Should I prepare my UK or my US tax return first?

Prepare the return of the country with the primary right to tax first, and the return of the relieving country second. For a UK resident with UK employment income, rental profit or gains on UK assets, that means the UK Self Assessment return leads and the US Form 1040 follows with a foreign tax credit on Form 1116. For US sourced income that the treaty allocates to the United States, the order reverses and the US figures feed the UK return.

Do UK payments on account count as foreign tax paid for the US foreign tax credit?

A payment on account is a payment towards a future bill, not a determination of tax for the year it is collected in. Each instalment is usually half of the previous year's tax and falls due on 31 January and 31 July. On the paid basis it is creditable in the US year the cash actually left your account. On the accrued basis it is the underlying UK liability, not the instalment, that drives the credit.

Should I elect the accrued basis on Form 1116?

The accrued basis matches UK tax to the income that generated it rather than to HMRC collection dates, which suits a settled UK resident with steady income. Two conditions matter. The election is made by checking the Accrued box in Part II of Form 1116 on a timely filed original return and must then be followed in all later years, and accrued taxes not paid within 24 months after that year end reduce the credit already claimed.

Does the automatic extension to 15 June also extend the time to pay US tax?

No. US citizens and resident aliens abroad receive an automatic two month extension to file, to 15 June for calendar year filers, claimed by attaching a statement to the return, and can request a further extension to 15 October on Form 4868. None of these extend the time to pay. Interest runs on any tax not paid by the regular 15 April due date, so an estimated payment should go out in April.

How long do I have to correct a UK Self Assessment return?

You can correct a Self Assessment return within 12 months of the Self Assessment deadline, which for an online filer means twelve months after the relevant 31 January. After that you must write to HMRC, and a claim for overpayment relief can be made up to four years after the end of the tax year it relates to. The UK window is much shorter than the US foreign tax credit window, so deal with the UK side first.

US UK Tax Returns Preparation: Which Return Comes First

Q: What happens if my UK tax changes after I have already filed the US return?

That is a foreign tax redetermination, and a redetermination of your US tax liability is required in most situations. It is generally made on Form 1040-X, and Schedule C of Form 1116 is filed with the current year return to summarise redeterminations relating to earlier years. Separately, a credit can be claimed, or a deduction changed to a credit, within the special ten year limitation period under section 6511(d)(3).

US UK tax returns preparation is a sequencing problem before it is a compliance problem, and the short answer is that for most UK resident US citizens the UK Self Assessment return is prepared first, because the US Form 1040 needs a settled UK tax figure before a foreign tax credit can be claimed on Form 1116. The order reverses whenever the income is US sourced and the treaty gives the United States the primary right to tax, because then it is HMRC that has to give the relief and HMRC that needs the finished number. Everything else in a dual filing year sits downstream of that one decision.

This is not a point about filing convenience. Order changes the tax. A credit taken in the wrong year is not simply deferred, it can be trapped by a limitation rule and expire unused. A US return filed on estimated UK figures creates an obligation to revisit that return when the real numbers land. And an election made once on Form 1116, in the very year you first get the sequence wrong, binds every year that follows. In the returns we prepare for UK based Americans, more tax is lost to sequencing errors than to any misunderstanding of the underlying rules.

Why US UK Tax Returns Preparation Is an Order of Operations Exercise

Two returns, two tax authorities, two tax years and one pot of income. The United States taxes its citizens on worldwide income wherever they live. The United Kingdom taxes residents on worldwide income, with residence determined by the Statutory Residence Test summarised at https://www.gov.uk/tax-foreign-income/residence. The double taxation agreement then decides which country taxes first and which country has to hand relief back. The country giving relief always needs the other country's number. That is the entire problem in one sentence.

So the working rule is easy to state: prepare the return of the primary taxing country first, and the return of the relieving country second. Applying it is harder, because a single dual filer usually holds income falling on both sides of that line in the same year. UK employment earnings, UK rental profit and gains on UK situated assets normally make the United Kingdom the primary taxing state. A US sourced pension lump sum, gains on US real property and certain other US source items push the other way. In practice you are not sequencing two returns. You are sequencing income streams, and then assembling two returns out of the results.

The Two Filing Calendars Side by Side

Before anything else, put both calendars on one page. Most dual filers only ever see them separately, which is exactly why the dependencies between them get missed.

