How to Choose US/UK Cross Border Tax Specialists: 9 Questions to Ask First

To choose US/UK cross border tax specialists, work through three things in order: whether the firm covers both tax systems inside one engagement, whether its US and UK credentials and registrations can be verified on official registers, and whether the engagement letter names every return and report it will file. How to test each of those is set out below, as nine questions to ask before you sign anything.

Most people looking for a cross-border adviser have already had the experience that sends them looking: a UK accountant who could not advise on the US side, or a year where two firms each assumed the other had dealt with something. Almost every firm calls itself a specialist, and the words on a website cannot be verified. Credentials, registrations and the scope of an engagement letter can be.

What do US/UK cross border tax specialists do that a single-country accountant does not?

A cross-border specialist owns the join between two tax systems rather than one side of it. That means deciding which country has the first right to tax each item of income, claiming relief on the other side so the same income is not taxed twice without credit, and handling the fact that the US taxes its citizens on worldwide income wherever they live while the UK taxes on a different basis and a different calendar.

The consequences show up in three places. Timing: the US tax year runs to 31 December while the UK tax year runs to 5 April, so the income and the foreign tax credits being matched sit in different periods. Treatment mismatches: a product that is efficient in one country can be expensive in the other, which is why UK investment funds and the US rules for passive foreign investment companies come up so often for Americans in the UK. And reporting: the US has an information-reporting layer with no UK equivalent, separate from the income tax return itself.

That reporting layer is where single-country engagements most often fall short. The FBAR — the Report of Foreign Bank and Financial Accounts, filed on FinCEN Form 114 — is required where a US person has a financial interest in or signature or other authority over foreign financial accounts whose aggregate value exceeded $10,000 at any time during the calendar year, and the IRS is explicit that it is filed electronically through FinCEN's BSA E-Filing System and not with your federal tax return. Form 8938 is different again: it is attached to the income tax return, and for taxpayers living abroad the thresholds start at more than $200,000 of specified foreign financial assets on the last day of the tax year, or more than $300,000 at any time during the year, for someone filing single — and more than $400,000 and $600,000 respectively for a married couple filing jointly. We set out the difference between the two in FBAR vs Form 8938.

A firm that only prepares the 1040 can be entirely correct on the return and still leave you with unfiled reports. Ask how the reporting layer is handled before you ask about anything else.

How to choose US/UK cross border tax specialists: the nine questions to ask first

Ask these in a first call, in this order. A firm that does this work daily will answer all nine without hesitating.

1. Who on the engagement is qualified in each country, and how is that verified? You want a named US credential and a named UK professional body or HMRC supervision, not a claim that the firm covers both sides.
2. Which specific returns and reports will you file for me this year? A good answer is a list: the federal return, any state return, foreign account reporting where it applies, and the UK Self Assessment return.
3. What is explicitly out of scope? Corporate filings, trust reporting, gift tax returns and state returns are common exclusions that clients assume are included.
4. How do you handle the timing mismatch between the US and UK tax years? Listen for a concrete method rather than reassurance.
5. Who reviews the cross-border positions, and does the same person see both returns? If the US and UK work never meets in one review, you are buying two single-country engagements in one invoice.
6. What are your fees, and what triggers additional charges? Fixed fee, hourly or per return — and what happens when a new account, a property or a second country appears mid-year.
7. Who signs the US return, and will you represent me if the IRS or HMRC opens an enquiry? Representation rights are a matter of credential, not willingness.
8. How do you want authorisation set up? The right answer involves the proper authorisation forms, never your own login credentials.
9. Who do I speak to in August, and what response time should I expect? Cross-border problems arrive outside filing season, and the IRS itself suggests choosing a preparer you can contact after the return is filed.

If the answers to questions two and three are vague, stop there. Scope disputes are the most common reason a cross-border relationship goes wrong, and they are avoidable in writing.

How do you check a US preparer's credentials?

