UK PAYE Coding Notices and Your US Withholding Plan

If you are a US citizen on a UK payroll, your UK tax is collected through a code that HMRC sets and can revise at any point in the year, while your US position runs on estimated payments and a foreign tax credit that depends on what UK tax you actually pay. A coding notice is therefore not only a UK document. It changes the number your US plan is built on, and it can do so in October for a year that is already half spent.

The expensive version of this is not a wrong code. It is a correct code that moves, and a US plan that never moves with it.

What a coding notice actually is

Your tax code is the instruction HMRC gives your employer. In gov.uk's own words, it "is used by your employer or pension provider to work out how much Income Tax to take from your pay or pension", and HMRC "will tell them which code to use". You do not choose it, and neither does your employer. Both of you are told.

The notice itself — the P2 — is the arithmetic behind that instruction: your tax-free allowances on one side, the deductions HMRC is making from them on the other, and the resulting code. Most people receive one before the tax year starts, then again whenever something changes. You can also ask to be told electronically; gov.uk notes you can "sign up for paperless notifications", which means HMRC "will email you when your tax code changes".

Two things follow for an American reader. The code is a forecast, built on what HMRC currently believes about your year. And because it is a forecast, it gets revised when the belief changes.

Why it changes part-way through the year

HMRC lists the triggers plainly. A code changes if, among other things, you "start a new job", you "start to get income from an additional job or pension", "the interest on your savings is more than your Personal Savings Allowance", your "employer tells HMRC you have started or stopped getting benefits from your job", you "claim Marriage Allowance", you "claim expenses that you get tax relief on", or — the one that matters most here — you have "been on the wrong tax code and owe tax". The full list is on gov.uk's page on why your tax code might change.

Notice what is in that list. A company car reported by your employer. A savings account that did better than expected. A prior year's shortfall. None of these are events you file anything about, and all of them can land mid-year.

The sharpest version is the K code. A K prefix means, per gov.uk, that you "have income or deductions which are higher than your tax-free Personal Allowance" and which are "not already being taxed". Among the examples gov.uk gives are "paying tax you owe from a previous year through your wages or pension" and company benefits such as a car. A K code does not change your tax rate. It adds notional income to be taxed, so more tax comes out of the same gross pay. There is also a statutory ceiling on how much a K code can take from any one payment, which that page sets out.

For 2026/27 the personal allowance is £12,570, tapered by £1 for every £2 of income above £100,000, and the England and Northern Ireland income tax bands run at the 20% UK basic rate, the 40% UK higher rate of income tax, and the 45% UK additional rate. A mid-year coding change does not alter any of those. It alters how much of your allowance survives into your code, and therefore how much UK tax is collected, and when.

Why a UK code change is a US planning event

Your US return does not care what your tax code says. It cares how much UK tax you paid or accrued, because that is what the credit is measured by. The IRS directs you to file Form 1116 to claim the foreign tax credit, and the credit is only ever as large as the foreign tax behind it.

So when a coding notice changes, an input to your US calculation has changed, usually in one of two directions:

• The code becomes less generous — a benefit in kind is added, allowances are withdrawn, a K code appears. More UK tax is collected across the rest of the year, so your creditable foreign tax for the year rises, and the US liability on the same income generally falls.
• The code becomes more generous — expenses relief is granted, an estimate is corrected downward. Less UK tax is collected, your creditable foreign tax falls, and the US gap it was covering widens.

That second case is the one that bites, because it feels like good news in sterling and arrives as a US shortfall months later.

This is where US estimated payments come in. Currently, the IRS expects estimated tax where you "expect to owe at least $1,000 in tax for the current tax year after subtracting your withholding and refundable credits". The penalty safe harbour is the smaller of two prongs — broadly 90% of the current year's tax, or 100% of the prior year's — with a 110% figure substituted where the prior year's adjusted gross income was above $150,000 ($75,000 if married filing separately). That 110% uplift appears in the IRS estimated tax FAQs. Payments fall due in four unequal instalments, on April 15, June 15, September 15 and January 15 of the following year.