15 April: the regular due date for a calendar year Form 1040, and the date from which interest runs on unpaid US tax even where an extension applies, per https://www.irs.gov/individuals/international-taxpayers/us-citizens-and-resident-aliens-abroad
15 April: the FBAR due date, with an automatic extension to 15 October, filed separately through the FinCEN BSA E-Filing System and not with the tax return, where foreign financial accounts exceeded 10,000 US dollars in aggregate at any point in the year
15 June: the automatic two month extension for US citizens and resident aliens living abroad, granted without any request, but claimed by attaching a statement to the return explaining which situation qualified you for it
15 October: the further extension available on Form 4868, which has to be filed before the automatic two month extension date runs out
Form 2350: a different instrument entirely, available only to those expecting to file Form 2555 who need more time to meet the bona fide residence or physical presence test, and generally granted for 30 days beyond the date on which you can reasonably expect to qualify

6 April to 5 April: the UK tax year, which will never align with the US calendar year
5 October: the date by which HMRC must be told that a return is needed for the previous tax year
31 October: the paper Self Assessment filing deadline
31 January: the online filing deadline, the balancing payment date, and the first payment on account for the following year
31 July: the second payment on account

Line those up against a single US calendar year and the structural gap becomes obvious. US calendar year 2026 covers the tail of UK tax year 2025 to 2026 and the first nine months of UK tax year 2026 to 2027. The 2025 to 2026 UK return is due online by 31 January 2027. The 2026 to 2027 UK return is not due online until 31 January 2028, which falls after the fully extended US deadline of 15 October 2027 for that same 2026 calendar year. For roughly three quarters of the income sitting on your US return, the UK liability is not legally required to be settled until months after the US return has to be filed. No amount of diligence removes that gap. You can only manage it. The UK dates are confirmed at https://www.gov.uk/self-assessment-tax-returns/deadlines and the instalment mechanics at https://www.gov.uk/understand-self-assessment-bill/payments-on-account.

Why the Mismatched Tax Years Force an Apportionment Before Either Return Is Final

Because HMRC measures 6 April to 5 April and the IRS measures 1 January to 31 December, no UK figure can be dropped into a US return unaltered, and no US figure can be dropped into a UK return unaltered. Something always has to be split. That apportionment is the first substantive task in the file, and it has to be completed before either return can be finalised. It is not one calculation but a series of them, and each income stream needs its own basis.

Employment income: apportioned by the period in which it was earned rather than the date the payslip issued, with bonuses traced back to the period they relate to instead of the month they were paid
UK tax deducted under PAYE: apportioned on exactly the same basis as the income it was withheld against, so that the credit follows the income
Rental profit: recomputed under each country's own rules on allowable expenses and capital allowances before any apportionment, because the two profit figures will not agree
Dividends and interest: allocated by payment date, usually the cleanest of the streams
Capital gains: allocated by disposal date, with acquisition cost translated under each country's own currency rules

The discipline that matters here is consistency. Whatever basis you use to move a number from the UK year into the US year, you must be able to use the mirror of it when you move the tax back the other way. In files we take over from other preparers, the most common defect is not an arithmetic error in the apportionment. It is that the income was apportioned on one basis and the tax credit on another, so the two returns no longer reconcile to each other and neither position can be defended if either authority asks.

The Foreign Tax Credit Chicken and Egg Problem

Form 1116 asks you to report foreign taxes paid or accrued. To answer that for a UK resident you need to know the UK liability. But the UK liability for the tax year covering most of your US calendar year may not be computed, agreed or paid by the time the US return is due. That is the chicken and egg, and it is structural rather than a symptom of leaving things late.

It is made worse by how the United Kingdom collects. What a UK resident actually pays across a calendar year is a mixture of PAYE deductions, a balancing payment on 31 January and payments on account on 31 January and 31 July, and those instalments are by design calculated on the prior year's liability rather than on the year they are collected for. Each payment on account is usually half of the tax owed the previous year. A payment on account is therefore a movement of cash, not yet a determination of tax. Treating the money that left your account during the calendar year as the foreign tax attributable to that year's income is one of the quickest ways to build a Form 1116 that cannot be reconciled to any UK document.

