HMRC changed an earlier UK year. The IRS calls that a redetermination, the duty to report it is yours, and the year you amend is rarely the current one.
If HMRC changes your UK tax for a year in which you claimed a foreign tax credit, US law treats that as a foreign tax redetermination under section 905(c), and the duty to report it is yours. HMRC does not tell the IRS, and the IRS will not adjust your credit for you. Whether you file a Form 1040-X depends on whether your US tax actually changes; whether you file a Schedule C (Form 1116) does not.
The question almost nobody answers is which US year the adjustment belongs to. For a UK filer it is rarely the year the letter arrived: the UK tax year does not sit inside a US one, and the regulation keys the year you amend and the year that sets your deadline to two different things. That, and the fork deciding whether anything reopens at all, is what follows.
The IRS describes a foreign tax redetermination as “any change in your foreign tax liability that may affect your U.S. foreign tax credit claimed”. The operative word is liability — and not every brown envelope changes one.
These generally do change the UK liability for a year, and so are capable of being a redetermination:
These, on their own, generally do not:
The second list carries most of the volume, and treating a reduced payment on account as a redetermination produces an amended US return that should never have been filed.
Two different US years are in play, and they do different jobs.
The year you redetermine is the US year in which the UK tax was taken into account — the year of payment on the paid basis, the year of accrual on the accrued basis. A UK year runs to 5 April, so the UK year ending 5 April 2024 accrues inside US year 2024, and a closure notice issued in 2026 about that UK year redetermines your 2024 return, not your 2026 one.
The year that sets your deadline is the year the redetermination occurred. Where the UK tax goes down and your US tax goes up, Treas. Reg. §1.905-4(b)(1)(ii) requires notification “by the due date (with extensions) of the original return” for the taxable year in which the redetermination occurs — 2026, in that example.
And the year that carries the summary is also 2026: the Schedule C instructions require you to file Schedule C “with your current-year tax return summarizing the foreign tax redeterminations that occurred that year that relate to prior tax years”.
So one HMRC letter puts three things in motion — an amended 2024 return, a Schedule C on the 2026 return, and a 2026 due date governing both. Which US year a UK year maps onto is worked through in the UK tax year straddles two US years.
Nearly everything written on section 905(c) assumes the accrued basis. If you file on the paid basis, read this before you reopen anything.
On the accrued basis, the whole UK year's liability sat in one US year. Change the liability and you have changed that year's accrued figure, so the earlier US year is genuinely redetermined.
On the paid basis, your credit followed the cash. Extra UK tax you actually pay in 2026 after an HMRC amendment is generally foreign tax paid in 2026, creditable in 2026 — not a reopening of an earlier year. The IRS says as much for a resolved dispute: “if you claim your foreign tax credit on the cash basis and you paid additional foreign income tax at the resolution of the contest, that is not a foreign tax redetermination”.
The asymmetry is the part to hold on to: a refund is a redetermination on either basis. If HMRC repays UK tax you have already credited, the credit for the year you claimed it was overstated, and the paid basis offers no escape. So UK tax down always reopens something; UK tax up often reopens nothing on the paid basis, because you have a credit in the year of payment instead.
The basis is not a free choice each year: the election to claim on the accrued basis is made on a timely filed original return and binds the years that follow, as accrued or paid on Form 1116 sets out.
Three outcomes, and only one is a Form 1040-X.
Your US tax changes. Then you “must generally file Form 1040-X” with a revised Form 1116 for the affected year. Where the change increases your US tax, the IRS asks for a specific sentence in the explanation of changes — for example at Form 1040-X, Part II: “This amended return and Form 1116 are for a change in foreign tax credit that increases U.S. tax liability.”
Your US tax does not change. Then “you don't need to file an amended return” — you notify instead, by attaching a completed Schedule C to the original return for the year the redetermination occurred.
