US UK Tax Returns Preparation: Why Two Returns Never Match

US UK tax returns preparation almost always produces two returns that report different income for what feels like the same year — and that is correct, not an error. The UK and US measure a different period, allow different reliefs, define assets differently and convert currency at different moments. A defensible reconciliation, not a matching total, is the goal.

Why do my UK and US tax returns show different income for the same year?

This is the single most common question sophisticated dual filers put to Jungle Tax, usually with a UK tax computation in one hand and a Form 1040 in the other, and usually with a note of alarm. The instinct is reasonable: one set of bank accounts, one salary, one portfolio, one year of life. Two documents that disagree must mean one is wrong.

They are both right. The two returns are not two attempts to measure the same quantity. They are two different measurements, taken over different periods, of differently defined things, expressed in different currencies, under systems that answer to different legislatures. Asking why they do not match is a little like asking why a thermometer and a barometer disagree about the weather.

What matters for a wealthy household — particularly one catching up several years at once through the IRS streamlined procedures — is not that the numbers agree. It is that a competent preparer can explain, line by line and in writing, exactly why they differ, and can show that every pound of UK income has been picked up somewhere on the US side and every dollar of creditable UK tax has been claimed in the right US year. That explanation is the asset. It is what survives an IRS examination, an HMRC enquiry, a lender's due diligence or a divorce disclosure. The matching total is a fantasy.

This guide sets out the four structural reasons the two returns cannot agree, then explains what a preparer actually reconciles, and why the reconciliation matters more than the totals when years are being brought current together.

The four structural reasons the numbers cannot agree

Every divergence a dual filer will ever see traces back to one of four causes. Almost every panicked email traces back to the first.

1. The periods are different: 6 April to 5 April against 1 January to 31 December

The UK tax year runs from 6 April to 5 April. The US tax year for individuals is the calendar year. That is roughly a three-month offset, and it means no UK year ever sits inside a single US year.

The practical consequences are larger than the arithmetic suggests. A UK P60 for the year ended 5 April reports pay and PAYE that belong to two separate US returns — roughly the last quarter of one calendar year and the first three quarters of the next. A UK Self Assessment computation covering April to April will similarly straddle two Forms 1040. A bonus paid in March, a dividend declared in February, a capital disposal in April: each of these lands in one UK year but must be sliced into the correct US year on the basis of when it was actually received, not when the UK year that contains it ended.

Any preparer who simply drops the P60 figure onto a Form 1040 is producing a wrong return. The correct method is to rebuild the calendar year from primary records — monthly payslips, contract notes, dividend vouchers, bank credits — and to treat the P60 as a control total for the UK year rather than an input to the US year. We cover the mechanics in detail in our guide to the UK/US tax year mismatch in catch-up filing.

2. The reliefs and allowances exist on one side only

Each system grants exemptions and allowances the other simply does not recognise. Where an item is exempt in one country and taxable in the other, the income lines diverge permanently — this is not a timing difference that reverses in a later year.

Common examples for a UK-resident American include the UK's personal savings allowance and dividend allowance, which shelter amounts from UK tax while the IRS taxes the same interest and dividends in full; the ISA, which HMRC treats as tax-free and the IRS treats as an ordinary taxable account (often holding passive foreign investment companies); and the UK's tax-free pension commencement lump sum, whose US treatment depends on the treaty analysis and the specific scheme rather than on HMRC's label. Running the other way, the US allows a foreign earned income exclusion and a standard deduction with no UK counterpart, and the UK denies relief for items a US return deducts freely.

The important consequence is that UK-exempt income frequently generates a US tax charge with no corresponding UK tax to credit against it. Investors are usually surprised by this. It is a recurring finding in the returns we prepare for high-net-worth clients, and it is one of the few places where a dual filer genuinely writes a cheque to the IRS.

3. The two systems measure assets differently

Beyond timing and allowances, the systems disagree about what an asset is and when it produces income. This is where cross-border returns become genuinely technical, and where generalist preparers on either side of the Atlantic tend to go quiet.

• Collective investments. A UK unit trust, OEIC, investment trust or UCITS ETF is an ordinary holding to HMRC. To the IRS it is a passive foreign investment company, taxed under a punitive default regime with its own annual reporting on Form 8621 and its own income figure that bears no relation to anything on the UK return.
• Pensions. A UK registered pension scheme normally grows without UK tax and is taxed on withdrawal. The US treatment of growth, employer contributions and lump sums turns on the treaty and on the scheme's characteristics, and the two systems can recognise income in entirely different years.
• Foreign currency itself. The US treats non-dollar currency as property. Repaying or refinancing a sterling mortgage, or moving material sums between GBP accounts, can create a taxable exchange gain on the US return that has no UK counterpart at all — an income item that literally cannot appear on the UK computation.
• Capital gains. The UK and US differ on rates, on annual exemptions, on the treatment of a main residence, on how losses are used, and on the base cost of assets acquired before arrival. The same disposal routinely produces two very different gains.
• Trust and company interests. A structure that is transparent for one system may be opaque for the other, pushing income onto one return years before — or after — it appears on the other.

