Seafarers' Earnings Deduction (SED)

The Seafarers' Earnings Deduction (SED) is one of the most financially significant benefits available to UK seafarers — and one of the most misunderstood. Get it right and you pay no UK income tax on your seafaring earnings. Get it wrong and HMRC will want the tax back, possibly with interest and penalties. This guide explains the rules clearly.

What Is the SED?

Under section 378 of the Income Tax (Earnings and Pensions) Act 2003, a UK-resident seafarer employed under a contract of employment wholly or partly abroad can claim a 100% deduction on their foreign employment income — effectively paying zero UK income tax on those earnings — if they meet the qualifying criteria. This is the SED.

It applies to employees only (not self-employed seafarers) and to employment income from duties performed wholly or partly outside the UK on a ship. It does not apply to offshore oil and gas workers on installations (who have separate arrangements).

The 183-Day Rule

To qualify for a full year, your period of foreign employment must include at least one voyage outside UK territorial waters during a qualifying period of 365 consecutive days that includes at least 183 days outside the UK.

More precisely: you must count days outside the UK across a 365-day period. Any day on which you were outside the UK at midnight counts. Days spent in transit through UK airports or ports generally do not count as UK days. The qualifying period does not need to be a calendar year.

The Half-Day Rule

Under the half-day counting rule, days of departure and arrival in the UK each count as half a day. So a voyage that departs UK on Monday and returns Wednesday counts as: half Monday + Tuesday + half Wednesday = 2 days outside UK. This catches some seafarers out when their count is close to the 183-day threshold.

What Counts as "Outside the UK"?

UK territorial waters extend 12 nautical miles from the baseline. If your vessel is within those 12 miles at midnight, that day counts as a UK day. When operating in UK coastal waters, rivers, or ports, most of your time may count against you. Deep-sea trades are significantly easier to qualify under.

How to Claim

Claim the SED on your self-assessment tax return. You will need to complete the HS205 helpsheet (available from HMRC) to calculate the deduction. The process requires you to:

1. Record every day you spent outside the UK during the qualifying period
2. Identify which days were spent on foreign employment duties
3. Calculate the deduction using the HS205 worksheet
4. Enter the deduction amount on your SA100 tax return

HMRC may request evidence — discharge book entries, joining and leaving dates, voyage records, or payslips showing the periods of employment. Keep these records for at least six years.

HMRC Benchmark Rates

If your employer does not pay a subsistence allowance for travel between the vessel and your home, HMRC publishes benchmark rates for subsistence and overnight expenses for seafarers that you can claim without receipts. These are published in the HMRC Employment Income Manual and updated periodically. Check the current rates at gov.uk.

Common Mistakes

• Not keeping a day-count diary. The most common reason claims fail. Start recording immediately.
• Confusing "outside the UK" with "at sea". A week in Rotterdam alongside is outside the UK and counts. A weekend in a UK port does not.
• Including offshore oil and gas days. Workers on offshore installations (rigs, FPSOs) cannot claim SED — different rules apply.
• Assuming the employer handles it. SED is a personal claim on your tax return. Some employers provide payslips that facilitate the claim, but the responsibility and the claim is yours.
• Mixing employed and self-employed periods. SED only applies to employment income, not income from self-employment or freelance contracts.

Partial Year Claims

If you only qualify for part of a tax year, you can still claim a partial SED. The deduction is proportionate to the qualifying days within the year. This is common for seafarers who return ashore mid-contract, change employers, or start their seafaring career partway through a tax year.

Getting Professional Advice

Tax situations involving multiple employers, mixed-sector employment, or complex voyage patterns benefit from professional advice. Several firms specialise in seafarer tax, including STL (Seafarers Tax Limited) and others listed by Nautilus International. The cost of an accountant who knows the SED rules is almost always repaid many times over by a correctly filed return.