts it is told about. The FBAR is required where a US person's foreign financial accounts exceeded $10,000 in aggregate at any time during the calendar year, and the IRS states it is filed through FinCEN's BSA E-Filing System and not with your federal tax return. Someone has to own the account list.

Residence

UK residence decides what the UK taxes; US citizenship or residence decides what the US taxes. If each firm reaches its own conclusion without telling the other, the returns can be internally consistent and mutually contradictory.

Currency

One return is in pounds and the other in dollars. Two firms using different conversion methods will produce two versions of the same income, which is hard to explain later.

Amendments

A correction in one country often changes relief in the other. With two firms, an amendment can be filed in one country and never reach the other.

Illustrative example: an American in London pays a UK balancing payment in January and a payment on account in July. Her UK accountant reports the UK year to 5 April; her US preparer reports the calendar year. Each is correct within its own return, but neither confirms which UK payments have been claimed against which US year. Two years later, a foreign tax credit is claimed twice for the same UK tax in one year and not at all in another. Nothing was wrong inside either engagement — the mistake was in the handover that nobody owned.

What one firm gives you that two cannot by default

One firm covering both sides gives you a single set of facts and one review. In practice that means: residence settled once; a single decision on where each item of income is relieved; one currency method; one account list feeding the US information returns; one timetable across both filing seasons; and one person to ask in August. The sequence is described step by step in the US/UK cross border tax preparation process, and the underlying work in what US UK tax accountants do.

None of that is impossible with two firms. It simply has to be arranged deliberately rather than assumed.

How do you make two firms work together?

If you are keeping both, set these seven things in writing before the filing season starts.

1. Name the lead. One firm owns the cross-border positions and has the final say where the two disagree.
2. Split the scope explicitly. List every return and report and put a firm's name against each, including the FBAR and any state return.
3. Give written permission to talk. Both firms should be authorised to share your information with each other directly, rather than through you.
4. Agree one data pack. One set of documents, one account list with highest balances, one travel calendar, shared with both.
5. Fix the sequence. Agree which return is drafted first, when the figures are exchanged, and the date the second firm needs them.
6. Assign the elections. Credit or exclusion, currency method, treaty positions: one firm decides and both apply the result.
7. Book a joint review. A short call with both firms and the two draft returns side by side catches most mismatches before filing.

If either firm will not do numbers three, five or seven, that tells you something useful before you commit to the arrangement.

What does each firm need from the other?

Most coordination failures are really missing information rather than disagreement. It helps to write down, once, exactly what crosses between the two firms and when. The list below is a reasonable starting point for an individual with income in both countries.

The US preparer needs from the UK firm	The UK firm needs from the US preparer
UK tax paid, with the dates of each payment, including balancing payments and payments on account	US tax paid or accrued on income the UK also taxes, by date
Employment figures from the P60 and any taxable benefits	US income by type — wages, dividends, interest, capital gains — for the UK tax year
The UK residence conclusion and whether a split year applies	The US filing status and any elections made
Pension contributions, employer contributions and any withdrawals	Treaty positions taken on the US return that affect UK reporting
The full list of UK accounts and investment holdings	Confirmation of which income has already been relieved in the US

Two details are worth pinning down in advance. First, the dates: because the UK tax year ends on 5 April and UK payments fall due on 31 January and 31 July, a single UK liability can be paid across two US tax years, so payment dates matter more than the UK year's total. Second, the direction of travel: agree which firm reports a figure first, so the second firm is working from a final number rather than an estimate that later moves.

What if you are already mid-arrangement?

If two firms are already in place and you are not sure the seam is covered, three questions usually settle it. Ask each firm which cross-border positions it believes it owns; ask both for their list of the returns and reports they file for you; and ask whether they have ever exchanged figures directly. If the two scope lists do not cover the FBAR, the information returns and the relief claims between them, that is the gap, and it is easier to close before a filing season than during one.

Authorisation: who each tax authority will actually deal with

Authorisation is where two-firm arrangements sometimes fall over administratively. In the UK, authorisation is given per tax: form 64-8 covers separate regimes such as Self Assessment, PAYE, VAT and Corporation Tax, so different firms can hold different ones. But GOV.UK is explicit that any new authority received will replace any existing authority. If a second firm submits a request for a tax your first firm already holds, the first firm can lose access without anyone intending it.

For certain HMRC services authorisation is given by a digital handshake from your own tax account, and GOV.UK is clear that you must not give your sign-in credentials to an agent. On the US side, Form 2848 authorises someone eligible to practise before the IRS to represent you, and Form 8821 authorises information access only. Representation is also a matter of credential: the IRS explains that attorneys, certified public accountants and enrolled agents have unlimited representation rights, while others are limited or have none.

Does two firms cost more?

Not necessarily in fees, but often in duplication. Both firms gather the same information, both ask the same questions, and the cross-border work is either done twice or skipped. The expensive outcome is rework: an amended return in one country usually means revisiting the other.

Compare scoped fees rather than headline prices. Ask each firm for the list of returns and reports it will file, what is excluded, and what it assumes the other firm is doing. Two quotes that each exclude the same task are not cheaper — they are a gap with a price attached. The IRS also suggests practical checks when choosing any preparer in Topic no. 254, including avoiding fees based on a percentage of your refund and choosing someone you can contact after filing.

What people get wrong about this decision

Assuming the firms are talking. Unless you have authorised it and asked for it, they usually are not.

Splitting by country instead of by question. Countries do not divide the work cleanly; residence, relief and reporting all span both.

Adding a second firm mid-season. Authorisation changes and half-finished figures at the same time is how deadlines get missed.

Treating the lead as a formality. The lead role only works if it comes with the final say and the time to exercise it.

Making the decision

Start with a plain question to each firm you are considering: which returns will you file, and who owns the positions that sit between them? One firm should answer for the whole file. Two firms should give you two answers that fit together without a gap. If they do not, the choice has been made for you. The wider checks to run on any firm are in how to choose US/UK cross border tax specialists.

US/UK Cross Border Tax — US CPAs and UK tax advisers working as one team; London, Manchester, New York, San Francisco. If you want one engagement covering both sides, our accountants for US and UK taxes handle the returns and the reporting together; Americans in the UK covers the most common position. If you would rather keep your existing adviser in one country, we are happy to work alongside them on the terms above — ask for a fee quote either way.