The same test as everyone else
The Statutory Residence Test does not carve Crown servants out into a special residence category, any more than it looks at nationality or domicile. You apply the tests in order for each tax year. If you were UK resident in any of the three preceding years you are a leaver, so fewer than 16 UK days makes you automatically non-resident, while an arriver has a 46-day threshold, and full-time work overseas with fewer than 91 UK days and UK workdays kept below 31 is the third automatic overseas route. Spend 183 or more days in the UK and you are automatically resident. Only if no automatic test settles it do you count ties, and leavers face the tighter table: at 16 to 45 UK days it takes 4 or more ties to be resident, at 46 to 90 days just 3. A spouse who stays UK resident, accommodation kept available for 91 days, 40 or more UK workdays, more than 90 UK days in either of the two prior years, and the country tie can each count against you.
Your Crown salary stays within UK tax either way
The reason Crown servants feel different from ordinary expats is the treatment of pay, not residence. Under section 28 of the Income Tax (Earnings and Pensions) Act 2003, general earnings from Crown employment performed outside the UK are treated as subject to United Kingdom tax unless they fall within an exception made by order, so the default is that a Crown salary earned in Nairobi, Brussels or anywhere else remains within UK income tax even for a non-resident. That makes the residence analysis a question about everything around the salary rather than the salary itself, and it also means moves between Crown employment and private contracting for the same posting can produce very different UK tax outcomes, so the employment status of each engagement is worth pinning down before you rely on it.
Why the answer still matters, and what to do
Residence still decides how the UK taxes the rest of your life: a non-resident is generally taxable only on UK-source income and certain UK assets, while a resident is taxed on worldwide income and gains. Rental income, investment portfolios, offshore accounts built up on a posting and the timing of any disposals can all turn on the answer, and the year you leave the UK or return may qualify for split-year treatment if you fall within one of the eight statutory cases. Keep contemporaneous day-count records, because the burden of proving your position sits with you, and remember that non-residents usually cannot use HMRC's free online return once the SA109 residence pages are needed. Horizon UK Tax Solutions handles residence reviews and returns for Crown servants and other cross-border clients on a fixed fee agreed upfront, starting with a free 30-minute clarity call at /book.