UK Self Assessment for Internationally Connected Filers

What Self Assessment is for

PAYE deals with most UK employees invisibly: tax comes out of salary and nothing more is asked. Self Assessment exists for everything PAYE cannot see — self-employment, rental income, foreign income and gains, larger investment income, and various one-off situations. You complete a return after the tax year ends, HMRC calculates (or you calculate) the liability, and you pay any balance.

The UK tax year runs from 6 April to 5 April, which matters more than it sounds for anyone also filing a US return on a calendar year. The same income sits in different "years" in each country, and lining up the two is one of the core disciplines of cross-border filing.

Who has to file

There is no single test; you file if HMRC has told you to, or if your circumstances require it. Common triggers for internationally connected people include:

• Self-employment income (a sole trader or partner)
• Foreign income or gains — dividends from a US brokerage account, US rental income, interest on US accounts
• UK rental income
• Claiming the split-year treatment or relief under the US–UK treaty
• Capital gains above the annual exempt amount, or where a return is otherwise needed
• High income alongside child benefit, or untaxed income PAYE cannot collect

For a US citizen resident in the UK, the "foreign income" trigger is the big one. Once you are UK resident, the UK generally taxes your worldwide income — so the US bank interest and brokerage dividends that felt domestic before the move are now foreign income on a UK return. New arrivals within their first four years may be able to elect into the foreign income and gains (FIG) regime, which changes that analysis but still typically involves a return.

Registering: the 5 October deadline

If you need to file for the first time, you must tell HMRC by 5 October following the end of the tax year in question. Register online; HMRC issues a Unique Taxpayer Reference (UTR), which you need before you can file anything. Allow time — the UTR arrives by post, and the process is slower for people without a UK National Insurance number or credit footprint, as new arrivals often discover.

Missing the registration deadline is not fatal if the tax itself is paid on time, but leaving registration late compresses everything that follows into the winter rush.

Filing and paying: 31 January

The online return for a tax year is due by 31 January following the end of that tax year, together with any balancing payment. Paper returns have an earlier deadline of 31 October. File even one day late and an automatic £100 penalty applies, with escalating penalties and interest thereafter.

The foreign pages (SA106) are where non-UK income is reported and where foreign tax credit relief is claimed for US tax properly paid on the same income. Which country gets first taxing rights on each type of income is determined by source rules and the US–UK treaty — this is precisely where double taxation relief is won or lost.

Payments on account

If your Self Assessment bill is large enough, HMRC requires two payments on account towards the following year — each half of the prior year's liability, due 31 January and 31 July. They surprise almost every first-time filer, because the first 31 January bill can effectively be one and a half years of tax at once: the balance for last year plus the first instalment for this year.

You can claim to reduce payments on account if you expect a lower liability, but underestimating attracts interest on the shortfall. For people with lumpy cross-border income — RSU vests, US capital gains, a one-off distribution — getting this projection right is worth real money in cash-flow terms.

How the UK and US returns interact

Two practical points dominate. First, credits flow both ways but not automatically: the country with primary taxing rights on each income type takes its tax, and the other gives credit — get the direction wrong and you can pay twice, or claim a credit the IRS or HMRC later unwinds. Second, timing mismatches are normal: a 2025/26 UK liability paid in January 2027 relates to income spread over parts of two US tax years. There are established ways of matching credits across the mismatch, but they need to be applied consistently year after year.

None of this is a reason to fear Self Assessment. It is a reason to treat the UK and US filings as one coordinated project — which is exactly how we prepare them.