Read those two prongs against a moving tax code and the problem is clear. The current-year prong depends on a UK number HMRC can revise after you have already made two instalments. The prior-year prong does not move — which is exactly why, in a year when your code is in flux, the prior-year prong is often the more defensible thing to aim at. Our calculators help with the UK side of the arithmetic, but choosing between the prongs is a judgement call rather than a sum.

The trap: a UK adjustment landing in the wrong US year

Here is the mismatch that the pages ranking on this subject do not join up.

The UK tax year ends on 5 April. The US tax year ends on 31 December. A UK underpayment or refund is usually settled long after the income arose, and often through the code rather than by a cheque. HMRC may issue a P800 tax calculation after the year ends, or simply collect the shortfall by adjusting a later year's code — which is the "paying tax you owe from a previous year through your wages or pension" line in the K code list above. gov.uk sets out how overpayments and underpayments are handled.

So UK tax relating to one period can be paid in a calendar year holding entirely different US income. Claim the credit on the paid basis and it can land in a US year with little for it to offset, while the US year that actually needed it has already been filed.

US law offers a lever and a correction here. Both have conditions.

The lever is the accrued basis. IRS Publication 514 explains that even a cash-method taxpayer "can choose to take a credit for foreign taxes in the year they accrue". The conditions are strict and worth reading twice: you make the choice "by checking the 'Accrued' box in Part II of Form 1116 on a timely filed original return", you "cannot make this choice on an amended return", and once made, "you must follow it in all later years". It is a one-way door, and not a thing to decide in the week a notice arrives.

The correction is the foreign tax redetermination. Publication 514 defines one as "a change in your foreign tax liability" and states that, generally, "you must take into account foreign tax redeterminations in the tax year to which the tax relates". That sentence is the answer to the question this whole topic circles around: when HMRC later changes the UK number, the change is generally carried back to the year the tax belongs to, rather than dropped into the year the letter arrived.

Two practical consequences. Excess credit in a year is not simply lost — there are carryback and carryforward rules, covered in foreign tax credit carryovers for US expats, and you should confirm the current periods rather than assume them. And National Insurance is not income tax; it does not become a foreign tax credit, which is a common and expensive assumption. Which UK taxes do count is set out in UK taxes that are creditable against US tax.

What a PAYE employee can and cannot control

Advice written for the self-employed sometimes suggests paying foreign tax early to pull the credit into the right year. A PAYE employee largely cannot. HMRC controls the code, the employer operates it, and the timing of collection is not yours to set. What you can do is narrower, and still worth doing:

• Read every P2 on arrival and check the deductions against reality — a company car you returned, an expenses claim double-counted, an estimate of savings interest that is simply wrong.
• Tell HMRC when a figure is wrong, because the code only improves once the belief behind it does.
• Record the date of every coding change, not just the new code. Which US year a change touches depends on when the tax is collected.
• Re-run the US estimate after a material change, rather than at year end.
• Decide the paid-versus-accrued question deliberately, in a year when you can still choose, with both years modelled.

Where we fit

The two halves of this problem are usually held by two people who never speak: a UK payroll or accounting contact who sees the code, and a US preparer who sees the return eleven months later. Neither is doing anything wrong. The gap is structural.

We work the UK code and the US estimate as one file — reading the coding notice when it arrives, pricing what it does to the credit, and adjusting the US instalments while there are still instalments left to adjust. Every filing we prepare is reviewed and signed by a licensed professional, an IRS Enrolled Agent who is also ACCA-qualified, so one person holds both sides rather than two advisers corresponding through you. More on this work is on our US-UK tax specialists page; you can also see what it costs or book a consultation.

If a treaty claim is also in play on US-source income, the companion piece on resourcing income by treaty covers that interaction.

This article is general information, not advice on your own facts. Rates, thresholds and the figures behind the US estimated tax rules should be confirmed for your own year before you act on them.