UK Self-Assessment Tax Return: A Guide for Expats and Internationally Mobile Individuals

The UK Self-Assessment system requires individuals to file a tax return and pay any tax due if their tax affairs cannot be settled through PAYE (Pay As You Earn) alone. For most UK residents with straightforward employment income, PAYE handles everything. For expats, non-residents with UK income, recent arrivers using the Foreign Income and Gains (FIG) regime, and individuals with overseas assets, Self-Assessment is typically required.

A note on the 2025 reforms. The remittance basis and the concept of "non-dom" taxation were abolished from 6 April 2025. They have been replaced by a residence-based system: a four-year FIG regime for new arrivers and a residence test for inheritance tax. References below to the remittance basis describe the pre-6 April 2025 position and the transitional rules that still apply to funds built up under it — the remittance basis is no longer available to elect for 2025/26 onwards.

The consequences of not filing when required — or filing incorrectly — include penalties starting at £100, interest on late payments, and potentially a formal investigation by HMRC. The Common Reporting Standard has significantly increased HMRC's visibility of offshore income and accounts, making compliance more important — and easier to enforce — than ever before.

Who Must File a UK Self-Assessment Return?

You are required to file a UK Self-Assessment tax return for a tax year (6 April to 5 April) if any of the following apply:

Non-residents with UK income:

• You are non-UK resident but have UK rental income
• You have UK employment income or director's fees from a UK company
• You receive pension income from a UK source
• You receive other UK-source income (royalties from UK intellectual property, interest from UK bonds)
• In general: if you are non-resident but have UK-source income that exceeds your personal allowance (£12,570 currently, subject to change and potential withdrawal for very high earners under the personal allowance taper)

UK residents with foreign income:

• Any individual UK-resident with income from overseas sources (dividends, interest, rental income, employment income from overseas) that is not fully covered by PAYE

Recent arrivers and those with offshore funds:

• If you are a recent arriver claiming relief under the four-year FIG regime (SA109 form required)
• If you have pre-6 April 2025 unremitted foreign income or gains and remit them, or use the Temporary Repatriation Facility to designate them

Other common triggers:

• Income from self-employment or a partnership exceeding £1,000
• Income above £100,000 (the personal allowance taper applies and must be settled through SA)
• Capital gains above the annual exempt amount (£3,000 from 2024/25)
• Receipt of child benefit where either you or your partner earns over £60,000 (the High Income Child Benefit Charge applies)
• A director of a UK company with further untaxed income to report (HMRC no longer treats company directorship alone as an automatic Self-Assessment trigger, but dividends and other untaxed income above the relevant allowances must be declared)

If in doubt, the default is to register for Self-Assessment and file. The cost of compliance is modest compared to the penalties for non-compliance.

Key Deadlines

Understanding the deadlines is important — HMRC's penalties system is automatic and does not wait for explanations.

• 5 October: Deadline to notify HMRC that you need to file a tax return for the previous tax year (if you have not filed before)
• 31 October: Deadline for paper SA returns
• 31 January: Deadline for online SA returns (this is the usual deadline for most filers)
• 31 January: Deadline for payment of all tax due for the previous tax year AND payment on account for the current year (the first of two payments on account)
• 31 July: Deadline for the second payment on account

Late filing penalty: £100 for a return filed up to 3 months late. Additional £10/day for up to 90 days thereafter (if more than 3 months late). Further penalties for very late filing. Interest accrues on unpaid tax from 31 January.

What to Declare: The Key Forms and Pages

A UK Self-Assessment return uses the main SA100 form plus supplementary pages depending on your circumstances:

• SA100: The main return (personal details, employment, state benefits, reliefs)
• SA102: Employment income (including P60 details)
• SA103: Self-employment income
• SA105: UK property income (rental income from UK property)
• SA106: Foreign income (overseas employment, overseas property, foreign dividends, foreign interest — filed by UK-resident individuals with foreign income)
• SA108: Capital gains (gains from selling assets above the annual exempt amount)
• SA109: Residence and the FIG regime (required for non-UK residents filing UK returns, and for recent arrivers claiming relief under the four-year FIG regime)

The SA109: Non-Residents and Recent Arrivers

The SA109 is the supplementary page that addresses your residency position. It is essential for:

Non-residents with UK income: You confirm your non-residence using the statutory residence test, confirm that you are filing only in relation to UK-source income, and claim any applicable treaty relief (if a double tax treaty reduces or eliminates UK tax on certain income).

