An S at the front of your tax code means your employer uses the Scottish income tax rates, and a C means the Welsh rates. The prefix follows where your main home is for most of the tax year, not where your employer is based or where you work. It changes the rates your pay runs through, not the tax-free allowance set out in the rest of the code.
What each prefix changes
The prefix is a tax regime rather than part of the code itself. HMRC's manual is explicit that the S or C sits outside the code and is set by the system from your residency. That is why the same code with and without an S gives you the same tax-free pay and a different tax bill.
S sends your pay through the Scottish rates, set by the Scottish Parliament. They run from a starter rate of 19% up to a top rate of 48%. There are more bands than in the rest of the UK and they change at different points, so two people on the same salary, with the same allowance, can take home different amounts. On £50,000 a year the gap is £1,496.05: £3,168.67 a month on the Scottish ladder against £3,293.34 on the rest-of-UK one.
C sends your pay through the Welsh rates, set by the Welsh Government. For 2026/27 the Welsh rates are the same as the rates in England and Northern Ireland, so the C prefix makes no difference to what you pay this year. It still matters: it records where your income tax is counted, and a future change in Wales would reach you through the same code. Codes with no prefix use the rates for England and Northern Ireland.
How HMRC decides
The test is where you live, not where you work. The prefix applies where you are resident in the UK for tax purposes and your sole or main place of residence is in Scotland, or in Wales, for more of the tax year than in another part of the UK.
Two points catch people out. Your employer's location is irrelevant: you can work for an Edinburgh company from Newcastle and have no S prefix, or work in London from a flat in Cardiff and have a C. And it is all or nothing for the year. Spend more of the tax year living in Scotland than elsewhere in the UK and you are a Scottish taxpayer for the whole of that year, not only for the months you were there.
If you have homes in two parts of the UK, the main one is usually where you live and spend most of your time, whether you own it, rent it or live there for free. GOV.UK says it can be the home where you spend less time, if that is where most of your possessions are, where your family lives, where you are registered for things like your bank account, GP or car insurance, or where you belong to clubs and societies. That is how it often falls for lorry drivers, offshore workers and people in the armed forces. Where it is genuinely unclear, GOV.UK's guidance counts days, and where you were at midnight decides the day.
Moving between nations
Tell HMRC your new address when you move. Whether the move changes your rates at all depends on the same whole-year test: GOV.UK says you pay the rates of the place you move to only if you live there for longer than anywhere else in the UK during that tax year. Move in February and you stay on the old ladder for the whole of that year, then start on the new one from the following 6 April.
Where the move does change your rates, GOV.UK says the new rates are backdated to the start of the tax year in which you moved, and the tax taken from your pay is adjusted so the year comes out right. Leave the address out of date and you can spend months on the wrong rate ladder before the correction arrives in one lump.
What the prefix does not change
Your Personal Allowance is set UK-wide at £12,570, and the number in your code is worked out the same way wherever you live.
National Insurance is the same everywhere in the UK, because it is not a devolved tax and it takes no notice of your tax code.
Dividends and savings interest are taxed at the UK rates even for a Scottish taxpayer. Student loan repayments follow the plan you are on rather than your tax code, so an S prefix does not change them either.
Prefixed second-job codes
The prefix rides along on every code, and the Scottish versions do not line up with the rest of the UK. SBR is the Scottish basic rate, which is straightforward. But SD0 is the Scottish intermediate rate, 21%. SD1 is the Scottish higher rate, 42%. SD2 is the advanced rate, 45%, and SD3 the top rate, 48%.
Read SD0 as the equivalent of D0 and you will be wrong by a wide margin on both the rate and the tax it takes.
My code has an S but I live in England. What now?
Update your address with HMRC. The prefix is set automatically from the address HMRC holds, so an out-of-date record is the usual cause. HMRC issues a corrected code to your employer, and the year is put right through your pay once the new code is in use.
Do I pay Scottish tax if I only work in Scotland?
No. Working in Scotland while your main home is elsewhere in the UK does not make you a Scottish taxpayer, and commuting across the border changes nothing on its own. The same test applies to pension income from a former employer: where you live sets the prefix, and the pension provider uses the code HMRC sends it.