UK Self Assessment for Expats: Forms, Deadlines, and Common Errors

The UK's self-assessment system is not designed with the internationally mobile individual in mind. Forms, supplementary pages, and filing requirements pile up quickly once cross-border elements are involved. For the expat who has recently left the UK, is currently non-resident, or has returned after a period abroad, the system presents a series of practical and technical challenges that, if mishandled, can result in penalties, interest charges, and HMRC enquiries.

This guide sets out the key elements of self-assessment that matter most for expats and internationally mobile individuals, and the errors most frequently encountered.

Do You Need to File?

Not every UK taxpayer needs to file a self-assessment return. HMRC operates a "tick the box" system — if you meet any of the criteria below, you must register for self-assessment and file a return:

• Self-employed income or partnership income
• Untaxed income over £1,000 (including rental income, savings interest above the personal savings allowance, or dividend income above the dividend allowance)
• Capital gains above the annual exemption (£3,000 in 2026/27)
• UK source income as a non-resident (rental income, employment income with UK workdays, UK pensions or interest)
• High income Child Benefit charge (income over £60,000)
• Any tax liability that cannot be collected through PAYE
• Claims for specific reliefs (e.g. loss relief, overseas workday relief)

For expats and non-residents with UK rental property, investment accounts, or any form of UK source income, self-assessment is almost always required.

Registration: If you are not already registered, you must notify HMRC by 5 October following the end of the relevant tax year (i.e. by 5 October 2026 for the 2025/26 tax year). Failure to notify on time can itself trigger a penalty.

The SA100 — The Main Return

The SA100 is the core self-assessment return. It records:

• Income from employment (boxes 1–10)
• Interest and dividends (boxes 11–17)
• Income from trusts and estates
• Reliefs and adjustments (pension contributions, gift aid, etc.)
• Tax already deducted at source

Most taxpayers complete the SA100 online via HMRC's Government Gateway or through commercial tax software. Paper returns are still accepted but the deadline is earlier (31 October vs 31 January for online).

SA109 — The Residence Pages

The SA109 supplementary pages are the most important — and most technically demanding — section for expats. They must be completed whenever your residence status needs to be determined, including if you:

• Were not UK resident for the full tax year
• Are claiming split-year treatment
• Are claiming overseas workday relief (OWR)
• Were claiming the remittance basis in a pre-2025/26 return (note: the remittance basis was abolished from 6 April 2025 and replaced by the four-year Foreign Income and Gains (FIG) regime — for 2025/26 onwards the SA109 is used to claim FIG relief instead)

Statutory Residence Test boxes: You must indicate which SRT tests you applied (Automatic Overseas Test, Automatic UK Test, or Sufficient Ties Test) and the number of UK days spent during the year.

Split-year treatment: If you left or arrived in the UK during the tax year and qualify for split-year treatment, you must claim this explicitly in the SA109. HMRC does not apply split-year treatment automatically. The relevant case must be ticked (there are eight cases — for UK leavers, the relevant ones are typically Cases 1, 2, and 3; for arrivals, Cases 4, 5, and 8).

Missing the split-year claim is a costly error: without it, HMRC will assess you as UK resident for the full year, taxing worldwide income for all 12 months.

Overseas workday relief (OWR): If you worked in the UK during the year but also worked overseas, and you qualify as a new UK resident (typically in the first three years of UK residence), you may be entitled to OWR, which reduces UK employment income for the proportion of workdays spent outside the UK. This relief is now only available for individuals within the "four-year foreign income and gains" (FIG) regime introduced in April 2025. The precise qualifying conditions and claim procedure should be verified with a specialist.

