The SA100 Boxes US Citizens Get Wrong

US citizens in the UK get the SA100 wrong in four predictable places: US interest and dividends typed into UK boxes instead of the Foreign pages (SA106), US fund gains reported as capital gains, US tax credited at the wrong amount, and residence claims missing from the Residence pages (SA109).

None of these are exotic. They happen because the main SA100 form is built for someone whose income is British, and HMRC's software will accept a US figure in a UK box without complaint. The return files, the calculation runs, and the mistake only surfaces when HMRC compares notes with the IRS under the automatic exchange of information, or when your US return and your UK return stop agreeing with each other. This guide walks through the boxes we most often have to correct for American clients, and where each item belongs instead.

Key takeaways

The main SA100 interest and dividend boxes are for UK-source income only. Interest from a US bank and dividends from US shares belong on the Foreign pages, SA106.
Most US mutual funds and ETFs are not UK "reporting funds". A gain on selling one is usually taxed as income, not as a capital gain, and does not belong on the Capital Gains summary, SA108.
Foreign Tax Credit Relief for US tax is limited. For a US citizen, the UK generally credits only the US tax the treaty would allow on a non-citizen, not the full US tax you paid.
Residence status, split-year treatment and the 4-year FIG regime all live in the return itself, mainly on the Residence pages, SA109. Leave them off and HMRC treats you as fully resident on everything.
Every US item must be converted to sterling using a consistent, defensible exchange-rate method, and the figures must reconcile to your US return.

Which supplementary pages does an American in the UK usually need?

The SA100 is only the core of the return. Most of the work for a US citizen happens on the supplementary pages, which HMRC's online service adds when you answer "yes" to the screening questions at the start.

What you have	Wrong place on the return	Right place on the return
Interest from a US checking, savings or money-market account	SA100 UK interest box	Foreign pages (SA106)
Dividends from US shares	SA100 UK dividends box	Foreign pages (SA106)
Gain on selling individual US shares	Left off, or reported in dollars	Capital Gains summary (SA108), in sterling
Gain on selling most US mutual funds or ETFs	Capital Gains summary (SA108)	Foreign pages (SA106), as an offshore income gain
Salary from a US employer for work done in the UK	Left off because "it was taxed in the US"	Employment pages (SA102)
Arrival or departure part-way through the UK tax year	Not claimed	Residence pages (SA109), split-year claim
US rental property	SA100 or UK property pages	Foreign pages (SA106)

HMRC publishes the Foreign pages (SA106) and their notes on GOV.UK, and the same publication lists every box. The notes are worth reading once, even if someone else prepares your return, because they define what HMRC treats as "foreign".

Why don't US interest and dividends go in the main SA100 boxes?

The interest and dividend boxes on the SA100 itself are labelled for UK sources: UK banks and building societies, UK companies, UK authorised unit trusts. Income from a US account or a US company is foreign income, and foreign income has its own pages because HMRC needs to know the country, the amount before any foreign tax, the foreign tax taken off, and whether you are claiming relief for it.

Putting US interest in the UK interest box has three practical effects:

Foreign Tax Credit Relief becomes impossible to claim. The relief is claimed on the Foreign pages, so if the income is not there, any US tax withheld on it is simply ignored.
The figures stop matching what HMRC receives from the US. Under information-exchange agreements HMRC receives data on accounts that UK residents hold abroad. A US account whose income appears nowhere on the Foreign pages is the kind of mismatch that generates a letter.
If you later claim the 4-year FIG regime, the income is in the wrong category. The regime exempts qualifying foreign income and gains. Income recorded as UK-source cannot be exempted, because on the face of the return it is not foreign.

The £500 dividend allowance for 2026-27 (gov.uk, Autumn Budget 2025 rates) applies to foreign dividends as well as UK ones, so reporting a US dividend correctly does not cost you the allowance. The £12,570 personal allowance likewise covers foreign income unless you claim the FIG regime, which removes it for that year.

Where do US capital gains go, and why are US funds different?

For individual US shares, the gain goes on the Capital Gains summary, SA108, and it is computed in sterling. That means converting the purchase price at the exchange rate on the purchase date and the sale proceeds at the rate on the sale date. A share that made nothing in dollars can produce a UK gain or a UK loss purely because sterling moved against the dollar in between. Americans who report their US cost basis and US proceeds, converted at a single rate, routinely get the UK gain wrong.

US mutual funds and ETFs are a separate trap. The UK has a special regime for "offshore funds". A fund that holds UK reporting fund status is taxed broadly like a share. A fund without it is a non-reporting fund, and the gain on selling it is an offshore income gain, taxed at income tax rates and reported on the Foreign pages rather than the Capital Gains summary. Most US-domiciled mutual funds and ETFs have never applied for UK reporting status, because their managers have no commercial reason to. HMRC maintains a list of reporting funds; if your US fund is not on it, assume the income treatment applies.

This interacts with the US side in an awkward way. UK-domiciled funds are usually PFICs for US purposes, and US-domiciled funds are usually non-reporting funds for UK purposes. An American in the UK often has no fund that is efficient in both countries, which is a structural question to settle before you invest, not at the return stage.

How do you claim credit for US tax on the SA100?

