UK Tax Return for Expats: A Complete Self-Assessment Guide

Filing a UK self-assessment tax return is rarely straightforward for someone living abroad. The rules governing what income you must declare, how relief is calculated, and which supplementary pages you need differ materially from those that apply to a UK-resident taxpayer. This guide explains the process from start to finish, with particular attention to the issues most commonly encountered by expatriates, non-residents, and internationally mobile individuals. All rules reflect HMRC's published guidance as of 2026 — seek professional advice before filing, as the rules may change.

Who Must File a UK Self-Assessment Return?

Not every expat needs to file. HMRC issues a notice to file to certain individuals automatically, but many non-residents fall into self-assessment without receiving one. You are generally required to register and file if, in a UK tax year (6 April to 5 April), any of the following apply:

• You were UK-resident for part or all of the year and had untaxed income or capital gains above the reporting threshold.
• You were a non-resident but received UK-source income not fully taxed at source (for example, rental income under the non-resident landlord scheme, or income from a UK partnership or trading business).
• You had UK employment income where the PAYE code did not cover your full liability.
• You realised a chargeable capital gain on a UK residential property — this is reportable within 60 days of completion even if no overall tax is due.
• You received taxable income in excess of £150,000, regardless of residency (HMRC raised the income trigger for filing to £150,000 from the 2023/24 tax year onwards).
• You are entitled to claim relief under a double taxation agreement and wish to reclaim UK tax deducted at source.
• You are a recent arriver claiming relief under the four-year Foreign Income and Gains (FIG) regime (which replaced the remittance basis from 6 April 2025).

If you are in any doubt, HMRC's online tool at gov.uk can help you determine whether registration is required. It is advisable to err on the side of registration: the penalties for late or non-registration are often more costly than the compliance itself.

Registering for Self-Assessment

If you have not previously filed a self-assessment return, you must register with HMRC. For most individuals, this is done online via the Government Gateway. You will need a National Insurance number and a UK address or the address of an authorised tax agent. HMRC will then issue a Unique Taxpayer Reference (UTR), which identifies you for all future correspondence.

The deadline for registering is 5 October following the end of the tax year in which your liability arose. So if you had UK rental income in the year to 5 April 2026, you must register by 5 October 2026. Registering late can itself attract a penalty, though HMRC has discretion in some circumstances.

Non-residents who do not have a UK National Insurance number face additional hurdles. HMRC will require evidence of identity and, in some cases, a covering letter from a tax agent. Allow extra time if you do not have an NI number.

Key Deadlines

• 31 January following the end of the tax year: deadline for online submission of the tax return and payment of any balancing payment. For the 2025–26 tax year, this is 31 January 2027.
• 31 July: deadline for the second payment on account (where payments on account apply).
• 60 days from completion: deadline for reporting UK residential property gains (a separate CGT return is required regardless of whether self-assessment is already in progress).
• 5 October: registration deadline for new filers (see above).

Time zones are not an accepted excuse for late filing. If you submit your return at 23:59 UK time on 31 January, it is on time. Filing from Sydney or Dubai does not extend the deadline.

The Return Itself: Main Form and Supplementary Pages

The core self-assessment return is the SA100. However, most expats will also need one or more supplementary pages. The most relevant are:

SA102 — Employment: for UK employment income, including directors' remuneration from UK companies.

SA105 — UK Property: for rental income from UK land and buildings. If you operate under the non-resident landlord scheme and tax has been withheld by your letting agent, you will reclaim the credit here.

SA106 — Foreign: for foreign income and gains, including interest, dividends, and employment income, together with foreign tax credit relief claims.

SA108 — Capital Gains: for disposals of UK assets, including UK residential property (though the 60-day return is separate), business assets, and shares.

SA109 — Residence, etc.: this is the most important page for internationally mobile individuals. It covers the statutory residence test, split-year treatment claims, claims under the four-year FIG regime for recent arrivers, and days counted in the UK. (The remittance basis and its annual charge were abolished from 6 April 2025; the page is now built around the residence-based rules.)

Filing the wrong supplementary pages, or failing to include SA109 when you should, is one of the most common errors HMRC identifies in non-resident returns.

Gathering the Necessary Information

Before beginning your return, collate the following:

• P60 or P11D from any UK employer.
• Bank statements for UK accounts, noting interest credited gross or net.
• Statements from UK letting agents showing gross rent received and allowable expenses.
• Broker statements for UK share sales and dividends.
• Foreign income receipts if you are resident in the UK and need to declare worldwide income.
• Details of any remittances to the UK of pre-6 April 2025 foreign income or gains if you previously used the remittance basis (the basis itself ended on 5 April 2025, but transitional rules still apply to those historic funds).
• Disposal completion statements for any UK property sold in the year.