The mirror image limit applies on the UK side. Foreign Tax Credit Relief is the lower of the foreign tax paid, or allowed by the double taxation agreement, on the income or gain and the UK tax liability on that same income or gain, as set out in helpsheet HS263 at https://www.gov.uk/government/publications/calculating-foreign-tax-credit-relief-on-income-hs263-self-assessment-helpsheet/relief-for-foreign-tax-paid-2026-hs263. HMRC will not relieve US tax that the treaty did not permit the United States to charge in the first place, which is precisely why a US return prepared without regard to treaty sourcing can generate US tax that nobody will ever credit.

Paid or Accrued: The Election That Decides Your Sequence

This is the lever almost nobody pulls deliberately, and it is the single most powerful sequencing tool a dual filer has. A cash method taxpayer claims foreign tax credits by default in the year the foreign tax is paid. Publication 514 at https://www.irs.gov/publications/p514 confirms that a cash method taxpayer may instead choose to take the credit in the year the taxes accrue, by checking the Accrued box in Part II of Form 1116 on a timely filed original return, and that once the choice is made it must be followed in all later years. It is not an annual toggle.

On the paid basis the credit follows the cash. UK tax paid during your US calendar year is creditable in that year, so PAYE deductions and the January and July instalments land in whichever US year they physically left your account, regardless of which UK year they relate to. Foreign taxes are translated at the rate of exchange in effect on the day paid or withheld.
On the accrued basis the credit follows the liability. UK tax attributable to the income reported on the US return is credited in the same year as that income, which removes the timing mismatch at source. Accrued foreign taxes are generally translated using the average exchange rate for the tax year to which the taxes relate.
The accrued basis carries a hard condition. If accrued taxes claimed as a credit are not paid within 24 months after the end of that tax year, the credit previously claimed must be reduced by the amount of the unpaid taxes.
The accrued election binds every subsequent year, so it should be made because it suits a long term pattern, not because it happens to rescue one difficult year.

For a settled UK resident with steady UK employment income, the accrued basis usually produces the cleaner match, because it lines the UK tax up with the income that generated it rather than with the calendar of HMRC's collection cycle. For someone in a transitional year, arriving in or leaving the United Kingdom, the paid basis is often safer, because the accrued figure is exactly the number that is hardest to pin down in a split year. Either way the choice should be made on paper, with reasons, before the first Form 1116 is filed.

A Month by Month Preparation Timetable for Dual Filers

Two lists of deadlines are not a plan. What a dual filer needs is a dependency chain, because most of these dates are blocked by work that has to be finished earlier. This is the timetable we run, expressed for a US calendar year that has just ended.

January: settle the UK position for the tax year that ended the previous 5 April. File the UK return by 31 January, make the balancing payment and the first payment on account, and record the split between them, because only one of those two numbers is a determination of tax.
January to February: collect the source documents both returns depend on. P60 and P11D, UK interest and dividend certificates, broker statements, completion statements for any disposals, and the full year of peak account balances needed for the FBAR.
February: build the apportionment. Convert the UK tax year figures onto the US calendar year, and separately compute the UK tax attributable to each apportioned stream. This is the gate. Nothing downstream can be finalised until it is done.
March: run the treaty sourcing analysis. Decide, stream by stream, which country is the primary taxing state and which is the relieving state, and confirm whether Form 8833 disclosure is needed for any position being taken.

Early April: run the credit versus exclusion comparison, and model the paid basis against the accrued basis. In a first year of dual filing this is where the accrued election is decided, and it is decided for good.
By 15 April: pay. This is the date that costs money. Interest runs on unpaid US tax from the regular due date whether or not an extension applies, so an estimated payment goes out now even though the return will not be filed for months. The FBAR is also due, with the automatic extension to 15 October behind it.
5 April: the UK tax year ends, which starts the clock on the UK return covering the last quarter of your US calendar year. Nothing about that return will be final for many months.
By 15 June: either file, or decide consciously not to. The automatic two month extension for citizens abroad is granted without a request but is claimed by attaching a statement to the return, so the supporting note belongs in the file now, not later.

31 July: the second UK payment on account. Log it, but do not mistake it for a determination of the UK liability for any particular year.
August to September: finalise. By now the UK figures for the year ended the previous 5 April are settled, and the current UK year is only months from ending, so the accrual position for the final quarter of the US year can be estimated from real data rather than assumption.
By 15 October: file the Form 1040 and the FBAR. This is the last date that does not require an amendment, and for most dual filers it is the target rather than the fallback.
31 October and the following 31 January: the UK paper and online deadlines for the UK year that closed the previous 5 April, which is where the final quarter of your already filed US year finally gets confirmed. If it moves, this is when the amendment question arises.