4. Currency is converted at different points, using different conventions

The UK return is prepared in sterling; the US return must be expressed in US dollars. The IRS position is that amounts are translated at the rate prevailing when the item is received, paid or accrued, with a yearly average accepted for many recurring items where that is a fair reflection. HMRC has its own published rates and its own conventions.

Two honest preparers using two defensible rate methodologies will produce two different dollar figures from the same sterling income. Convert a year of salary at a monthly rate and you get one number; convert it at the annual average and you get another; convert a March bonus at the date of receipt and it differs again. None is wrong. What is wrong is switching methods between years, or between the income line and the tax-credit line, without documenting why. We set out our approach in the guide to exchange rates in a multi-year catch-up.

US and UK returns compared: the same year, measured two ways

Feature	United Kingdom (HMRC)	United States (IRS)
Tax year measured	6 April to 5 April	1 January to 31 December
Basis of taxation	Residence (and, historically, domicile / now the FIG regime for new arrivals)	Citizenship and lawful permanent residence, wherever you live
Return vehicle	Self Assessment (SA100 plus SA106 for foreign income and other schedules)	Form 1040 plus international schedules and forms
Filing deadline	31 January following the end of the tax year (online)	15 April, with automatic and elective extensions for those abroad
ISA income	Exempt	Fully taxable; underlying funds usually PFICs
UK collective funds	Ordinary investment income and gains	PFIC regime, Form 8621, punitive default treatment
Main residence gain	Private residence relief may exempt in full	Limited statutory exclusion; excess is taxable
Foreign currency	Not itself an asset producing income	Property; disposals can create taxable exchange gain or loss
Double tax relief	Foreign tax credit relief within the Self Assessment computation	Foreign tax credit on Form 1116, by income category, with carryback and carryforward
Reporting of accounts	No general account-disclosure regime for residents	FBAR and Form 8938 disclosure of foreign accounts and assets

What does a preparer actually reconcile between the two returns?

A proper cross-border engagement does not attempt to make the two returns agree. It produces a reconciliation workpaper — a bridge — that starts with the UK figures and walks, with a stated reason for every step, to the US figures. When we prepare returns for both jurisdictions, that bridge is a deliverable in its own right, and it is retained with the file.

Step one: build the calendar year from primary records

We rebuild each US calendar year from source documents rather than from UK year-end summaries: monthly payslips, employer bonus statements, contract notes and dividend vouchers, rental statements, and bank and custodian records. The P60, P11D and UK computation become control totals used to prove nothing has been dropped, not inputs to the US return.

Step two: identify the permanent differences

Every item exempt on one side and taxable on the other is listed separately, with the reason. UK personal savings and dividend allowances, ISA income, exempt lump sums, PFIC inclusions, section 988 currency gains, differences in capital gains base cost and residence relief — each becomes a labelled line on the bridge. These never reverse and should never be presented as "reconciling items" that will wash out.

Step three: identify the timing differences

Items taxed by both systems but in different years — the April-to-April slice of employment income, a bonus straddling the boundary, an accrued but unpaid UK liability, income recognised on receipt in one system and on entitlement in the other. These do reverse, and the bridge should show where.

Step four: fix the currency methodology and apply it consistently

We record which rate source is used, whether an item is translated at spot or at the yearly average, and why. That methodology is then applied identically to the income line and to the foreign tax credit line, and consistently across every year in a catch-up. The IRS sets out the underlying principle in its guidance on foreign currency and currency exchange rates.

Step five: match UK tax to the correct US year and category

This is the step that goes wrong most often, and it is the one with the largest cash consequence. UK tax is credited on the US return either when paid or, if the accrual election is made, in the year to which it relates. The election is binding for future years and cannot be casually reversed. It also determines whether a January Self Assessment balancing payment for the UK year ended the previous 5 April supports a credit in the earlier US year or the later one.

Alongside the timing question, each pound of UK tax must be allocated to the correct US income category — general, passive and the rest — because credits do not move between categories. Excess credit in one category does not shelter income in another; it carries back or forward within its own. Our guide to the foreign tax credit on the paid versus accrued basis works through the election, and the companion guide to baskets and carryovers covers the allocation. The IRS overview of Form 1116, including the Schedule B carryover reconciliation, is the primary source.

