Which Tax Return To File First, The US Or The UK?

For most Americans living in the UK, the UK return is prepared first and the US return second: the US return needs a UK tax figure to compute the foreign tax credit, while the UK return needs nothing from the US side. The complication is that the UK tax year runs 6 April to 5 April while the US tax year is the calendar year, so every UK year straddles two US years — and the sequencing, not the arithmetic, is where most people lose money.

Key takeaways

• The UK tax year runs 6 April to 5 April; the US tax year is the calendar year. One UK year sits across two US years, and no UK document lines up with a US return.
• Prepare the UK return first in the ordinary case. The dependency runs one way: Form 1116 needs a UK number, Self Assessment needs nothing American.
• The order reverses where the UK position depends on a US figure — chiefly when relying on the foreign earned income exclusion, or where US-source income is taxed in the US first.
• The paid-versus-accrued choice on Form 1116 decides which US year a UK tax belongs to. Choose accrual and you must follow it in all later years, and only on a timely filed original return.
• Unused foreign tax credits carry back one year and forward ten.
• An extension of time to file is never an extension of time to pay; interest runs from the original due date.

Why the two calendars will not reconcile

The gov.uk Self Assessment deadlines page sets out the cycle: UK 2025-26 began on 6 April 2025 and ended on 5 April 2026, with the online return and the balancing payment both due by 31 January 2027. The US year is January to December, and for the 2025 tax year the Form 1040 due date was 15 April 2026, as IRS Topic 301 states.

The consequence is underestimated: a single UK tax year feeds two different US returns. Of the 365 days in UK 2025-26, 270 fall in US calendar 2025 and 95 in US calendar 2026. Nothing HMRC sends you is cut to a calendar year, so every cross-border file contains a translation layer — and its quality separates a defensible foreign tax credit from a guess.

This article goes deep on the sequencing decision itself; for the month-by-month view of the whole compliance year, our pillar guide for accountants for US and UK filers carries the full calendar.

Why the UK return goes first

The dependency is asymmetric, and that is the whole argument.

A UK Self Assessment return is computed under UK rules on UK-year figures. It does not ask what you paid the IRS. Even UK-side double tax relief is driven by US tax on US-source income — a narrow part of the picture for someone living and earning in Britain.

A US return is the opposite. Form 1040 taxes worldwide income, and the relief that stops the same pound being taxed twice is the foreign tax credit on Form 1116, which needs a UK tax amount, correctly characterised and correctly allocated to a US year. You cannot complete Form 1116 without the UK number; you can complete the UK return without the US one. So the default order is to finalise UK income and UK tax, then build the US return on top — which also means filing Self Assessment early rather than in the January crush, because the US clock does not wait for HMRC.

When the order reverses

Three situations flip the sequence:

• You are relying on the foreign earned income exclusion. If the exclusion rather than credits is doing the work, the US computation has its own logic and may need modelling first; our comparison of FEIE versus the foreign tax credit covers that choice.
• You have significant US-source income taxed in the US first. US rental, partnership or certain pension income is taxed in the US and the UK side then gives relief for that tax, so the UK return waits on a US number.
• Your first or last year of UK residence. In an arrival or departure year both sides move at once and neither can be finished alone; the moving-year US and UK return guide covers it.

The sequencing table

| Task | Which system | When | What it needs from the other side |

|---|---|---|---|

| Register for Self Assessment (first UK year) | UK | By 5 October after the UK year ends | Nothing |

| Gather payslips, P60, P11D, investment data | UK | From late April; P60 due by 31 May | Nothing |

| File the UK return online | UK | By 31 January after the UK year ends | Nothing, ordinarily |

| Pay UK balancing payment and first instalment | UK | 31 January | Nothing |

| Pay UK second payment on account | UK | 31 July | Nothing |

| Estimate and pay the US liability | US | By 15 April — interest runs from here | A UK tax estimate, even if provisional |

| File Form 1040 with Form 1116 | US | 15 June automatic if abroad; 15 October with Form 4868 | The UK tax figure for the period |

| Correct a changed UK figure | US | When the final UK amount is known | Final UK calculation or payment proof |

Apportioning a UK tax year across two US years

Two distinct things get split, and conflating them is the commonest technical error.

Income is reported on a calendar-year basis. Form 1040 reports what you earned between 1 January and 31 December, so UK employment income must be recut from UK-year records into calendar months. Self-employment, rental and investment income need the same treatment.