Recent arrivers using the FIG regime: From 6 April 2025, the remittance basis is no longer available. New arrivers who were non-UK resident for the previous 10 tax years can instead claim relief under the four-year Foreign Income and Gains (FIG) regime, which exempts qualifying foreign income and gains from UK tax for the first four years of residence. The claim is made on the SA109, and qualifying amounts must still be reported. Unlike the old remittance basis, there is no Remittance Basis Charge — that annual charge (formerly £30,000 or £60,000 depending on years of residence) was abolished alongside the remittance basis on 6 April 2025.

Transitional Remittances: Pre-April 2025 Funds

The remittance basis no longer applies for 2025/26 onwards, but funds built up under it before 6 April 2025 remain relevant:

• UK income must always be declared — the remittance basis never protected UK-source income, and UK income remains fully taxable.
• Pre-April 2025 foreign income and gains remitted to the UK must still be declared — bringing previously unremitted funds into the UK can trigger a charge, unless they are designated under the Temporary Repatriation Facility (TRF), which allows them to be brought in at a reduced rate of 12% in 2025/26 and 2026/27, rising to 15% in 2027/28, with the facility closing on 5 April 2028.
• Foreign income and gains that remain offshore retain their pre-April 2025 character and are taxed only if and when remitted.

What counts as a "remittance" is technically defined and includes more than simply bank transfers. Using offshore credit cards to pay UK costs, having overseas companies pay UK expenses on your behalf, or making gifts to UK residents from offshore accounts can all be remittances. The rules are detailed and HMRC has issued extensive guidance — professional advice before assuming something is not a remittance is strongly recommended.

What HMRC Already Knows: The CRS Factor

The Common Reporting Standard (CRS) — the automatic exchange of financial account information between tax authorities — has fundamentally changed HMRC's capacity to cross-reference tax returns against offshore financial data.

Over 100 jurisdictions participate in CRS, and financial institutions in those jurisdictions are required to:

• Identify account holders who are tax resident in other CRS countries
• Report account balances, interest, dividends, and gross proceeds annually
• Transmit this data to the account holder's home tax authority

HMRC receives CRS data and uses it through its Connect system to identify discrepancies between reported income and known offshore account activity. If you hold significant assets in an overseas bank account and do not declare the income on your UK return, the probability that HMRC will identify this has increased dramatically.

This does not mean that offshore accounts are illegal — they are entirely lawful. It means that offshore income must be correctly declared (now that the remittance basis has been abolished, UK residents are taxed on worldwide income unless within the four-year FIG window) and that the "HMRC won't find out" approach is unreliable.

The Mixed Fund Rules: A Common Complication for Former Remittance-Basis Users

Those who used the remittance basis before 6 April 2025 often have overseas accounts that contain a mix of:

• Clean capital (the original investment, pre-arrival in the UK, or funds that were already in the UK and then moved abroad)
• Foreign income
• Foreign capital gains

When any of these funds are remitted to the UK, HMRC applies a strict ordering rule: foreign income is treated as remitted first, then foreign capital gains, then clean capital. This means that an attempt to remit "clean capital" from a mixed account will trigger a tax charge if there is income or gains in the same account.

The practical solution: segregate. Keep income, gains, and clean capital in separate accounts. This is straightforward advice to give but requires discipline to maintain over time — particularly as investment returns accumulate in existing accounts.

Record-Keeping Requirements

HMRC can investigate returns for up to 4 years (careless errors), 6 years (non-deliberate but involving foreign income), or 20 years (deliberate). Keep:

• Bank statements for all UK and overseas accounts
• Investment account statements and contract notes
• Rental income accounts and expense receipts
• Evidence of remittances and their source
• Overseas tax returns and assessments (relevant for double tax treaty credit claims)

For internationally mobile individuals with complex affairs, 6 years of comprehensive record-keeping for all jurisdictions is the minimum.

How Global Investments Can Help

UK Self-Assessment compliance for expats and internationally mobile individuals is a specialist area where getting things right matters significantly. Global Investments works with UK-qualified tax advisers who specialise in non-resident returns and the post-2025 residence-based regime, and we can help you understand your filing obligations, maximise available reliefs, and ensure your SA return is consistent with your CRS-reported offshore financial data. Contact us to discuss your UK tax return requirements in confidence.

This article is for general information only and does not constitute tax advice. UK tax law is complex, changes frequently, and depends on individual circumstances. Always seek professional advice from a qualified UK tax adviser. HMRC rules and deadlines mentioned are correct as at 2026 but should be verified.