US Tax Return Preparation for Expats: UK Basis Period Reform

Basis period reform pushed many UK sole traders and LLP partners into a 2023/24 tax year containing more than twelve months of profit. The US calendar year did not move. The result is a year in which UK tax paid spikes without matching US taxable income, distorting Form 1116 foreign tax credits across every year of a US catch-up filing.

If you are a US citizen or green card holder trading through a UK sole trade, partnership or LLP, this is the single most under-diagnosed problem in US tax return preparation for expats right now. It does not show up as an error. It shows up as foreign tax credits that stubbornly refuse to cover a US liability they should have covered comfortably — and, in a multi-year catch-up, as credits stranded in the wrong years entirely. Jungle Tax has spent the last two filing seasons unpicking it for founders, consultants, barristers, LLP partners and creative-sector principals on both sides of the Atlantic.

What basis period reform actually did to one UK tax year

Until 2023/24, unincorporated UK businesses were taxed on the "current year basis": the profits of the twelve-month accounting period ending in the tax year. A business with a 30 April year end was taxed in 2022/23 on the year to 30 April 2022 — a lag of almost a full year. From 2024/25 onwards, everyone is taxed on the "tax year basis": profits arising between 6 April and 5 April, apportioned if the accounting date does not align.

Bridging those two worlds required one transitional year: 2023/24. HMRC's guidance splits that year's basis period into two components:

• The standard part — the normal twelve months beginning immediately after the 2022/23 basis period ended (for a 30 April year end, the year to 30 April 2023).
• The transition part — everything from the end of the standard part up to 5 April 2024 (for that same business, 1 May 2023 to 5 April 2024: eleven further months).

Both parts are taxed in 2023/24. A business with a 30 April year end therefore reported roughly twenty-three months of profit in a single tax year. Businesses with 31 December, 30 June or 30 September year ends faced smaller but still material extensions. Only those already on 31 March to 5 April were unaffected.

Overlap relief and five-year spreading

Two reliefs soften the blow. First, overlap relief — profits taxed twice in the business's opening years — is deducted from the transition part. HMRC is explicit that this was the last chance to use it: overlap relief cannot be carried beyond 2023/24. Second, the remaining transition profit is spread equally across five tax years, 2023/24 through 2027/28, with at least 20% taxed in 2023/24. A taxpayer may elect to accelerate more into an earlier year, but cannot defer beyond the five.

HMRC's own guidance on this is at Work out your transition profit. It is competent UK guidance. It says nothing about the United States, because it was never written for someone who also files a Form 1040.

Why does the mismatch only hurt cross-border filers?

A UK-only taxpayer sees basis period reform as a cash-flow event: more profit taxed sooner, softened by spreading. Painful, but internally consistent — the income and the tax move together within the same system.

A US filer has two systems that must be reconciled year by year. The US taxes worldwide income on a 1 January to 31 December basis and grants relief for foreign taxes under the foreign tax credit rules of Form 1116. That reconciliation only works when the foreign income reported on the US return and the foreign tax claimed against it describe roughly the same economic period. Basis period reform severed that link for one year, and the spreading rules keep it severed for four more.

Feature	United Kingdom (HMRC)	United States (IRS)
Tax year	6 April to 5 April	1 January to 31 December
Basis of trading profit	Tax year basis from 2024/25; transitional year 2023/24	Calendar year, unchanged; Schedule C or Schedule E accounting period
Effect of basis period reform	Up to ~23 months of profit taxed in 2023/24	None — twelve months of profit reported, as always
Deferral mechanism	Transition profit spread over five years to 2027/28	No equivalent; no US recognition of the spreading election
Relief for the other country's tax	Foreign Tax Credit Relief (HS263), rarely relevant for UK-source trade	Form 1116 credit, or deduction under section 164
Excess relief treatment	Generally lost	Carried back one year, forward ten years, by category
Timing of foreign tax	Payable 31 January and 31 July	Cash basis: year of payment. Accrual basis: year the tax relates to

How transition profit breaks Form 1116

The foreign tax credit is not a refund of foreign tax. It is a credit capped by a limitation fraction: broadly, your US tax multiplied by the ratio of foreign-source taxable income in a category to total taxable income. Trading profits from a UK sole trade or LLP interest fall in the general category. Three separate things go wrong.

1. The income numerator does not move

Your US Schedule C or partnership reporting continues to describe a normal twelve months of trading. Basis period reform did not create economic income; it accelerated the recognition of income that had always been deferred by the old lag. So the foreign-source general category income on your 2024 Form 1040 looks exactly as it would have without reform.

2. The tax denominator balloons in one year

The UK income tax and Class 4 National Insurance attributable to a twenty-three month basis period is far larger than a normal year's. If you claim foreign taxes on the cash basis — the default — that spike lands in the US calendar year in which you actually pay it. For 2023/24, the balancing payment falls due 31 January 2025, and payments on account for 2024/25 fall on the same date. Both are cash movements in US calendar year 2025.

