What is the Seafarers' Earnings Deduction?
The Seafarers' Earnings Deduction — usually shortened to SED — is a UK tax relief for people who work aboard ships. If you qualify, it lets you deduct 100% of your eligible seafaring earnings from your UK income tax. In plain terms, it can reduce the tax on your sea earnings to zero.
It has been part of UK tax law for many years, yet a surprising number of seafarers who could claim it never do — usually because they don't know it exists, aren't sure they qualify, or find the rules off-putting. The relief is generous, but it is not automatic: you have to claim it, and you have to meet the conditions.
The three things you need
- You work on a ship — Your duties must be performed on a ship. This is important, because an offshore installation — such as a fixed oil or gas platform — is specifically not a ship in the eyes of the law. More on that in our guide on vessel status.
- You work outside the UK — Your duties must be carried out wholly or partly outside the UK, and your employment must include at least one voyage in each tax year that begins or ends at a foreign port.
- You build a long enough qualifying period — You must string together an “eligible period” of at least 365 days, made up mostly of days spent outside the UK. This is where most of the detail — and most of the mistakes — live.
How the deduction is claimed
SED is claimed through Self Assessment. You report your seafaring earnings on your tax return and enter the deduction in the seafarers' section of the additional information pages. If tax has already been taken off your wages through PAYE, claiming SED is how you get it back.
You must claim within four years of the end of the tax year the earnings relate to. So, for example, a claim for the 2025/26 tax year needs to be made by 5 April 2030. Leave it later than that and the relief is lost, even if you qualified.
The mistakes that cost people their relief
- Miscounting days in and out of the UK — the day you are outside the UK at midnight is the day that counts, and getting this wrong can break a qualifying period.
- Spending too long back in the UK and breaking the “half-day” rule (covered in our qualifying-period guide).
- Assuming an offshore installation counts as a ship — it doesn't, and days spent working on one don't earn the deduction.
- Poor record-keeping — HMRC can ask you to prove you were outside the UK, and the burden of proof is on you.
- Missing the four-year claim window entirely.
Keeping the right records
You don't have to send HMRC your evidence when you claim, but you should keep it in case they ask. Useful records include your discharge book, a voyage or travel log, air tickets and boarding passes, and copies of your seafarer contracts. A simple, consistent record of every date you left and re-entered the UK is the single most valuable thing you can keep.
Is it worth claiming?
For a qualifying seafarer, SED can be one of the most valuable reliefs in the UK tax system — often worth thousands of pounds a year. The rules reward careful record-keeping and punish guesswork, which is exactly why specialist help pays for itself. If you think you might qualify, the best time to start keeping proper records is now, not at the end of the year.