The extra US tax is absorbed. If a carryback or carryover of unused foreign tax soaks it up, Publication 514 is explicit: “you do not have to amend your tax return for the year affected by the redetermination.” But excess credits only move within the same category of income, so a credit stranded in the wrong one will not help — as why your Form 1116 credit cannot cross over explains.
In all three cases Schedule C is the vehicle: it “must be filed irrespective of whether the foreign tax redetermination changed your U.S. tax liability”, with a separate one per separate category of income. Where a UK liability is genuinely in dispute it is filed annually “up to and including the tax year in which the contest is resolved” — so a long enquiry reports every year it runs.
The regulation is not symmetrical, which is the most useful thing to know about timing.
UK tax down, US tax up. A hard deadline — the due date, with extensions, of the original return for the year the redetermination occurred — and missing it carries a penalty.
UK tax up, US tax down. You are on nobody's deadline; you are making a claim for refund within the period provided by section 6511. Nobody will chase you, and if you are late you simply lose the money. For a claim attributable to creditable foreign taxes the window is ten years, measured from the due date for the return for the year the foreign taxes were actually paid or accrued — not from the year of the overpayment, which is the error people make.
There is a carve-out worth knowing before you accept a UK repayment. Under section 905(c)(5), no interest is assessed on an underpayment resulting from a refund of foreign tax “for any period before the receipt of the refund, except to the extent interest was paid by the foreign country” on that refund for that period. From receipt onwards, interest runs normally until you pay.
The UK hook is obvious once stated: HMRC commonly pays repayment interest on overpaid tax. A repayment with interest attached switches on the exception, and US interest can then reach back over the period before the money arrived; a repayment without it leaves the pre-receipt period generally clean. Underpayment rates reset quarterly, so the arithmetic belongs to the date you do it, not to an article.
Not every difference needs reporting. Where the whole of it is “solely attributable to fluctuations in the value of the foreign currency relative to the dollar”, no redetermination is required and the IRS need not be notified, provided the net amount for that country is less than the lesser of $10,000 or 2% of the total foreign income tax initially accrued for that country for that year.
Three cautions. It is the lesser of, so for almost every individual the binding figure is the 2% — quoting the dollar amount as “the threshold” is how this gets misapplied. It covers currency movement only: a change in the UK liability never qualifies, however small. And the difference does not vanish — an appropriate adjustment is made in the year the redetermination occurs.
One redetermination arrives without a letter. Accrued foreign income tax still unpaid 24 months after the close of the year it relates to is itself a redetermination — the credit comes back out. That reaches further into UK facts than people expect: Self Assessment paid very late, or a disputed amount held back while an enquiry runs, can cross the line while everyone is still arguing about the number. Paying it later is a further redetermination in your favour, and the notification must carry the foreign-currency amount and the exchange rate used — which is why the original workpapers matter years afterwards.
Section 6689 adds 5% of the deficiency attributable to the redetermination for a failure of up to a month, and a further 5% for each additional month or part month it continues, to a maximum of 25%. It does not apply where the failure is “due to reasonable cause and not due to willful neglect”. Two points of precision: the base is the deficiency, not the credit and not the UK tax, so “25% of the credit” overstates it — sometimes badly; and it accrues by the month the failure continues, so it grows quietly from a missed deadline rather than from the day anybody notices.
A workable sequence:
Whether a particular HMRC document changed a liability, and whether the limitation absorbs the difference, are judgement calls on your facts rather than look-ups. Every return and notification we prepare is reviewed and signed by a licensed professional — an Enrolled Agent on the US side, an ACCA-qualified accountant on the UK side. If you are holding a closure notice or a repayment and are not certain which US year it touches, our US-UK tax specialists handle this sequence routinely; book a consultation or see pricing.
General guidance, not advice on your circumstances. Whether an HMRC adjustment is a foreign tax redetermination, and which US year it affects, turns on your facts and should be settled by a licensed professional before anything is filed.