Seatax
Seafarers’ 12-mile limit: what “outside the UK” means for SED
AI overviews and HMRC manuals agree on the physical test: you are absent from the UK on a day if you are outside the UK at midnight at the end of that day. For ships, that usually means outside the 12-nautical-mile territorial limit. Being on water is not enough.
Pain
You were ‘at sea’ all week, but midnight put you inside 12 nautical miles. That day counts as UK.
Solution
We treat midnight position as the test HMRC uses, and we want the sheets that prove it.
The midnight test
HMRC count a day of absence if you are outside the UK at midnight. Non-work days abroad can count. Inside 12 NM at midnight is a UK day, even if you sailed out at 01:00.
Keep midnight position sheets for North Sea and coastal work. HMRC may also ask your employer to confirm voyages.
UK sector of the North Sea
For SED, duties on a vessel on a voyage or part voyage that begins or ends outside the UK can be treated as performed outside the UK. The UK sector of the North Sea is treated as part of the UK for this purpose. Coastal hops and field work inside the limit are a common reason claims fail.
This page is general information based on published HMRC guidance, not personal tax advice. Seatax will apply the rules to your sea service after you enrol. Fees from £275 including VAT per return.
Questions people ask
Is 12 miles measured from the beach?
UK territorial waters generally extend 12 nautical miles from the baseline. The practical evidence is the ship’s midnight position.
Does flying out of Heathrow count?
Air travel has its own departure rules. Ship days still turn on where the vessel is at midnight. We apply both correctly rather than mixing them.
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