US/UK Cross Border Tax Services Onboarding: What the Best Providers Do in Your First 30 Days

US/UK cross border tax services onboarding is the first month of work before any return is prepared: identity checks and a signed scope, IRS and HMRC authorisations, a review of what has already been filed in both countries, and a written map of every US and UK deadline for the next twelve months. The best providers finish that groundwork in about 30 days, so the filing season starts with no surprises.

Choosing a firm is the step most people think hard about. The weeks after signing get less attention, yet they decide whether the first set of returns goes smoothly. A firm that prepares both US and UK returns has to collect two sets of authorisations, two sets of records and two filing histories, and it has to spot problems in either country before they turn into penalties. This guide sets out what that first month should look like, week by week, so you can tell whether a new adviser is doing it properly. If you are still choosing, read our guide on how to choose US/UK cross border tax specialists first.

What happens during US/UK cross border tax services onboarding?

During US/UK cross border tax services onboarding, the firm confirms who you are, agrees in writing exactly which returns and reports it will handle in each country, gets permission to see your records at the IRS and HMRC, and reviews your recent filings for gaps. The output is a plan, not a return.

That sequence matters because cross-border work depends on history. UK payments on account are based on last year's bill. Unused US foreign tax credits carry from one year to the next. The FBAR account list should match the accounts that produce income on the UK return. A provider that skips straight to this year's return without looking back can file a technically correct return that is inconsistent with everything filed before it.

The table below is the 30-day outline we would expect any serious provider to follow. Exact timing varies with how quickly records arrive, but the order should not.

Week	What the provider does	What you do	What you should have at the end
Week 1	Identity checks, conflict check, engagement letter, initial questionnaire	Send ID and proof of address, sign the letter, complete the questionnaire	A signed scope naming every US and UK return and report
Weeks 1 to 2	Sends IRS Form 8821 or 2848 and an HMRC authorisation request	Sign the IRS form, accept the HMRC link within 21 days	Working access to IRS and HMRC records
Weeks 2 to 3	Pulls IRS transcripts and HMRC records, reviews prior returns and FBARs	Send prior returns, the previous adviser's file and account details	A complete filing history for both countries
Week 3	Gap review: missed filings, residence, accounts, pensions, funds	Answer follow-up questions about moves, days and accounts	A written note of any gaps and the recommended fix
Week 4	Builds the deadline map and the document request for the season	Agree the timetable and who sends what, and when	A 12-month calendar of US and UK dates

Week 1: identity checks, conflicts and the engagement letter

The first week is administrative, and it should be. UK firms that provide accountancy and tax services are covered by the Money Laundering Regulations. GOV.UK explains that customer due diligence means identifying customers and checking they are who they say they are, typically using your name, a photograph on an official identity document, your residential address and your date of birth. The firm must also understand the purpose and intended nature of the relationship, which is why you may be asked where your income and assets come from.

Alongside the identity checks, the firm runs a conflict check and sends an engagement letter. For cross-border clients the engagement letter is the most important document of the month. It should list, country by country, each return and report the firm will prepare. On the US side that usually means the federal return and any information returns and reports, such as the FBAR. On the UK side it means the Self Assessment return and any supplementary pages. If something is outside scope, such as a US state return or a trust return, the letter should say so plainly.

The questionnaire that shapes everything else

Expect a detailed first questionnaire: citizenship and immigration status in each country, where you have lived and when you moved, family members, employers, self-employment, property, pensions, investments and any trusts or companies. It feels long, but each answer decides which forms apply. A move date, for example, affects UK residence and whether split-year treatment might apply, while a UK investment fund may raise US reporting questions that a domestic preparer would never ask.

How does a new provider get access to your IRS and HMRC records?

A new provider gets access to your records by asking you to sign an IRS authorisation form and to accept an HMRC authorisation request. Neither country lets an adviser see your records without your permission, so this step depends on you responding quickly.

On the US side: Form 8821 or Form 2848

The IRS offers two forms, and they do different jobs. Form 8821 authorises a designee to inspect or receive your confidential tax information for the types of tax and the years or periods listed on the form. It is an information authorisation. Form 2848 is a power of attorney: the IRS says it is used to authorise an individual to represent you before the IRS, and that individual must be eligible to practise before the IRS. A good provider tells you which form it is asking for and why. Either form can be submitted online, where the IRS accepts several kinds of electronic signature; forms sent by fax or post need a handwritten ink signature.

Once the authorisation is processed, the firm can order IRS transcripts. The IRS lists the transcript types: the tax return transcript and the record of account transcript cover the current and three prior tax years, the wage and income transcript covers the current and nine prior tax years, and a verification of non-filing letter confirms that the IRS has no record of a processed return for a year. For a client who is not sure what was filed while abroad, these transcripts are often the fastest way to find out.

On the UK side: the digital handshake

For many HMRC services, the firm signs in to its agent services account, chooses to ask a client to authorise it and sends you an authorisation request link. You accept by signing in with the Government Gateway user ID you used for that tax service. GOV.UK says the client should respond within 21 days and that the link expires after 21 days; if it is not used in time, the agent has to start the request again. Unanswered links are one of the most common reasons onboarding runs late.

Weeks 2 and 3: records and the gap review

With access in place, the provider assembles your filing history for both countries and compares it with your life over the same years. If you are moving from another adviser, ask for their complete file early; our guide to switching US UK tax accountants mid-year covers what to request and how to move authorisations without missing a date. The document side of the season is covered in the US/UK cross border tax preparation checklist.