The US side is checkable. Anyone paid to prepare federal tax returns must hold a Preparer Tax Identification Number (PTIN), and must sign the return and include that PTIN on it. The IRS runs a public Directory of Federal Tax Return Preparers listing those who hold a credential it recognises or an Annual Filing Season Program record of completion.

The distinction that matters most is representation rights, which the IRS sets out in its guide to preparer credentials and qualifications:

Preparer type	Rights before the IRS
Attorney, certified public accountant (CPA) or enrolled agent	Unlimited representation — may represent you on audits, payment and collection matters, and appeals
Annual Filing Season Program participant	Limited — only for returns they prepared and signed, and only before revenue agents and customer service representatives
PTIN holder with no credential	No authority to represent clients before the IRS, for returns filed after 1 January 2016

An enrolled agent is licensed directly by the IRS after passing a three-part Special Enrollment Examination, and must complete 72 hours of continuing education every three years. A certified public accountant (the US equivalent of a chartered accountant) is licensed by a US state. Either is a legitimate answer; "our US team", with no named credential, is not.

The IRS also publishes plain warnings in Topic no. 254: avoid preparers who base fees on a percentage of your refund or who offer to deposit any part of your refund into their own accounts, never sign a blank tax form, and make sure you receive a copy of the completed return. A preparer who will not sign the return they prepared — a ghost preparer, in the IRS's own phrase — should end the conversation.

How do you check a UK adviser's registration?

The UK side became more checkable in 2026. Two separate requirements apply, and a cross-border firm should satisfy both.

Anti-money laundering supervision

Accountancy service providers must be supervised for anti-money laundering purposes, either through a designated professional body or by registering with HMRC directly. GOV.UK is blunt about it: a business must not trade without registering under the regulations, and trading while not registered is a criminal offence. You can look a firm up on HMRC's Supervised Business Register, bearing in mind HMRC's caveats: a newly registering business may not appear immediately, a firm may be listed under a different name, and being listed is not an endorsement.

Mandatory tax adviser registration

Under rules introduced this year, an adviser who interacts with HMRC about someone else's tax affairs and is paid for it must register with HMRC for an agent services account and meet minimum standards. HMRC's guidance sets the windows out in stages: most advisers need to register now; those who already have a Self Assessment or Corporation Tax agent account register from 18 August 2026; third-party payroll service providers who do not interact with HMRC in any other way register from 18 November 2026; and financial services organisations register from 31 December 2026. Once a window opens, an adviser has three months to apply. HMRC warns that an adviser who needs to register and does not may be unable to interact with HMRC on behalf of clients, and may be subject to sanctions.

So ask directly in 2026: has the firm registered, or when does its window open? Alongside registration sits the HMRC standard for agents, last updated on 9 February 2026, which requires integrity, professional competence and due care, professional behaviour, and tax planning based on a realistic assessment of the facts and a credible view of the law. Where an agent falls short, HMRC may restrict access to agent services, issue conduct notices, or refuse to deal with the agent at all.

Scope and deadlines: what the two systems demand

Ask a prospective adviser to walk you through the year. If the dates below are not immediately familiar to them, they do not do this work often.

Obligation	Standard date	Extension
US federal income tax return (calendar year)	15 April	Automatic two months to 15 June where your tax home is abroad; to 15 October on Form 4868
FBAR (FinCEN Form 114)	15 April	Automatic to 15 October — no request needed
UK Self Assessment — telling HMRC you need to file, 2025 to 2026 tax year	5 October 2026	—
UK Self Assessment — paper return	31 October 2026	—
UK Self Assessment — online return and payment	31 January 2027	—

Two points advisers are asked about constantly. The US extension to 15 June for taxpayers whose tax home is abroad is automatic, but it extends the filing date rather than the payment date: the IRS states that interest still runs on any tax unpaid by the regular April due date. The UK dates above are those published on GOV.UK for the 2025 to 2026 tax year, with the online return and the payment both due by 11:59pm on 31 January 2027.

