Common Tax Mistakes Expats Make in the UK, Ireland and Northern Ireland and How to Avoid Them

If you live abroad, move to the UK or Ireland, or split your life between countries, it is very easy to make the wrong assumption about tax. A lot of expats think the rules are simple: once you leave a country, your tax position there ends; once you arrive somewhere new, you only need to think about what you earn there. In reality, your tax position usually depends on your residence status, where your income comes from, when you moved, and how the relevant tax authority expects that income to be reported. In the UK, including Northern Ireland, residence is tested under the Statutory Residence Test. In Ireland, residence is generally based on day-count rules and related concepts such as ordinary residence and domicile. 

That is why expats often run into problems without meaning to. The issue is usually not dishonesty. It is assuming the answer is obvious when it is not. If your financial life crosses borders, getting proper support from a specialist in Cross-Border Accounting & Tax can help you stay compliant and avoid paying more tax than you need to. 

1. Assuming tax residence is just about where you live

One of the most common mistakes is believing tax residence is simply based on where your home is. In the UK and Northern Ireland, that is not how it works. HMRC looks at things like how many days you spend in the UK, whether you work here, whether you have accommodation available, and what ties you still have to the UK. You may be resident if you spend 183 days or more in the UK, and in some cases you can also become resident under the sufficient ties test. You may be automatically non-resident if, for example, you spend fewer than 16 days in the UK, or fewer than 46 days if you have not been a UK resident in the previous 3 tax years.

In Ireland, the test is different. Revenue says you are generally tax resident if you are present in Ireland for 183 days or more in a tax year, or 280 days or more across the current and previous tax year together, subject to a minimum of 30 days in either year. Ireland also works on a calendar-year tax year, not the UK tax year running from 6 April to 5 April. 

The best way to avoid problems is to track your days carefully and review your position before the end of the relevant tax year. If you wait until filing season, it is much harder to rebuild the facts accurately. If you need help getting your reporting in order once your status is clear, Tax Compliance support can make that process far more manageable. 

2. Thinking foreign income does not matter if it has already been taxed abroad

A lot of expats assume that if foreign tax has already been paid, their home tax authority does not need to know about that income. That is a risky assumption. In the UK and Northern Ireland, UK residents are generally taxed on worldwide income and gains unless a specific relief or regime applies. HMRC’s SA106 pages are used to declare foreign income and gains and to claim foreign tax credit relief where appropriate.

In Ireland, if you are resident and domiciled there, you are generally chargeable on worldwide income, subject to any relevant Double Taxation Agreement. But Ireland also still uses domicile and ordinary residence in a way that can affect how foreign income is taxed. Revenue also states that certain non-domiciled individuals may be taxed on a remittance basis for relevant foreign income, which is very different from the current UK position.

The practical answer is simple: keep proper records for all foreign income, all tax deducted overseas, and the exchange rates or currency values used in your return. If your records are spread across different accounts and countries, Digital Bookkeeping can help you create a cleaner system.

3. Missing split-year treatment when you move in or out of a country

The year in which you arrive or leave is often the year that causes the most confusion. Many expats assume the tax authority will automatically split the year between resident and non-resident periods. That is not something you should assume.

In the UK and Northern Ireland, split-year treatment can apply when you move in or out of the UK, but only if the conditions are met. HMRC makes clear that it is not automatic and that other conditions apply.

In Ireland, split-year treatment exists too, but Revenue states that it applies to employment income only in your year of arrival or departure, provided the conditions are met. That is an important difference, and one that can easily be missed if you read UK guidance and assume it works the same way in Ireland.

If you are moving between the UK, Northern Ireland, and Ireland, the arrival or departure year needs special attention. It is often worth reviewing that year separately rather than treating it like a standard annual return.

4. Relying on old non-dom advice

This is one of the easiest ways to get caught out now. In the UK, including Northern Ireland, the old non-domicile regime for income tax and capital gains tax was replaced from 6 April 2025 by the 4-year Foreign Income and Gains regime for qualifying new UK residents. Broadly, eligibility depends on having been a non-UK resident for at least 10 consecutive tax years before becoming a UK resident.

But that is a UK and Northern Ireland point, not an Irish one. Ireland still uses domicile as part of how foreign income can be taxed, and Revenue guidance still refers to remittance basis treatment for certain non-domiciled individuals.

So if you are relying on an older article, a forum thread, or advice aimed at a different country, there is a good chance it no longer reflects the current rules. This is exactly where a technical review through Specialist Tax can be useful. 

5. Filing the wrong return, the wrong supplementary pages, or not filing at all

Expats also make mistakes with the return itself. Some assume they do not need to file because tax is dealt with through payroll. Others do file, but leave out the residence or foreign income sections.

In the UK and Northern Ireland, HMRC uses supplementary pages depending on your circumstances. SA106 is used for foreign income and gains, while SA109 covers residence and related matters. HMRC’s notes also say that if you are resident abroad and complete SA109, you should not complete SA106.