SA106 — Foreign Income

The SA106 pages cover:

• Income from overseas employment
• Overseas pension income
• Income from overseas property (rental income from foreign properties)
• Foreign savings and investment income (interest, dividends from overseas sources)
• Foreign tax paid (used to calculate Foreign Tax Credit Relief)

The most common errors on SA106 include:

• Reporting net rather than gross income: Always declare the gross amount (before withholding tax), then claim the credit
• Wrong currency: All amounts must be reported in sterling, converted at the appropriate exchange rate
• Missing notional distributions: Accumulation units in reporting funds generate taxable income even without a cash payment

Non-Resident Landlord Scheme (NRLS)

If you own UK rental property and are non-UK resident (or intend to spend more than six months per year abroad), you should register with HMRC's Non-Resident Landlord Scheme. This allows you to receive rental income gross (without deduction of basic rate tax at source by your agent or tenant).

Without registration under the NRLS, your letting agent is legally required to deduct 20% basic rate tax from the rent before passing it to you. While you can reclaim this through self-assessment, it creates a cash flow disadvantage.

Registration is made via Form NRL1 and takes approximately three to five weeks. Approval is not automatic — HMRC must be satisfied that your UK tax affairs are up to date.

You must still file self-assessment returns and declare the rental income, even if no tax is deducted at source through the NRLS.

The Penalty Regime

HMRC's penalty regime for late filing, late payment, and inaccurate returns has become progressively more punitive in recent years. Key points:

Late filing penalties (online SA return, due 31 January):

• £100 penalty for returns filed up to 3 months late
• Additional daily penalties of £10 per day for returns 3–6 months late (up to £900)
• Further penalties of 5% of tax owed or £300 (whichever is higher) at 6 months and 12 months

Late payment penalties: 5% of unpaid tax at 30 days, 6 months, and 12 months after the due date, plus interest at HMRC's published rate (set at the Bank of England base rate plus 4%, which is 7.75% per annum as of June 2026).

Inaccuracy penalties: For offshore matters — which include any non-UK income or assets — penalties of up to 200% of the unpaid tax can apply in the most serious cases. For careless (non-deliberate) inaccuracies relating to offshore income, voluntarily disclosed, penalties typically range from 10%–30%.

The statutory excuse of "reasonable care" is a defence to inaccuracy penalties — but must be genuine and documented. Reliance on professional advice that turns out to be incorrect can constitute reasonable care if the adviser was qualified and the reliance was appropriate.

Common Errors for Expats

Based on our experience, the most frequent and costly errors in expat self-assessment returns include:

1. Not filing at all — particularly common among non-resident landlords who believe NRLS registration satisfies all obligations
2. Failing to claim split-year treatment in the year of departure or arrival
3. Incorrectly calculating UK days — HMRC's day counting rules require a midnight count; partial days in the UK may or may not count depending on circumstances
4. Missing the SA109 entirely — some third-party software and self-completed returns miss these pages if the "foreign" tick-boxes in the SA100 are not correctly set
5. Claiming OWR without meeting the qualifying conditions — particularly following the April 2025 reforms to non-dom and FIG regime rules

Keeping Records

HMRC expects taxpayers to retain records supporting their self-assessment returns for a minimum of:

• 5 years from the 31 January filing deadline for employees and investors
• 5 years 10 months for the self-employed

For offshore matters, HMRC's discovery window extends to 20 years in cases of deliberate behaviour. Keeping thorough records — bank statements, tenancy agreements, exchange rate evidence, day count diaries — is important protection in the event of an enquiry.

How Global Investments Can Help

Self-assessment for expats sits at the intersection of UK domestic tax law, international treaty provisions, and specific offshore reporting obligations. At Global Investments, our tax specialists work with internationally mobile clients to ensure their UK returns are complete, accurate, and filed on time. We co-ordinate with local advisers in clients' countries of residence to ensure that UK and overseas filings are consistent and that treaty relief is properly claimed. If your self-assessment affairs are not fully up to date, or if you are unsure whether you need to file, contact us for a confidential review.

This article is for informational purposes only and does not constitute regulated financial or tax advice. Tax rules are complex and change frequently. Seek professional advice specific to your circumstances.