Step six: check the disclosure forms against the reconciled figures

Finally, the reconciled US figures are tested against the information-return requirements: FBAR, Form 8938, Form 8621 for each PFIC, Forms 3520 and 3520-A where a foreign trust or certain pension arrangements are in point, Form 5471 for a controlled UK company. Penalties for these are assessed per form and per year, and they are the reason a technically small underpayment can become an expensive problem. Our FBAR penalty calculator gives an indication of the exposure before you speak to anyone.

Why does a defensible reconciliation matter more than matching totals?

When a single year is filed on time, the reconciliation is largely internal housekeeping. When several years are being brought current at once — the usual position for an accidental American, a returning executive, or a founder who discovered the US filing obligation late — it becomes the centre of the engagement.

The reason is procedural. Relief under the IRS streamlined procedures, including the Streamlined Foreign Offshore Procedure, depends on a certification of non-wilfulness. That certification is a narrative statement, and its credibility rests on whether the accompanying returns look like the work of someone making a genuine, careful effort to get it right. A file that shows a consistent methodology, a documented currency convention, a clear list of permanent and timing differences, and UK tax credited in reasoned years reads as exactly that. A file of round numbers lifted from P60s, with income that mysteriously matches the UK computation to the pound, does not — and invites the examiner to ask how the calendar-year figures were derived.

There is a second, quieter reason. In a multi-year catch-up, small methodological choices compound. Choose the accrual basis in the first year and you have chosen it for all of them. Use a monthly rate in one year and an annual average in the next and your foreign tax credit carryovers stop tying together, which Schedule B will expose. Misallocate a category once and the error propagates through every subsequent carryforward. The reconciliation is what keeps six or eight years internally coherent, and coherence is what makes the position defensible. We do this work as part of our IRS streamlined filing engagements.

Which figures should I stop trying to reconcile?

Clients often spend hours attempting to force agreement where none is possible. It is worth naming the items that will never tie:

• Total income. The headline figures will not match and should not be expected to. They cover different periods and different definitions.
• Total tax. The UK liability and the US liability are computed under different rate structures with different reliefs. Even where the foreign tax credit eliminates the US charge, the two totals are unrelated numbers.
• Investment income. ISA income, exempt allowances, PFIC inclusions and reporting-fund status make this the least comparable line on the return.
• Capital gains. Different base costs, different exemptions, different residence relief, different loss rules.
• Pension figures. Contributions, growth and withdrawals are recognised on different triggers.

What should tie, and what a good preparer will prove ties, is narrower and more useful: every source of income appears somewhere on both sides across the relevant years; every item of UK tax is claimed once, in one US year, in one category, and never twice; the currency methodology is the same throughout; and the carryover balances roll forward without a break.

How the UK side is affected in return

The traffic is not one-way. Where a US citizen is UK resident, the UK return must pick up worldwide income including US-source items, and HMRC's own foreign tax credit relief mechanism applies within the Self Assessment computation. That relief is limited to the UK tax attributable to the same income and, importantly, to the US tax properly payable under the treaty rather than the US tax actually withheld — a distinction that catches out anyone who has suffered excess withholding on US dividends without filing the appropriate treaty documentation with the payer. HMRC's overview of tax on foreign income is the starting point, and the reforms replacing the remittance basis with the four-year foreign income and gains regime for new arrivals have made the residence question materially more consequential for anyone who has recently moved.

For a newly arrived American executive or founder, this creates a period in which the two returns diverge more sharply than at any other point: the UK may be relieving foreign income entirely while the US taxes it in full, and the interaction determines whether the household pays UK tax at all in those years. Getting both returns prepared by the same team, from the same reconciliation, is the only reliable way to avoid claiming a credit on one side for tax that was never properly due on the other. Our UK tax services and US tax services are run from a single file for precisely this reason.

Practical documentation to keep

Whatever preparer you use, the quality of the reconciliation is limited by the quality of the records. For each year we ask for monthly payslips rather than only the P60; the full Self Assessment return and HMRC tax calculation; HMRC statements of account showing the date each payment was actually made; contract notes and dividend vouchers rather than annual consolidated summaries alone; custodian statements identifying fund holdings by ISIN so PFIC status can be determined; pension scheme statements including contribution splits; and year-end balances for every account for FBAR and Form 8938. Payment dates matter as much as amounts, because they drive the foreign tax credit year.

Speak to a specialist cross-border preparer

If you are looking at a UK computation and a US return that disagree, the useful question is not which one is wrong. It is whether anyone has written down why they differ, and whether that explanation would satisfy an examiner. For most people who come to us mid-catch-up, no one has.

Jungle Tax prepares both returns from a single reconciled file, for individuals and families whose affairs sit on both sides of the Atlantic. If you have missed US returns, unreported UK pensions, ISAs or investment accounts, or simply two returns you cannot square, contact our cross-border team for a confidential consultation. We will tell you plainly what needs to be filed, in what order, and what the reconciliation will show.