What Self Assessment is
Self Assessment is the system HMRC uses to collect tax from people whose tax isn't fully handled through PAYE — and to give relief to people who are owed it. For seafarers, it's not just an obligation; it's usually the route to claiming the Seafarers' Earnings Deduction and getting back tax already paid.
When you'll usually need to file
You'll generally need to complete a Self Assessment return if any of the following apply:
- You want to claim the Seafarers' Earnings Deduction — this is claimed through your tax return.
- You have foreign earnings, or you're paid by an overseas employer without UK tax deducted.
- You have untaxed income — for example rental income from a property, or self-employed or freelance work.
- You need to sort out your residency position for the year, or claim relief under a double taxation agreement.
- HMRC has sent you a notice to file — once they ask, you must, even if you think you owe nothing.
The key deadlines
The UK tax year runs from 6 April to 5 April. For a given tax year, the main online filing deadline is the following 31 January. So the 2025/26 tax year (6 April 2025 to 5 April 2026) must be filed online by 31 January 2027, and any tax owed is due by the same date.
- Register — If you've not filed before, register for Self Assessment in good time — ideally by 5 October following the end of the tax year.
- File online — By 31 January after the tax year ends.
- Pay — Any tax due by 31 January too.
Remember the separate SED point: even though you file the year by 31 January, you have up to four years after the end of a tax year to claim SED for it — so overlooked earlier years can often still be reclaimed.
What happens if you don't file
If you were required to file and didn't, HMRC charges an automatic penalty for a late return — and further penalties and interest build up the longer it's outstanding, on top of any tax due. Just as importantly, not filing usually means not claiming, so you may be leaving a valuable SED refund unclaimed.
In other words, skipping the return can cost you twice: once in penalties, once in relief you never received.
Filing is how you get money back
For most qualifying seafarers, the tax return isn't a bill — it's a refund. Tax taken through PAYE on earnings that qualify for SED is reclaimed by filing. Not filing simply leaves that money with HMRC.
The takeaway
If you're a seafarer with foreign or qualifying earnings, the safe assumption is that you probably should be filing — and that doing so is likely to work in your favour. If you're unsure whether you need to, or you've missed previous years, it's worth checking sooner rather than later, while refunds are still in reach.