In Ireland, self-assessed individuals generally use Form 11, and Revenue requires residence and domicile information to be completed there where relevant.

That means there is more to getting it right than simply submitting a basic return. If your position is international, your filing needs to be approached that way too.

6. Forgetting that tax can still apply after you leave

Leaving a country does not always mean leaving its tax net completely. In the UK and Northern Ireland, HMRC says non-residents generally pay UK tax on UK income, while UK residents are normally taxed on worldwide income. That means UK property income, certain UK workdays, and some other UK-source income may still be taxable after you move abroad.

Ireland works in a similar broad sense, but under its own rules. Revenue says that if you are neither resident nor domiciled in Ireland, you are generally chargeable on Irish-source income and foreign employment income where the duties are carried out in Ireland.

This is why it helps to list your income by source rather than just by country of residence. UK property income, Irish-source income, overseas salary, foreign investments, and pensions should each be reviewed properly. If you are also running a business, SME Business Solutions can help you keep the wider picture under control.

7. Keeping poor records and rough currency conversions

Another common problem is using estimates instead of clean records. Expats sometimes rely on rough exchange rates, incomplete statements, or old spreadsheets that no longer match what actually happened. That can lead to inaccurate returns, missed reliefs, and unnecessary queries from HMRC or Revenue.

A much better approach is to keep statements, tax certificates, dates of receipt, and evidence of the exchange rates used. Good records do not just protect you if a tax authority asks questions. They also make it easier to spot planning opportunities earlier. That is part of the wider joined-up approach SCC Chartered Accountants takes across areas such as Corporate Finance, Forensics & Investigations, Recovery & Restructuring, Internal Audit, and Sustainability. 

8. Waiting until filing season to think about tax

This is often the mistake behind all the others. By the time a filing deadline is close, you are trying to piece together travel dates, foreign tax deductions, old payslips, and bank statements from multiple countries. That is when expensive mistakes happen.

The smarter approach is to review your affairs early, ideally before the end of the tax year or shortly afterwards. In the UK and Northern Ireland, the tax year runs from 6 April to 5 April. In Ireland, the tax year runs from 1 January to 31 December. That difference alone is enough to catch people out when they move between the 2 systems.

Reviewing things early gives you time to gather documents, check residence properly, and understand whether any reliefs or reporting obligations apply. You can also stay up to date through News & Insights and find practical support through Resources. 

UK, Ireland and Northern Ireland at a glance

For readers dealing with more than 1 jurisdiction, these are the main practical differences to remember:

UK and Northern Ireland

• Same tax system for these purposes, administered by HMRC.
• Tax year runs from 6 April to 5 April.
• Residence is based on the Statutory Residence Test and sufficient ties.
• The old remittance basis for income tax and capital gains tax was replaced from 6 April 2025 by the 4-year Foreign Income and Gains regime for qualifying new residents.

Ireland

• Separate tax system, administered by Revenue.
• Tax year runs from 1 January to 31 December.
• Residence is generally based on 183 days in 1 year or 280 days over 2 years, subject to the 30-day minimum rule.
• Split-year treatment is available in certain cases, but Revenue says it applies to employment income only.
• Ireland still uses domicile and can still apply a remittance basis to certain non-domiciled individuals.

FAQs

Do expats always have to file a tax return?

No, not always. But many do. If you have cross-border income, residence matters to declare, or foreign income that needs to be reported, a return may be required. The exact filing method depends on whether you are dealing with HMRC in the UK or Revenue in Ireland.

If I pay tax abroad, does that mean I will not pay tax again?

Not necessarily. You may still need to report the income in the country where you are resident, even if tax has already been paid elsewhere. Relief may be available under domestic rules or a Double Taxation Agreement, but it depends on the facts.

Does Northern Ireland have different expat tax residence rules from the rest of the UK?

No. For these income tax and residence points, Northern Ireland follows the same UK-wide HMRC rules as England, Scotland, and Wales.

Has the remittance basis ended?

In the UK and Northern Ireland, the old non-dom remittance basis for income tax and capital gains tax was replaced from 6 April 2025 by the Foreign Income and Gains regime for qualifying new residents. In Ireland, remittance basis treatment can still apply in some circumstances for non-domiciled individuals.

Can I still owe tax after moving abroad?

Yes. You may still have tax obligations in the country you left if you continue to receive income sourced there, such as rental income, work income connected to that country, or other taxable receipts.

Where should I start if I think I have made a mistake?

Start by reviewing your residence status, where each source of income arose, and which tax years are affected. Once you know which country’s rules apply to each part of your income, it becomes much easier to correct the reporting properly.

If you want straightforward advice on your tax position as an expat dealing with the UK, Ireland, or Northern Ireland, speak to SCC Chartered Accountants through the Contact Us page and get clear, practical support before a small mistake becomes a much bigger problem.