Foreign tax is allocated according to your Form 1116 basis. On the paid (cash) basis — the default — a UK tax belongs to the US year in which it was actually paid or withheld. On the accrual basis, it belongs to the year in which it accrued. That is where the straddle bites.

Why a P60 will not do the job

A P60 shows the tax paid on your salary for the tax year 6 April to 5 April, and gov.uk confirms employers must provide it by 31 May. It is an excellent document and the wrong shape: two P60s overlap any given US year, and together they are fifteen months of data.

The usable source is the monthly payslip run, because payslips are dated. Twelve calendar-month payslips give gross pay, PAYE withheld and National Insurance for exactly the US year.

Evidence to keep

Build the file as you go, not in October as the extension expires:

• Every payslip for the calendar year, in date order, with both overlapping P60s as a cross-check.
• The HMRC tax calculation for each UK year the US year touches.
• Date and amount for every payment to HMRC — bank statements or the HMRC account view, not the return.
• Your exchange-rate working. On the paid basis, the Form 1116 instructions direct you to the rate in effect on the day you paid or the day the tax was withheld; on the accrual basis, generally the average rate for the year to which the taxes relate.
• A one-page note on the apportionment method used, so next year's return is consistent with this one.

That last item matters more than it looks. The published guidance gives no UK-specific apportionment formula; what is expected is a reasonable method, documented and applied consistently.

The paid-versus-accrued election

This is the most consequential choice on a US-UK return, and it is made by ticking a box.

Paid basis. You claim the credit in the US year the UK tax was actually paid or withheld. PAYE withheld monthly maps neatly onto calendar months, which is why the paid basis suits straightforward employees. The problem is the balancing payment: UK tax for 2025-26 paid on 31 January 2027 is creditable in US calendar year 2027 — well after some of the income it relates to was reported.

Accrual basis. You claim the credit in the year the tax accrued, which generally tracks the year the income arose. With meaningful non-PAYE income this matches tax to income far better and avoids the January lag.

The constraints are strict, and IRS Publication 514 is explicit on each:

• It is made by checking the "Accrued" box in Part II of Form 1116 on a timely filed original return — never on an amended return.
• It applies to all foreign taxes qualifying for the credit; you cannot take some when paid and others when accrued.
• Once made, you must follow it in all later years and credit foreign taxes in the year they accrue.

Treat it as a one-way door: make it deliberately, in the first year it is available, with the next five years in view — not as a tidy-up in a year when the timing happens to suit.

Two further accrual rules bite later. If accrued taxes are not paid within 24 months of the close of the year they relate to, the credit previously claimed must be reduced, and no credit is allowed until payment. And where accrued taxes when paid differ from the amount credited, that is a foreign tax redetermination: Publication 514 requires you to notify the IRS, generally on Form 1040-X.

Carrying excess credits

Where UK tax exceeds the US tax on the same income, the excess is not wasted: unused foreign taxes carry back one year and forward ten years following the year in which they arose. Publication 514 also allows ten years to claim a refund of US tax where you find you paid or accrued a larger foreign tax than you credited. Tracking carryovers by income category, year by year, is unglamorous and valuable; the mechanics are in our foreign tax credit carryover guide.

When the UK liability is not known by the US deadline

This is the normal state of affairs, not an edge case. A UK year ending 5 April 2026 need not be filed until 31 January 2027 — but the US 2026 return is due 15 April 2027, and payment is expected by then.

The extensions available. US citizens and resident aliens abroad on the regular due date get an automatic 2-month extension to file, moving 15 April to 15 June, per the IRS. Filing Form 4868 before that expires takes you to 15 October. Form 2350 separately serves those abroad who expect to file Form 2555 and need time to meet the residence or physical presence tests.

What extensions do not do. The IRS states that an extension of time to file is not an extension of time to pay, and that you will owe interest if the tax owed is not paid by the original due date. The guidance for Americans abroad repeats it: even with the automatic extension, you will pay interest on any tax unpaid at the regular due date.

So the practical sequence is: estimate, pay, then file. Build a provisional UK figure in March or April from payslips and known income, compute the US position on it, and pay anything due by 15 April. File on the 15 June or 15 October date with better numbers — our note on paying the IRS from abroad covers getting the money there.

Then correct. On the paid basis you report what you actually paid in the year, so a provisional estimate creates no redetermination. On the accrual basis, a difference between the accrued amount credited and the amount eventually paid is a redetermination requiring notification, generally on Form 1040-X.