3. The limitation truncates the credit

Foreign taxes exceeding the limitation are not lost, but they are not usable now either. They carry back one year and forward ten, general category only. If your US liability in the surrounding years is already fully sheltered — which it usually is, because effective UK rates comfortably exceed US rates — those carryovers may never be absorbed. You have converted real cash tax into a paper asset that expires. The IRS Instructions for Form 1116 set out the limitation and carryover mechanics in full.

Meanwhile the four years of spread transition profit — 2024/25 through 2027/28 — produce UK tax with no corresponding US income at all. Each of those years shows an inflated UK tax charge sitting against an ordinary twelve months of US trading income. You get four consecutive years of excess credits, in a category where excess credits are hardest to use.

Cash basis or accrual basis: the election that decides the outcome

A cash-method US taxpayer may elect under the foreign tax credit rules to claim foreign taxes on the accrual basis — matching foreign tax to the year in which the underlying income arose, rather than the year of payment. For UK filers this is frequently the better answer even without basis period reform, because UK self-assessment payments lag the income they relate to by up to nineteen months. With basis period reform in the picture, the election becomes decisive.

Three cautions, all of which we see mishandled:

• It is effectively permanent. Once made, the accrual election binds you for subsequent years. It is not a lever to pull in a single opportunistic year and then abandon.
• The transition year still needs manual work. Switching from cash to accrual creates a year in which either a period of UK tax is claimed twice or a period is claimed not at all. That has to be identified and reconciled deliberately, not left to software.
• Redeterminations follow you. Accrued foreign tax that is later amended, refunded, or unpaid within the statutory window triggers a foreign tax redetermination and, in many cases, an amended US return. HMRC enquiries into transition-year computations make this more likely, not less.

	Cash basis (default)	Accrual basis (election)
UK tax on 2023/24 transition profit	Credited when paid — typically US 2025	Credited against the year the profit relates to
Alignment with US income	Poor; lag of one to two calendar years	Materially better
Effect on spread instalments	Four further mismatched years	Spread instalments still need mapping, but to the right income
Reversibility	Default position	Binding for later years
Suits	Aligned year ends, stable profits	Non-March year ends, transition profit, catch-up filings

The multi-year catch-up problem

Where this becomes genuinely expensive is inside an IRS streamlined filing catch-up. The Streamlined Foreign Offshore Procedures require three years of delinquent or amended returns and six years of FBARs. If those three years straddle 2023/24 — and for most current catch-ups they do — you are choosing your foreign tax credit method once, retrospectively, for a period you can no longer influence.

The practical consequences:

• You cannot elect accrual for a year you have not filed and then treat later years inconsistently. The method must be coherent across the catch-up window and forward.
• Carrybacks are constrained. Excess credits carry back one year — but only into a year that is within the amendment window and, in a streamlined submission, already part of the package. Sequencing matters.
• The certification narrative must survive scrutiny. A streamlined submission requires a non-wilfulness certification. A return showing a large, unexplained swing in foreign tax with no movement in foreign income invites questions. The transition profit computation should be explained in the file, with the HMRC basis period computation attached.
• Spreading elections made in the UK cannot be undone to suit the US. The 2023/24 acceleration election, once made, is a UK fact. It is far better to model both systems before that election is finalised. Our cross-border tax planning work exists precisely for this sequencing.

Worked illustration

Consider a UK-resident US citizen consultant with a 30 April accounting date, trading profits of roughly £300,000 a year, and modest overlap relief from the 1990s.

• 2023/24 UK: standard part (year to 30 April 2023) plus transition part (1 May 2023 to 5 April 2024) — approximately twenty-three months of profit, less overlap relief. The transition element is spread across 2023/24 to 2027/28.
• US 2023 and 2024: Schedule C reports twelve months of calendar-year profit in each. Nothing unusual.
• US 2025 on the cash basis: a very large UK payment lands on 31 January 2025 — the 2023/24 balancing payment inflated by transition profit, plus payments on account. Foreign taxes paid may exceed the general category limitation by a wide margin. Excess credits arise.
• US 2026 to 2029: each year carries an extra slice of transition profit in the UK tax charge with no corresponding US income. Excess credits repeat.
• Outcome: a large general-category carryover that the ten-year window may never absorb, because subsequent normal years generate their own sufficient credits.

Run on the accrual basis with proper apportionment, the same facts often produce credits that sit close to the limitation each year and a materially smaller stranded balance. The economics did not change. The mapping did.