Foreign Tax Credit Relief is claimed on the Foreign pages, item by item, and HMRC's helpsheet HS263 on relief for foreign tax paid sets out how the credit is computed. For most taxpayers the rule is simple: the UK credits the foreign tax up to the UK tax on the same income.

For US citizens there is an extra layer. The US-UK double taxation convention lets the US keep taxing its citizens as if the treaty did not exist, but it also limits what the UK has to credit. On US-source dividends and interest, the UK generally gives credit only for the US tax that the US could have charged a UK resident who was not a US citizen. The balance of your US tax is then relieved on the US side, where the US re-sources part of the income so that a UK credit can be claimed against it on Form 1116.

The practical consequences:

You cannot simply copy the US tax shown on your Form 1040 into the UK foreign-tax box. The UK figure is the treaty-limited amount, which is usually lower.
The two returns depend on each other. The US credit for UK tax and the UK credit for US tax have to be worked out together, which is why the order you prepare them in matters. Our guide on which tax return to file first, US or UK works through that sequencing.
Claiming too much UK relief is the more expensive error. Over-claimed relief is recovered with interest, and HMRC can charge a penalty if the claim was careless.

Which pages handle residence, split-year and the FIG regime?

The Residence pages, SA109, are where you tell HMRC whether you were UK resident under the statutory residence test, whether the year is a split year, and other residence-based claims. HMRC's Residence, remittance basis etc pages (SA109) and notes explain each box.

Three situations make these pages essential for Americans:

The year you arrived. If you moved to the UK part-way through the tax year that runs from 6 April to 5 April, split-year treatment can keep the US portion of the year outside UK tax. It is not automatic; it has to be claimed on the return. If you are still within your first year, start with registering for UK Self Assessment as a newly arrived American, which covers the 5 October registration deadline.
The year you leave. The same split-year logic applies on departure, and is easy to forget once you are back in the US.
A claim under the 4-year FIG regime. New arrivals who qualify can claim to exempt foreign income and gains for up to four tax years, but the claim is made on the return and it removes the personal allowance and the Capital Gains Tax annual exempt amount for that year. For a US citizen the US still taxes the same income, with no UK tax to credit against it, so the claim needs modelling across both returns before you tick the box.

What about US pensions, Social Security and US salary?

Three more items regularly land in the wrong place:

401(k) and IRA distributions are overseas pension income and go on the Foreign pages. Growth inside the plan is not reported each year; the treaty generally lets you defer UK tax until money comes out. Roth accounts are a grey area whose UK treatment turns on the treaty's pension article, so confirm it before you take money out.
US Social Security paid to a UK resident is, under the treaty, generally taxable only in the US. It should not be taxed again on the UK return, which is the opposite of what many people assume.
Salary from a US employer for days worked in the UK is UK employment income even when no UK PAYE was run and US tax was withheld. It belongs on the Employment pages, SA102, and any US withholding on it is a matter for the US return, because the UK has the primary right to tax work physically performed in the UK.

What happens if a US item is in the wrong box?

Online returns can be amended within 12 months of the 31 January filing deadline (31 October for paper). After that, an error has to be corrected by a separate overpayment relief claim if you paid too much, or by a disclosure if you paid too little. HMRC's penalty for an inaccuracy depends on whether it was careless or deliberate and whether you told HMRC before it found out, so correcting a known mistake early, unprompted, is almost always the cheaper route.

The more common consequence is quieter: HMRC queries a mismatch between the return and the overseas account data it receives, and the answer requires reconstructing several years of US income in sterling. That is far easier to do once, correctly, at the time.

How Next Tax Source prepares the SA100 for Americans

We prepare the SA100 and its supplementary pages alongside your US return, so that every US item is on the right UK page, converted at a consistent rate, and credited at the treaty-limited amount. Every return is reviewed by our IRS Enrolled Agent, who is also ACCA-qualified, and you approve and submit it yourself. If you want both sides handled by one team, our US-UK tax specialists can take on the full annual cycle, or you can book a consultation well before 31 January.

Frequently asked questions

Where does a US citizen report US bank interest on a UK tax return?+

Interest from a US bank account is foreign income, so it goes on the Foreign pages (SA106) of the UK Self Assessment return, not in the UK interest box on the main SA100. Reporting it on SA106 is also what allows Foreign Tax Credit Relief to be claimed for any US tax on it.

Are gains on US ETFs taxed as capital gains in the UK?+

Usually not. Most US-domiciled mutual funds and ETFs do not have UK reporting fund status, so a gain on selling them is an offshore income gain taxed at income tax rates and reported on the Foreign pages (SA106), not on the Capital Gains summary (SA108). Funds on HMRC's reporting fund list are treated more like shares.

Can a US citizen in the UK claim credit for all the US tax they paid?+

Generally no. Under the US-UK tax treaty, the UK credits US tax on US-source dividends and interest only up to the amount the US could charge a UK resident who is not a US citizen. The remaining US tax is relieved on the US return instead, so the two returns need to be prepared together.

Where do I claim split-year treatment on a UK Self Assessment return?+

Split-year treatment is claimed on the Residence pages (SA109) of the Self Assessment return. It applies to the tax year in which you arrive in or leave the UK if you meet one of the statutory cases, and it is not given automatically: if you do not claim it, HMRC treats you as UK resident for the whole tax year.