Payments on account, and why they confuse the US picture

UK payments on account are instalments towards the next year's bill, due by midnight on 31 January and 31 July, each usually half of the tax owed the previous year. gov.uk sets out two exceptions: you do not make them if last year's tax was under £1,000, or if more than 80% of it was paid at source, typically through PAYE.

The confusion is structural. A payment made on 31 January 2027 is, in UK terms, partly a balancing payment for 2025-26 and partly an instalment against 2026-27. One bank debit, two UK years — and on the paid basis all of it lands in US calendar 2027. Splitting it between UK years, then deciding which US year it credits, is the work that gets skipped.

Three disciplines prevent it:

• Reconcile each HMRC payment to the UK year it settles before touching the US return.
• Do not treat a payment on account as creditable against the income it anticipates unless you are on the accrual basis and the tax has genuinely accrued.
• You can ask HMRC to reduce payments on account using form SA303 or your online account if income has fallen — which changes your cash-basis US credit, so model both sides first.

A worked cycle, in date order

An illustrative UK-resident American, employed under PAYE with some freelance income. Assume the UK liability for 2025-26 settles at £18,400 — £14,900 withheld through PAYE across the year, £3,500 falling due as a balancing payment. Of the 365 days in UK 2025-26, 270 fall in US calendar 2025 and 95 in US calendar 2026.

1. 6 April 2025 — UK 2025-26 begins. Payslips start accumulating; they, not the P60, are the US evidence base.

2. 31 December 2025 — US calendar 2025 closes. Income for the US return is the April-to-December slice of UK 2025-26 plus the January-to-April slice of UK 2024-25.

3. 15 April 2026 — US 2025 return and payment due. UK 2025-26 is nine months from its deadline, so the US position is computed on estimated UK figures and any US tax paid now, because interest runs from this date.

4. 31 May 2026 — the 2025-26 P60 arrives. Use it to reconcile payslip totals, not to drive them.

5. 15 June 2026 — automatic extended US filing date for those abroad.

6. 15 October 2026 — extended US 2025 filing date with Form 4868. The UK picture is usually firm enough by now to file properly.

7. 31 January 2027 — UK 2025-26 return and balancing payment due; the £3,500 is paid alongside the first 2026-27 instalment. On the paid basis that £3,500 becomes creditable in US calendar year 2027.

8. 15 April 2027 — US 2026 return due, picking up the 95-day slice of UK 2025-26 and the 270-day slice of UK 2026-27.

9. 31 July 2027 — second UK payment on account. Another dated payment to log.

The pattern is plain: the UK return for a year is filed after the US return that first reports part of its income. That is the structure, not a planning failure — the answer is estimating well and documenting the estimate.

Common mistakes

• Using a P60 as a calendar-year figure. It covers 6 April to 5 April and cannot support a US year alone.
• Treating UK National Insurance as creditable income tax. It is a social security contribution, dealt with under the totalisation agreement rather than Form 1116.
• Switching between paid and accrued informally. The accrual choice can only be made on a timely filed original return, and it binds later years.
• Crediting a payment on account against the income it anticipates. On the cash basis the credit follows the payment date, not the intention.
• Letting carryovers lapse. Ten years is generous and it does expire; unused credits need a schedule that survives a change of adviser.
• Treating an extension as permission to pay late. Interest runs from the original due date regardless.
• Changing the apportionment method quietly. Documented consistency beats precision achieved differently each year.

Sources

• Self Assessment deadlines — gov.uk
• Payments on account — gov.uk
• PAYE forms: P45, P60, P11D — gov.uk
• Topic no. 301 — IRS
• U.S. citizens and resident aliens abroad — IRS
• Instructions for Form 1116 — IRS
• Publication 514 — IRS
• About Form 2350 — IRS

When it is worth getting help

Sequencing is cheap to get right in the first year and expensive to unwind later — particularly the accrual election, which binds the years that follow. If your position involves non-PAYE income, a move in either direction, or carryovers already in play, both returns are better modelled together than prepared in sequence by two unconnected preparers. Our tax specialists for US and UK filers work both sides on one file; a licensed CPA or Enrolled Agent signs off the US return and an ACCA-qualified accountant the UK one. See what falls due when on our deadlines page, or book a consultation to map the order of play to your facts.

General information on how the two systems interact, not advice on your circumstances.