If you use a UK tax agent, they will provide a data-gathering questionnaire. Even so, it is your legal responsibility to ensure the return is accurate and complete. Never sign off a return without reviewing it carefully.

Claiming Double Taxation Relief

If you are resident in a country with which the UK has a double taxation agreement (DTA), you may be entitled to relief on UK-source income. The most common situation for expats is where UK bank interest, dividends, or pension income has been taxed at source in the UK but is also taxable in your country of residence.

Relief can be claimed in two ways: via an exemption at source (using form DT-Individual, which you submit to HMRC before income is paid) or as a credit against your UK tax liability on the self-assessment return. In most cases, the DTA provides that the state of residence gives credit for UK tax; however, some treaties allocate taxing rights exclusively to one state. The specific article of the relevant DTA will determine which applies.

HMRC publishes a full list of DTAs in force. As of 2026, the UK has comprehensive double taxation agreements with over 130 countries, though the scope of each agreement varies. Seek advice specific to your country of residence — professional guidance is particularly important for US citizens, given the complexity of the UK-US treaty.

Common Errors and How to Avoid Them

Not disclosing non-UK income if UK-resident: if you are UK-resident, you are taxable on worldwide income unless you are a recent arriver within the four-year FIG regime. Many expats mistakenly assume that foreign income received in a foreign bank account is outside HMRC's reach — it is not.

Failing to file after property disposal: since 2015, non-residents must report disposals of UK residential property within 60 days. HMRC uses Land Registry data to identify disposals. Penalties for late filing start at £100 even if no tax is due.

Incorrect treatment of split years: if you left or arrived in the UK during the year, you may be entitled to split-year treatment. This effectively divides the tax year into a UK-resident part and a non-resident part. The claim must be made explicitly on SA109; it does not apply automatically.

Mixing up payments on account: if your prior year liability exceeded £1,000, HMRC will issue payments on account for the following year (each equal to 50% of the prior year liability). Non-residents often forget to account for these when budgeting.

Mishandling foreign income reporting: with the remittance basis abolished from 6 April 2025, UK residents (outside the four-year FIG window) report foreign income on the arising basis on the SA106. Recent arrivers claiming FIG relief must make the claim correctly on the SA109; missing the four-year window or failing to claim has consequences that should be planned in advance.

Penalties for Late Filing and Payment

HMRC operates a tiered penalty regime. A return filed one day late attracts an automatic £100 penalty. Further daily penalties of £10 apply from three months after the deadline (up to a maximum of £900). After six months, a further 5% penalty applies to the tax unpaid at that date; a further 5% applies after twelve months. In cases of deliberate non-compliance or concealment, penalties can rise to 200% of the tax due for offshore matters.

Interest accrues on unpaid tax from the due date. As of 2026, the rate is based on the Bank of England base rate plus 2.5 percentage points. Expats living in countries with less visible HMRC enforcement activity sometimes take a relaxed approach to compliance — this is a significant error, as HMRC has access to financial account information from over 100 jurisdictions through the Common Reporting Standard.

When to Seek Professional Advice

Self-assessment is manageable for straightforward cases — for example, a non-resident with only UK rental income and no other complications. However, the following situations call for professional advice:

• You are a recent arriver assessing the four-year FIG regime, or you hold pre-April 2025 funds that interact with the remittance-basis transitional rules and the Temporary Repatriation Facility.
• You have complex offshore structures (trusts, foundations, corporate holdings).
• You are a US citizen or green card holder, given the interaction between US and UK tax obligations.
• You have exercised employment-related securities or share options.
• You have an HMRC enquiry open or anticipate one.
• You have failed to file returns for multiple years and wish to regularise your position.

A specialist international tax adviser will often save more in tax planning than their fees cost, particularly for HNW individuals with complex cross-border affairs. The key is to engage an adviser who understands both HMRC's requirements and the tax rules of your country of residence.

How Global Investments Can Help

Global Investments works with internationally mobile HNW clients across major property markets worldwide. Our advisers understand the intersection of UK self-assessment obligations and the tax regimes of your country of residence. We can coordinate with your local tax adviser to ensure your return is filed accurately and on time, that all available reliefs are claimed, and that your broader financial structure is optimised for your circumstances. Contact us to discuss your situation before the next filing deadline.