Take home pay drops for no reason you can see, and the only clue is a few characters on the payslip nobody has ever explained. Most bad codes come from one of six things, and all of them are fixable in one place.
Find the tax code. If it is 1257L with nothing after it, you have the full Personal Allowance on a cumulative basis and the code is probably right. If it carries W1, M1 or X, if it starts with a letter, or if the number is a long way off 1257, something has told HMRC that your year does not look the way you think it looks. That is the thing to go and find.
The number is your tax free pay for the year with the last digit dropped, so 1257 means £12,570. The letter says what kind of allowance it is. At 2026/27 rates the standard Personal Allowance is £12,570, which is where 1257L comes from.
| Code | What it means | Worth checking? |
|---|---|---|
| 1257L | Full Personal Allowance, cumulative | Normal for one job |
| 1257L W1 / M1 / X | Full allowance, but each pay run taxed in isolation | Yes, if it lasts past a couple of paydays |
| BR | Everything taxed at the basic rate, no allowance here | Normal on a second job, wrong on your only one |
| 0T | No allowance at all, taxed across the bands | Almost always yes |
| D0 / D1 | Everything at the higher or additional rate | Yes, unless your other income really is that high |
| K at the front | Deductions exceed your allowance, so income is added | Yes, find out what the deduction is |
The rest of the payslip is worth knowing too. Our guide to payslip abbreviations covers every code and column you will meet.
HMRC builds your code from what it believes about your whole year. It is rarely lying. It is usually working from something out of date, and the shape of the error points straight at the cause.
| Cause | The tell |
|---|---|
| You started a job with no P45 and no starter checklist | 0T on a week 1 or month 1 basis from your first payday |
| HMRC thinks you still have an old job | BR or 0T on the job you actually have |
| A company benefit was added, changed or ended | The number moves by roughly the value of the benefit |
| Tax underpaid in an earlier year is being collected | A lower number, or a K code, with no change in your pay |
| HMRC's estimate of your income is stale | D0 or a reduced allowance after a pay cut or a part year |
| Two sources of income, split the wrong way | Both jobs taxed as if the other one is the main one |
Priya, a lab technician in Swansea, is a fair example. She joined a mining equipment manufacturer in month 3 without a P45. She had been out of work for the first two months with no benefits, so she should have ticked statement A on the starter checklist and got 1257L cumulative. She ticked B instead, got 1257L on a month 1 basis, and stayed on it because nothing in the system knew she had not worked since April. Her code was giving her a twelfth of the allowance every month and quietly ignoring the two twelfths she had not used. Nothing was broken. Nobody had told HMRC anything.
Payroll cannot help you here, and it is not being awkward. An employer runs the code HMRC issues and has no authority to choose another one. The change has to come from HMRC, which then sends the employer a P6 coding notice telling them to switch.
This is the part people brace for and it is usually the easiest. If the overpayment happened in the current tax year and the corrected code is cumulative, you do not claim anything. The next payroll run works out the whole year again, sees that too much has gone, and the difference comes back in your pay. On a big correction that payslip can look startling. It is right.
An earlier tax year works differently. HMRC reconciles after 5 April and sends a P800 tax calculation if you have paid too much or too little, and those go out between June and March of the following tax year. A P800 is not a bill or a cheque, it is a statement of position, with the repayment or the collection arranged from there.
Underpayments are the same story in reverse. HMRC will usually collect the shortfall through your tax code the following year rather than ask for it in one go, which is exactly what a K code often turns out to be. That only works if you owe less than £3,000 and earn enough to cover it. If it can't be collected that way, HMRC writes to you about paying it instead, for example with a Simple Assessment bill.
A DPNI scheme is a real PAYE scheme, so it gets coding notices like any other. What changes is who is watching. The scheme is registered in your name, so there is no UK finance team reading the HMRC inbox, and a P6 can sit unopened for months while the wrong code keeps running. If you are on one of these, checking the code yourself once a quarter is not paranoia, it is the job.
If all of this is new, our guide for employees on direct payment schemes is the place to start, and there is a separate piece on whether you also need to file a tax return.
No, and you would not want them to. They run the code HMRC gives them. Asking payroll is still worth doing, because they can tell you which code they are actually running and whether it is on a week 1 or month 1 basis, and that is half the diagnosis.
Fifteen working days for HMRC to update the code and notify both sides, then two or three pay cycles before it lands on a payslip. If you have just started a job, wait 35 days before contacting HMRC, because the starter information usually arrives on its own.
Not for the current year on a cumulative code. It comes back through the payroll automatically. For an earlier year, wait for the P800, or contact HMRC if you are confident you overpaid and nothing has arrived.
Because your employer did not have your full picture. With no P45, the starter checklist decides it: statement B (another job or certain benefits since 6 April) means the emergency code, which at 2026/27 rates is 1257L on a week 1 or month 1 basis. It gives you the full allowance but ignores everything earlier in the year, so it can overtax anyone who has had a gap. With no P45 and no checklist at all, HMRC tells employers to use 0T on a week 1 or month 1 basis, which gives no allowance at all. A completed starter checklist before your first payday is the fastest way out of it. After that, your employer keeps the code until HMRC sends a new one. The payroll glossary explains the rest of the forms.
General guidance for people employed in the UK, correct for the 2026/27 tax year at the time of writing. It is not personal tax advice, and HMRC's own guidance is the authority on any figure or deadline here. Scotland sets its own income tax bands, so a Scottish code carries an S and the arithmetic differs. For your own position, check it or ask us.
If your employer is overseas and your PAYE runs in your own name, we pick the notices up, apply them and tell you in plain English why your take home moved. Tell us the situation and we will tell you which scheme we expect to ask HMRC for and a fixed monthly fee.
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