UK Seafarers Earnings Deduction explained

What SED is and why it matters

The Seafarers Earnings Deduction (SED) is a UK tax relief that allows eligible seafarers and offshore workers to deduct 100% of their qualifying offshore earnings from their UK taxable income. In practice, this means paying zero UK income tax on earnings from qualifying offshore employment.

For a worker earning €550/day on a 2/2 rotation (182 days/year), SED can save €20,000–€30,000 per year in income tax — a substantial real-world impact on net income.

Who qualifies for SED

SED is available to UK resident taxpayers who meet the following conditions:

• You are employed (not self-employed) under a contract of employment — SED does not apply to limited company contractors
• Your employment involves working on a ship or vessel
• You have been outside the UK for at least 183 days in any 365-day period (the 'half-year test')
• Your foreign earnings are from duties performed outside the UK

The 183-day rule explained

The core qualifying test is that you must be outside the UK for at least 183 days in any 365-day rolling period. This is not a calendar year — it's a rolling 365-day window.

Important details about how days are counted:

• A 'day outside the UK' means midnight outside the UK — if you're offshore at midnight, it counts as a qualifying day
• Travel days count if midnight is spent outside the UK
• Days at home in the UK do not count as qualifying days
• The 183 days do not need to be consecutive — they accumulate across multiple trips
• You can fail the test in one 365-day period and pass in another — check every rolling period separately

What counts as qualifying employment

• Employment on a vessel operating in international waters or in foreign waters
• The vessel must be classified as a 'ship' under HMRC's definition
• Fixed offshore platforms (jackets, jack-ups) are NOT vessels for SED purposes — platform-based offshore workers on PAYE employment may not qualify for SED via this route
• FPSO and other floating production units — specialist advice is needed as classification varies
• Crew transfer vessel (CTV) employment typically qualifies as the CTV is a vessel in international waters

How to keep records that support a SED claim

HMRC can and does challenge SED claims. Without clean records, you cannot substantiate your qualifying days and risk losing the entire deduction.

Records to maintain:

• Flight manifests and boarding cards — evidence of every departure from and return to the UK
• Offshore crew sheets and muster logs — official records of which days you were aboard the vessel
• Contract documentation — copies of all employment contracts for the period, showing vessel name and employment terms
• Payslips — all payslips showing offshore earnings and PAYE deductions
• Offshore medical and certificate records — indirect evidence of offshore activity

How to claim SED

• Complete a UK Self Assessment tax return (SA100) each tax year — SED is claimed here even if you would not otherwise need to file
• Complete the SA102 (Employment) supplement and the SA109 (Residence, Remittance Basis etc.) pages
• Claim the deduction on the 'Foreign income' section — enter your qualifying earnings and the deduction
• Submit by 31 January following the end of the tax year
• Consider using a specialist offshore or seafarers tax accountant — errors in SED claims can result in penalties and a tax bill equal to the full deduction claimed

Common mistakes that invalidate an SED claim

• Claiming SED as a limited company director — SED only applies to PAYE employment
• Failing the 183-day test because of excess days in the UK — even one extra day at home can tip you below the threshold
• Inadequate travel records — being unable to evidence qualifying days with contemporaneous documentation
• Confusing SED with other tax reliefs — SED is specific; don't conflate it with general overseas worker rules
• Filing without specialist advice when the situation is complex — mixed employment types, periods of self-employment, or vessel classification uncertainties all require specialist input