What Blocks What: The Dependency Chain in One View

Stripped to its skeleton, the chain runs in one direction only, and every link is blocked by the one above it.

Residence status blocks apportionment, because a split year under the Statutory Residence Test changes which months count on which side of the line
Apportionment blocks the treaty sourcing analysis, because you cannot allocate a taxing right over an amount you have not yet measured
Treaty sourcing blocks the choice of which return goes first, because it identifies the relieving country for each stream
The relieving country decision blocks the credit computation, because the credit belongs on one return and not the other
The credit computation blocks the paid versus accrued election, and that election, once made, blocks every future year
The US payment obligation on 15 April blocks nothing at all, which is exactly why it is the deadline dual filers most often miss: it falls due before any of the analysis above is complete

Amend, Extend or Accrue: Costing the Three Routes

Once you accept that a quarter of your US year will not be settled in the United Kingdom before the US return is due, there are exactly three honest responses. Each carries a different cost, and choosing between them is a pricing decision rather than a matter of principle.

Route one is to extend and wait. File Form 4868 to push the US filing deadline to 15 October and use the extra four months to get closer to a real UK number. The cost is interest, not penalty, provided a sound estimate of the US liability was paid by 15 April, because an extension of time to file has never been an extension of time to pay. This is the cheapest route for anyone whose UK position will be substantially clear by early autumn, which covers most salaried UK residents.

Route two is to file on estimates and amend. File the Form 1040 on the best available UK figures, then correct it on Form 1040-X once the UK liability is settled. The cost is a second filing, a second fee and a second point of exposure, but the timing rules here are unusually generous. Where the change is a foreign tax redetermination, the instructions to Form 1116 at https://www.irs.gov/instructions/i1116 confirm that a redetermination of your US tax liability is required in most situations, that it is generally made on Form 1040-X, and that Schedule C of Form 1116 is filed with the current year return to summarise redeterminations relating to prior years. Separately, a credit can be claimed, or a deduction changed to a credit, at any time before the end of the special ten year limitation period described in section 6511(d)(3). Ten years is a long runway. This route suits years where the UK number could still move materially and you would rather have a filed return on record than an open extension.

Route three is to elect the accrued basis and stop chasing cash altogether. Accruing removes the mismatch structurally instead of managing it annually. The cost is the loss of flexibility, because the election binds all later years, together with the 24 month payment condition, because an accrued credit left unpaid beyond two years has to be given back. This is the right route for a stable long term UK resident and the wrong route for anyone whose UK residence is about to change.

The comparison that settles it is straightforward. Set the interest cost of the extension route against the professional cost of the amendment route and against the permanence of the election. For most of the dual filers whose returns we prepare, the answer is a combination: extend as a matter of routine, accrue where the UK position is stable, and hold the amendment route in reserve for the years when an HMRC enquiry or a late adjustment moves the number after the US return has already gone in.

One asymmetry deserves flagging, because it catches people out. The two countries do not stay amendable for the same length of time. A UK Self Assessment return can be corrected within 12 months of the Self Assessment deadline, and after that you are writing to HMRC and relying on overpayment relief, which runs to four years after the end of the tax year it relates to, per https://www.gov.uk/self-assessment-tax-returns/corrections. The US side, for foreign tax credit purposes, stays open far longer. So when a late adjustment lands, the UK window is usually the one about to close, and the UK return is the one to deal with first.

A Worked Scenario Where the Order Changes the Answer

The following is an illustration only. The pattern is drawn from real files but the facts are simplified, no figures are given as amounts, and any real engagement would apply the appropriate published exchange rates rather than a single assumed rate.

A US citizen has been UK resident for six years. She is employed in London, and in the US calendar year in question she has UK employment income taxed under PAYE across the whole year, a UK share disposal in May, and a US source distribution in November on which US tax is charged at source under the treaty. Her preparer files the US return first, in April, using the UK tax actually paid during that calendar year as the Form 1116 figure.

