Do Irish Tradespeople Working in Britain Need to File a UK Tax Return?

Many Irish tradespeople who have worked in Britain assume that once they leave the UK and come home, their UK tax obligations are finished. The CIS was deducted from their payments, the contractors kept them right, and that is the end of it.

This is not correct. In most cases, an Irish tradesperson who has earned income in the UK construction industry has obligations that continue after they return to Ireland - and often significant money owed to them by HMRC that they have not claimed.

Who Needs to File a UK Tax Return

You are required to file a UK self-assessment tax return if any of the following apply:

You were self-employed in the UK (including as a construction subcontractor under CIS). Your UK income from self-employment was more than 1,000 pounds in the tax year. You received UK income that was not taxed at source, or was taxed under CIS and you wish to claim the deductions as a credit.

For an Irish tradesperson who worked in the UK on a construction site and had CIS deductions made, the answer is straightforward: you need to file a UK self-assessment return. The CIS deduction means HMRC has received money on your account. The only way to reconcile this against your actual tax liability - and to claim a refund if you are owed one - is through a return.

Failing to file does not mean HMRC forgets about the CIS deductions. It means they hold your money indefinitely, and you have no mechanism to claim it back.

The UK Tax Year

The UK tax year runs from 6 April to 5 April the following year. This is different from both the Irish calendar year (January to December) and from what most people intuitively expect.

UK tax year 2024-2025 covers 6 April 2024 to 5 April 2025. If you worked in the UK during this period, the income goes on the 2024-2025 UK return.

The filing deadline for a UK self-assessment return filed online is 31 January following the end of the tax year. So the 2024-2025 return is due by 31 January 2026.

Paper returns have an earlier deadline of 31 October. Given that most returns are filed online, the January 31 deadline is the relevant one for most people.

What Happens If You Don’t File

Missing the UK self-assessment filing deadline attracts automatic penalties:

A 100 pound penalty for being one day late. A further 10 pounds per day penalty for returns more than three months late (up to 90 days, meaning up to 900 pounds additional). A further 300 pound or 5% of the tax due penalty at six months. Further penalties at 12 months.

For a tradesperson who is owed a CIS refund and simply did not know they needed to file, receiving penalty notices from HMRC on top of having their own money held is a frustrating outcome that is entirely avoidable.

HMRC is increasingly efficient at identifying individuals who had CIS deductions but did not file returns. The CIS deduction data flows from contractors to HMRC automatically, so HMRC knows who had deductions made and can issue penalties for non-filing.

Registering for UK Self-Assessment

To file a UK return, you must first be registered for UK self-assessment. If you are not already registered, you do this through gov.uk by completing the online registration process.

You will need:

Your UK National Insurance number (if you have one from previous UK work). If you do not have a UK National Insurance number, you need to apply for one through the UK Department for Work and Pensions - this takes several weeks, so start early. Your personal details including your Irish address. Details of your UK income and the period you worked.

Once registered, HMRC issues you a Unique Taxpayer Reference (UTR) number. You need this to file the return.

The Irish Side of the Equation

Filing the UK return handles your HMRC obligations. It does not handle your Irish obligations.

As an Irish tax resident, you are required to declare worldwide income - including your UK earnings - on your Irish income tax return (Form 11). The UK tax paid on those earnings (reduced by any CIS refund received) is credited against the Irish tax liability on that income under the Ireland-UK double taxation agreement.

In practice, because the UK personal allowance is 12,570 pounds and the allowable expenses of UK construction work are often significant, the UK tax after expenses is typically very small. The Irish credit for UK tax is correspondingly small, but the income must still appear on the Irish return.

A tradesperson who had 25,000 pounds of UK earnings, 8,000 pounds of allowable expenses, and a UK personal allowance that covers most of the remainder will have a near-zero UK tax liability after the credit and a near-zero Irish tax liability on the UK income. But both returns need to be filed.

How Many Years Can You Go Back?

UK self-assessment returns can be filed within four years of the end of the relevant tax year. After that, the right to reclaim CIS deductions is lost.

The four-year window means that if you worked in the UK in 2021-2022, 2022-2023, 2023-2024, and 2024-2025 without filing returns, all four years are still recoverable. Working backwards through the years and claiming all outstanding CIS refunds at once is a common reason Irish tradespeople engage a cross-jurisdictional accountant.

Beyond four years, HMRC will not process the return and the deductions are not recoverable.

Getting Help

The combination of UK self-assessment filing, CIS refund claims, and Irish Form 11 declarations is not straightforward for someone who has not navigated it before. The practical complexity is compounded if multiple years are involved or if the expense records from UK jobs are incomplete.

An accountant who understands both the Irish and UK systems can manage the full process - registering with HMRC if needed, gathering CIS statements from contractors, calculating allowable expenses, filing the UK returns, and ensuring the Irish return is correctly updated.

The refund obtained will often substantially exceed the accountant’s fee for the work.