Best US UK Cross Border Tax Advisor: A Buyer's Checklist

The best US UK cross border tax advisor for you is the one that passes a short written checklist: a named US professional with unlimited IRS representation rights, a UK practice registered with HMRC, both returns prepared on one file, and an engagement letter that lists every form and the fee. No authority publishes a ranking, so the checking falls to the buyer.

This checklist is written for Americans in the UK, British nationals in the US and anyone who files in both countries. It is organised in the order you will meet each decision: before you contact anyone, on the first call, when the engagement letter arrives, during the work and after filing. A note on spelling: "advisor" is the common American form and "adviser" the British one used by HMRC. This article uses the British spelling from here on.

What makes the best US UK cross border tax advisor for your situation?

Fit. A firm can be excellent at US returns for salaried Americans abroad and a poor choice for a landlord with property in both countries. Before comparing firms, write down what you actually have, because that list decides which adviser is best for you and it is the first thing a good adviser will ask for.

Before you contact anyone

1. List your filing position in each country. Citizenship or green card status, where you live, where you work, and which returns you filed last year.
2. List what you own. Bank and investment accounts, pensions in each country, property, company shares and anything held in trust. The more of these cross a border, the more forms are involved.
3. Note anything overdue. Missed US returns, missed FBARs or a late UK return change the kind of adviser you need. Say so at the start.
4. Decide what you want done. Annual compliance only, or planning as well: a move, a property sale, a pension decision.

If you are unsure which returns you need, our overview for Americans in the UK sets out the usual pattern.

What should you ask a cross border tax advisor on the first call?

Ask about people, registration and method. The answers to these six questions separate firms faster than any brochure, and each can be checked afterwards.

5. Who signs the US return, and what credential do they hold? The IRS says enrolled agents, certified public accountants and attorneys have unlimited representation rights, covering audits, payment and collection issues, and appeals. Other preparers can represent only clients whose returns they prepared and signed, and not on appeals or collections.
6. Is the firm registered with HMRC as a tax adviser? GOV.UK says a business that deals with HMRC about someone else's tax affairs for payment must register, through the online service introduced on 18 May 2026. Overseas businesses are included. Registration is phased, so a firm may tell you which window applies to it.
7. Who supervises the firm for anti-money laundering? In the UK the answer is HMRC or a named professional body. Expect the firm to ask you for proof of identity before it starts work.
8. Are both returns prepared by one team? Ask who prepares the UK return, who prepares the US return and who reviews them side by side. A foreign tax credit on one return is only as good as the tax figure on the other.
9. How many clients like me do you act for? You are asking about pattern, not names. A firm that regularly handles UK pensions, ISAs and UK funds held by Americans will say so without hesitation.
10. How will we exchange documents? The IRS reminds US tax professionals that they are legally required to have a written information security plan. A firm that asks you to email passport scans and bank statements as ordinary attachments has answered the question.

Good answers and warning signs

Question	A good answer	A warning sign
Who signs the US return?	A named enrolled agent, CPA or attorney you can look up	"Our team" with no name, or a preparer with a PTIN only
HMRC registration?	Registered, or a clear statement of which window applies	"That does not apply to us" from a firm that files UK returns for a fee
Both returns?	One team, one file, a joint review before filing	The UK side is referred to an unconnected firm you must instruct yourself
Fees?	A fixed fee against a written list of forms	A fee based on a percentage of the refund
Outcome?	An explanation of the rules and the range of results	A promise of a larger refund than anyone else can obtain
Documents?	A secure portal and an identity check	Unencrypted email and no identity check

The fee and outcome rows come straight from the IRS. Topic no. 254, reviewed on 7 October 2026, tells taxpayers to be wary of preparers who claim they can obtain larger refunds than others and to avoid preparers who base their fees on a percentage of the refund.

Reading the engagement letter

The engagement letter is where a conversation becomes a commitment. Read it against these five points before signing.

11. Every return is named. The federal return, any state return, the UK Self Assessment return and each supplementary page or information form.
12. The FBAR is addressed. The IRS explains that the FBAR is filed through FinCEN's BSA E-Filing System and not with the federal tax return, and that it is required when the combined value of foreign accounts exceeded $10,000 at any time during the calendar year. Because it is a separate filing, check that the letter says who prepares it.
13. Extras are priced. Find out now what an additional pension, a property sale or a foreign fund holding adds to the fee. Our guide to what a cross-border tax accountant costs explains the usual fee drivers.
14. The authority is stated. For the IRS, Form 2848 authorises an individual eligible to practise before the IRS to represent you, while Form 8821 lets a designated person or firm inspect or receive your tax information. For HMRC, GOV.UK explains that you can authorise an agent such as an accountant or tax adviser to manage your tax affairs.
15. Deadlines are owned. The letter or its covering email should say when the firm needs your documents to meet each date. For the 2025 to 2026 UK tax year, GOV.UK gives 31 October 2026 for paper returns and 31 January 2027 for online returns. The FBAR is due on 15 April with an automatic extension to 15 October.

While the work is in progress

16. Notice the questions you are asked. Topic no. 254 says good preparers will ask to see your records and receipts. A cross-border adviser who never asks about pension contributions, account balances or days spent in each country is working from too little.
17. Read each return before it is filed. Ask for an explanation of the foreign tax credit, any treaty position and the figures carried between the two returns. Never sign a blank form.
18. Check the signature block. The IRS expects a paid preparer to sign the return and enter a PTIN, and to give you a copy.

After filing

19. Keep everything. Hold copies of both returns, the FBAR confirmation, the workings and the documents you supplied. Topic no. 254 reminds taxpayers that they are ultimately accountable for the accuracy of every item on the return.
20. Know who answers next year. The IRS suggests considering whether the individual or firm will be around for months or years after filing, and choosing someone you can contact if the return is examined. Ask who handles correspondence from the IRS or HMRC and whether that is inside the fee.

Illustrative example: a British engineer who became US resident in March speaks to two firms. The first quotes a low fixed fee for "your US return" within minutes. The second asks for his UK payslips up to the move, his pension statements and the dates of his visits back to the UK, then sends a letter listing a US federal return, a state return, a UK Self Assessment return for the year of departure, an FBAR and a note that a treaty disclosure may be needed. The second quote is higher. It is also the only one that covers the year he actually had. This example is illustrative and does not describe real firms or a real client.

What people get wrong when they look for the best adviser

Shopping on price before scope. Two quotes are only comparable when they list the same forms. Our article on how to compare US/UK cross-border tax services providers includes a scorecard for lining them up.

Treating a ranking as verification. A place on a list is not a credential. The post on what marks out top US UK tax specialists explains which credentials carry authority and which are only a minimum.

Hiring two unconnected firms. A US preparer and a UK accountant who never speak will each produce a defensible return and, together, a double-tax problem. The value is in the join.

Assuming the adviser carries the risk. The HMRC standard for agents holds advisers to integrity and due care, but the return and the liability stay with the taxpayer in both countries.

Waiting for a deadline to choose. A firm taken on a week before a filing date has no time to ask the questions that make a cross-border return right.

Using the checklist

Send the six first-call questions to every firm on your shortlist in writing and compare the replies. A firm that answers plainly, names the people involved and sends a letter listing every form has told you most of what you need. The process each side should then follow is described in our guide to the US/UK cross-border tax preparation process.

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 on one file. We would sooner be tested against this checklist than described as the best, so start at our US UK cross border tax advisor home page or put the questions to us through the contact page.