Three problems follow immediately. First, the UK tax paid during that calendar year included a January balancing payment for the previous UK tax year and a January payment on account computed on the previous year's liability, so a large part of the credit claimed relates to income that is not on the US return at all. Second, the May disposal falls in a UK tax year whose liability will not be settled until the following 31 January, so the UK tax on that gain is missing from the credit entirely. Third, the November US source distribution should have been dealt with on the US return first and relieved on the UK return, but the UK return was prepared afterwards without reference to the US figure and no Foreign Tax Credit Relief was claimed against it.

Sequenced properly, the same facts produce a different result. The employment income and the gain are apportioned onto the US calendar year and the UK tax attributable to them is computed on an accrued basis, so the credit matches the income it belongs to. The US source distribution is handled US first, and the US tax on it is carried across as Foreign Tax Credit Relief on the UK return, capped at the UK tax on that income. Nothing about the underlying facts changed. Only the order did, and the combined tax paid across both countries fell as a result.

Treaty and Totalization Items to Settle Before Either Return Is Signed

A handful of positions have to be resolved before either return can be finalised, because they determine what goes on the return rather than merely how it is presented.

Residence and any split year treatment under the Statutory Residence Test, since this fixes the UK reporting period and therefore the whole apportionment
Whether the four year foreign income and gains regime applies, which is open to a qualifying new resident within their first four years of UK residence following at least ten years of non-UK residence, and which replaced the remittance basis from 6 April 2025
Treaty sourcing for each income stream, and whether a treaty based return position needs disclosing on Form 8833
Social security. A certificate of coverage obtained from the home country's social security agency and given to the employer is what prevents contributions being charged twice, and it has to be in place before the payroll runs rather than reconstructed at return time
Whether the foreign earned income exclusion is in play at all, since claiming it on Form 2555 removes income from the US base and with it removes the foreign tax on that income from the credit computation

That last point deserves emphasis, because it is where the exclusion and the credit collide. The maximum foreign earned income exclusion is 130,000 US dollars for 2025 and 132,900 US dollars for 2026, and the physical presence test requires at least 330 full days in a foreign country during a period of 12 consecutive months, per https://www.irs.gov/individuals/international-taxpayers/figuring-the-foreign-earned-income-exclusion. For a high earning UK resident paying UK tax at rates above the US equivalent, the credit almost always outperforms the exclusion, and the exclusion actively wastes creditable UK tax that could otherwise have been carried back one year or forward ten. But where meeting the tests is the reason a return cannot be filed on time, Form 2350 exists for precisely that case, and is described at https://www.irs.gov/forms-pubs/about-form-2350.

Where the Sequence Breaks Down in Practice

Four failure modes account for most of the damage we see when taking over a dual filing engagement from another preparer.

The calendar year cash mistake: treating the UK tax that left the bank account during the US calendar year as the creditable foreign tax, which mixes years and inflates or deflates the credit essentially at random
The default election: never consciously choosing paid or accrued, so the paid basis applies by inertia and the mismatch is baked into every subsequent year
The one way street: preparing the UK return second and never carrying US tax back as Foreign Tax Credit Relief, so US source income the treaty allocates to the United States ends up taxed twice
The unmanaged extension: extending to 15 October without paying an estimate on 15 April, which converts four free months into interest bearing ones

Penalties on the UK side compound quickly once the sequence slips far enough to miss a deadline. A late Self Assessment return draws an initial 100 pounds penalty, then daily penalties of 10 pounds a day up to 900 pounds after three months, then a further 5 percent of the tax due or 300 pounds if greater at six months and the same again at twelve months, with late payment penalties of 5 percent of the tax unpaid at 30 days, six months and twelve months, plus interest. The detail is at https://www.gov.uk/self-assessment-tax-returns/penalties. None of that is triggered by complexity. It is triggered by running out of time, which is what a badly sequenced file does.

Locking the Sequence In for Every Future Year

Sequencing is not something to rediscover each spring. Fix the order once, document why, and run the same chain every year: residence, apportionment, treaty sourcing, relieving country, credit computation, election, file, and only then consider amending. The two calendars will never align, and no election makes them align. What good sequencing does is make the misalignment predictable, so that the credit lands where it belongs and the cash goes out on the dates that actually carry a cost. Our US filing work is at https://us-uktax.com/us-tax-services, the UK Self Assessment side at https://us-uktax.com/uk-tax-services, and combined dual filer engagements at https://us-uktax.com/cross-border-tax-planning.