Payments on account: the second-order distortion

Basis period reform did not alter UK payment dates, but an inflated 2023/24 liability inflates the payments on account for 2024/25 that are calculated from it. For a US filer on the cash basis, those payments are foreign taxes paid in a US calendar year, further concentrating credit in a year without matching income. Where a payment on account is later reduced or refunded on submission of the actual return, a cash-basis claim requires an amended US return; an accrual-basis claim requires a foreign tax redetermination. Neither is optional.

What about self-employment tax and National Insurance?

A frequent and costly confusion. Under the US–UK totalisation agreement, a self-employed individual resident and working in the UK is generally covered by the UK system and pays UK National Insurance rather than US self-employment tax. The exemption is claimed by reference to a certificate of coverage from HMRC, and the IRS explains the framework at Self-employment tax for businesses abroad.

Two points specific to transition profit. First, Class 4 National Insurance is a social security contribution, not an income tax, and is not a creditable foreign income tax on Form 1116. Including it in the foreign tax pool is a common and material overstatement — and one that becomes larger, not smaller, in a transition year. Second, transition profit does not create additional US self-employment income, so it cannot be argued into the picture on that side either. Your UK computation and your US foreign tax pool must be reconciled line by line, separating income tax from NIC.

LLP partners: the extra layer

Members of a UK LLP face everything above plus partnership-specific complications:

• Individual, not firm-level, transition. The LLP itself did not change how it prepares accounts. Each member moved to the tax year basis individually, so two members of the same firm can have quite different transition profiles depending on when they joined and what overlap relief they carried.
• Overlap relief records are often lost. Members who joined a firm fifteen or twenty years ago frequently cannot evidence their overlap profit. Without it, transition profit — and therefore the UK tax spike — is overstated, and so is the foreign tax pool you are trying to map.
• Entity classification. A UK LLP is fiscally transparent for UK purposes, but its US treatment depends on its classification. A default or elected classification that differs from the UK treatment can put income and tax in different US years or different taxpayers altogether — a mismatch that basis period reform amplifies.
• Non-UK source profits within the partnership. Where the firm has US-source or third-country profits, the general category apportionment on Form 1116 must be done on the underlying sourcing, not on the UK tax charge.

What if you have already filed?

Most people reading this filed something. The question is whether it can still be improved.

• Re-examine the foreign tax pool first. Strip out Class 4 NIC, student loan repayments and any payment on account later refunded. This alone corrects a surprising number of returns.
• Check whether the accrual election was made, and whether it should have been. Where returns are within the amendment window, there may be scope to correct the method and re-map credits.
• Re-run the carryover schedule. Excess general category credits should be tracked year by year with their expiry. Many preparers do not carry the schedule forward at all.
• Confirm the UK side is right. An overstated transition profit — usually from missing overlap relief — overstates UK tax, which then propagates into every US year. Amending the UK return may be the higher-value fix.
• If years are missing entirely, the streamlined procedures remain the orderly route. See our guides library and our US-UK tax accountants page for how we structure that work.

Documents your preparer should be asking for

• UK tax computations for 2022/23 through 2027/28, showing standard part, transition part and overlap relief separately.
• The accounts for every accounting period touching the transition basis period, so profit can be apportioned to the US calendar year.
• HMRC statements of account showing the date and composition of every payment — income tax, Class 4 NIC, student loan, payment on account, balancing payment.
• The 2023/24 return page recording the spreading position and any acceleration election.
• Partnership statements and member allocation schedules for LLP members.
• Prior-year Forms 1116 with the carryover schedule by category.
• HMRC's helpsheet HS263 position where any relief is claimed on the UK side for US tax.

Mistakes we see most often

• Treating the UK tax spike as a genuine increase in tax and quietly accepting a wasted credit.
• Claiming the whole 31 January payment as creditable foreign income tax without separating NIC.
• Making the accrual election in one year and reverting in the next.
• Reporting transition profit as additional US taxable income — it is not; it is a UK timing construct.
• Ignoring the four spread instalments after the transition year, on the assumption the problem ended in 2023/24.
• Letting UK and US advisers work in isolation, so the UK spreading election is finalised before anyone models the US consequence.

The principle worth remembering

Foreign tax credits are a matching exercise. Basis period reform was a UK-only change to what a UK tax year contains, and the United States neither recognised it nor adjusted for it. Everything that goes wrong flows from that single fact, and almost everything that can be recovered is recovered by mapping UK tax back to the economic period it belongs to — then filing consistently across every year of the catch-up. Done properly, the exercise usually reduces stranded credits, sometimes materially, and always produces a defensible file.

If you are a UK sole trader, partner or LLP member with a US filing obligation and a transition year in your recent history, the position is worth reviewing before another spread instalment passes. Contact our cross-border team for a confidential consultation. We will look at your UK computations and your existing US returns together, quantify what has been stranded, and set out the corrective route — whether that is an amended return, an accrual election, or a full streamlined submission. There is no obligation, and the conversation is entirely private.