The gap review is where a specialist earns the fee. It is not a re-preparation of past returns. It is a structured check of the points that most often go wrong for people with ties to both countries:

1. Missing US returns. US citizens and green card holders file on worldwide income wherever they live. Transcripts show which years were processed.
2. Missing FBARs. The IRS says the FBAR is required where the aggregate value of foreign financial accounts exceeded $10,000 at any time during the calendar year. It is filed electronically through FinCEN's BSA E-Filing System, not with the tax return, so it is easy to miss.
3. UK residence. GOV.UK notes that you may be automatically UK resident if you spent 183 or more days in the UK in the tax year, but the full statutory residence test has several other tests. Arrival and departure years need particular care.
4. Accounts that behave differently in each country. ISAs, UK pensions and UK investment funds can be treated very differently on the US return from the way they are treated in the UK.
5. Consistency of relief. The UK tax claimed as a credit on the US return should match what the UK return shows, and vice versa.

When the review finds a gap, the provider should explain the options in writing before preparing anything new. For US taxpayers living abroad whose failures were non-willful, one route the IRS offers is the Streamlined Foreign Offshore Procedures, which involve tax returns for the most recent three years with passed due dates and FBARs for the most recent six years, together with a signed non-willful certification on Form 14653. Whether that route fits depends on the facts; our guide to the Streamlined Foreign Offshore Procedures explains eligibility in full.

Week 4: the deadline map

The final deliverable of the month is a calendar that puts every US and UK date in one place. The two tax years do not align, which is why dual filers miss dates that each country's own residents rarely think about.

Obligation	Date	Source
UK paper Self Assessment return for 2025 to 2026	31 October 2026	GOV.UK
UK online Self Assessment return and balancing payment for 2025 to 2026	31 January 2027	GOV.UK
UK payments on account	31 January and 31 July	GOV.UK
US federal return for calendar year 2026	15 April 2027, or 15 June 2027 with the automatic two-month extension for those who qualify as living abroad	IRS
US return with a Form 4868 extension	15 October 2027 to file; tax is still due by the original date	IRS
FBAR for calendar year 2026	15 April 2027, with an automatic extension to 15 October 2027	IRS / FinCEN

The UK dates are from GOV.UK's Self Assessment deadlines page and its guidance on payments on account, which also says payments on account are not needed if last year's Self Assessment bill was under £1,000 or more than 80% of your tax was paid in other ways. The US dates follow the IRS rules on the automatic two-month extension for citizens and residents abroad, which still charges interest on tax paid after the regular due date, and on filing extensions, which extend the time to file but not the time to pay. A good deadline map also notes which figures cross from one return to the other, so the return that supplies a number is finished before the one that uses it.

Illustrative example: an American who moved to London in 2024 signs with a new cross-border firm on 2 November 2026. In week one the firm completes identity checks and a scope covering the US federal return, the FBAR and the UK Self Assessment return. In week two the client signs Form 8821 online and accepts the HMRC link. Transcripts in week three show the US returns were filed but no FBARs were, even though the client's UK accounts exceeded $10,000 in aggregate. The firm sets out the options in writing, and the deadline map shows the UK online return for 2025 to 2026 due on 31 January 2027, because the paper deadline of 31 October 2026 has already passed.

What do the best US/UK cross border tax services providers do differently?

The best US/UK cross border tax services providers do the same onboarding steps as everyone else but finish them in writing, in order, and with both countries looked at together. The difference shows in a few habits:

• One scope for both countries. A single engagement letter names every US and UK return and report, so nothing falls between two firms.
• They explain the authorisation they ask for. You know whether you are signing an information authorisation or a power of attorney, and for which years.
• They look backwards before forwards. Gaps in past filings are found in month one, not after this year's return has been filed on top of them.
• They write things down. You receive a gap note and a deadline map, not just verbal reassurance.
• They give you one contact. Questions that touch both returns go to one person, who can answer for both.

That is how we approach new clients at US/UK Cross Border Tax — US CPAs and UK tax advisers working as one team, with offices in London, Manchester, New York and San Francisco. You can read more about how the team works on our page about the best US/UK cross border tax services providers and what to expect from us. For the stages that follow onboarding, see the US/UK cross border tax preparation process from start to finish.

What people get wrong about onboarding

Most onboarding delays are avoidable. These are the most common patterns.

• Leaving the HMRC link in the inbox. The request expires after 21 days, and a new one has to be issued.
• Assuming the old adviser filed everything. Transcripts and the previous file show what was actually filed. FBARs, filed separately from the return, are the item most often missing.
• Sending this year's documents only. The provider needs last year's returns in both countries to calculate payments on account, carry forward credits and check consistency.
• Treating the engagement letter as a formality. If a return is not named in the scope, assume nobody is preparing it.
• Starting in January. Onboarding takes weeks. Starting in the autumn leaves room to fix problems before the UK online deadline on 31 January.

Getting started

If you are about to appoint a new firm, ask at the first meeting what you will have in writing at the end of the first 30 days. A clear answer, with a named scope, both authorisations, a gap review and a deadline map, is a good sign that the rest of the year will run the same way. You can see what our US and UK individual tax return service covers, how we handle UK Self Assessment for Americans, and how we help clients catch up through the IRS streamlined procedures. When you are ready, book a consultation and we will set out the onboarding plan for your situation.