Why SeaClaim exists

The problem

SED eligibility depends on three interlocking rules: the 365-day minimum period, the half-day rule, and the 183-day UK visit limit. Every time you return to the UK, HMRC checks whether your cumulative UK days have exceeded half the total days in your eligible period. Breach it and your period ends - not on the day you returned, but on the previous return date.

Most seafarers handle this one of two ways. Track it on a spreadsheet and hope the arithmetic is right. Or pay an accountant £300-400/year to do it for them. The spreadsheet approach is error-prone. The accountant approach works but means paying a professional to do something software can handle better.

A few SED tracking apps exist, but most give you a pass/fail result with no explanation. You're told you qualify or you don't, with no visibility into which rule mattered, how close you came to a breach, or what would happen if you stayed ashore another week.

What SeaClaim does differently

SeaClaim implements the full HMRC HS205 working sheet methodology. Not a simplified estimate - the actual column-by-column calculation that HMRC uses to verify your claim. Every eligibility result comes with a plain-English explanation of why: which rule was applied, how far short you were, and what changed the outcome.

The dashboard shows your position in real time. Days in hand before a rule break. Your predicted fail date. How close you are to the 183-day threshold. This means you can make informed decisions about shore leave and contracts based on data rather than guesswork.

When you're ready to file, SeaClaim generates a completed HS205 working sheet as a PDF - columns A through H - ready for your Self Assessment or your accountant.

How we built it

The algorithm was built from primary HMRC sources: the HS205 helpsheet, the Employment Income Manual (EIM33001), and the relevant sections of the Income Tax (Earnings and Pensions) Act 2003. Every edge case we could find - mid-period breaches, 183-day truncation, complex multi-year qualifying windows, combined periods - is handled.

The implementation has been tested against hundreds of real-world scenarios. If you want to verify a result, the full working is visible inside the app. Compare it against HMRC's own HS205 guidance yourself.

What it doesn't do

SeaClaim does not provide tax advice. A complete SED claim also requires evidence of earnings (P60, payslips), evidence of voyages (discharge book, ship records), and the rest of your Self Assessment return. SeaClaim handles the most complex, error-prone piece - qualifying day tracking and HS205 generation - so you arrive at your accountant or your Self Assessment with the hard bit already done.