Because the US year closes on 31 December and the UK year on 5 April, a firm working across both needs a schedule that gathers information once and uses it twice. Ask what that looks like in practice. Our own view of the annual cycle is in the US filing requirements for Americans in the UK.

Fees, engagement letters and the scope trap

Cross-border fees are higher than single-country fees because there are more returns, more information to gather and a review spanning both systems. What matters is not the headline number but what it buys. Get the following in writing before you engage:

• The exact returns and reports included for the year, named by form number where one exists.
• What is excluded, and the basis on which excluded work would be charged.
• Whether enquiry or examination support is included, and at what rate if not.
• How new accounts, a property purchase or a mid-year move between countries affect the fee.
• Who prepares and who reviews, and the turnaround once you have supplied complete information.

Illustrative example: an American who moves to Manchester in March engages a UK firm for Self Assessment and a US firm for the federal return. The UK firm handles the UK return; the US firm prepares the 1040. Neither is told about two accounts opened on arrival, because neither asks about the other country's paperwork, and the fixed fee each quoted covered only the return named in its own letter. The reporting is discovered the following year. Nothing was done wrong inside either engagement — the gap was between them.

Closing that gap is what a scoped engagement letter is for. Where reporting has already been missed, the route is usually a defined catch-up procedure: see our guide to the streamlined procedures.

Authorisation: how an adviser is permitted to act for you

Authorisation tells you what an adviser can actually do on your behalf, and it is where a small number of firms cut corners.

In the US, Form 2848, Power of Attorney and Declaration of Representative, authorises an individual who is eligible to practise before the IRS to represent you, including receiving and inspecting your confidential tax information. Where you want someone only to receive information rather than to represent you, Form 8821, Tax Information Authorization, is the narrower alternative. Note that Form 2848 requires a representative eligible to practise before the IRS — which returns you to the credential question above.

In the UK, an agent is authorised through an agent authorisation such as form 64-8 or, for certain services, a digital handshake: the agent sends you a link, you sign in to your own tax account and approve it, and the link expires after 21 days. GOV.UK is explicit on two points no legitimate adviser will ask you to breach — you must not give your sign-in credentials to your agent or anyone else, and you are responsible for your own tax affairs even if you authorise someone to act on your behalf. The IRS says the same thing in different words: you are ultimately accountable for the accuracy of every item reported on your return.

What people get wrong when choosing a cross-border firm

Four patterns come up repeatedly, and all four are avoidable at the point of hiring.

Treating "we work with US clients" as the same thing as dual coverage. Many UK firms have American clients and prepare only the UK side, quite properly. Ask which returns the firm itself files, and for which country.

Choosing on price without comparing scope. A cheaper quote covering one return is not cheaper than a quote covering four. Compare the lists, not the numbers.

Assuming investment advice has been checked against both tax systems. A product that is sensible for a UK taxpayer can carry heavy US reporting consequences, and the reverse is true for US products held by someone who becomes UK resident. If a firm advises on structure, ask who signs off the position in the other country.

Leaving earlier years out of the first conversation. If previous years are incomplete, say so on the first call. It changes the engagement, and it is far cheaper handled deliberately than discovered in year two.

Making the decision

Run the nine questions past two or three firms and the differences appear quickly. One will send a scoped engagement letter naming the forms; another will send a price. Neither the IRS nor HMRC will judge how you chose, but both will hold you to the outcome — which is why the checkable items, credentials, supervision, registration and scope, deserve the most weight.

US/UK Cross Border Tax — US CPAs and UK tax advisers working as one team; London, Manchester, New York, San Francisco. Our cross border tax service page sets out what an engagement covers, Americans in the UK covers the most common position, you can read about the firm, and you can ask for a fee quote through contact. If you are weighing firms generally, the checks above apply to any of the US/UK cross border tax specialists you are considering, including us.