If your tax code is wrong, the quickest fix is to update the information HMRC holds about you in its Check your Income Tax online service or the HMRC app. HMRC then sends a corrected code to you and your employer within 15 working days. Any tax overpaid is usually refunded through your next or following payslip if you are paid monthly, or by your third payslip if you are paid weekly. Underpaid tax under £3,000 is normally collected through next year's code. Where HMRC failed to use information it already had, it can sometimes write the arrears off under ESC A19.
How to tell if your tax code is wrong
Find your code on your payslip, in the HMRC app, in your personal tax account or on a Tax Code Notice. Then compare it with your circumstances. For 2026/27, 1257L is the code for most people with one job or pension and the standard £12,570 Personal Allowance. Common warning signs include:
- An emergency suffix. W1, M1, X or "NONCUM" still showing weeks after starting a job;
- 0T or BR on your main job. These give no allowance, which is usually right only where the allowance is used elsewhere or has been tapered away;
- A benefit you no longer get. A company car or medical insurance still being deducted from your allowance after it ended;
- Estimated income that is too high or too low. HMRC estimates income such as savings interest or part-time earnings and adjusts the code for it;
- The wrong letter. An S or C prefix if you do not live in Scotland or Wales, a missing M or N for Marriage Allowance, or a K code you do not understand.
HMRC says a wrong code is "usually because HMRC has incorrect or missing information". HMRC tells your employer or pension provider which code to use. Corrections therefore start with HMRC's records, not with payroll.
How to update your tax code
- Sign in to Check your Income Tax (or the HMRC app) and review your employment, pension, estimated taxable income, company benefit and expenses details;
- Update anything wrong or missing. This includes jobs that have ended, a change in pay, a benefit that has stopped, or employment expenses you can claim relief for;
- Chase a missing P45. If you left a job and did not get one, ask the old employer for it. This helps HMRC close the old employment;
- Wait for the new code. HMRC updates the code and tells you and your employer within 15 working days;
- Check your payslip. Monthly-paid staff should see it on the next or following payslip, and weekly-paid staff on the third. If it has not appeared, ask payroll whether the notice has arrived.
If you cannot use the online service, you can contact HMRC. If you have just started a new job, HMRC asks you to wait 35 days so it can receive your income details first. Agents use the PAYE Coding Notice query form. Friends or family can help as a registered "trusted helper".
Paid too much tax?
When HMRC updates your code and has your full income details for the year, it works out the difference and asks your employer or pension provider to refund it through pay. This usually happens when the new code is applied. If HMRC does not have your details yet, the refund waits until it does, often after the tax year ends.
After 5 April, HMRC reconciles employees' tax and sends a P800 tax calculation letter between June and March of the following tax year. If you are owed money, you can usually claim it online and receive it within 5 working days. If the letter says a cheque is coming, it arrives within 14 days of the date on the letter. People registered for Self Assessment do not get P800s. The return is adjusted instead.
An emergency code is a frequent cause of overpayment for new starters. Our emergency tax code guide explains why.
Paid too little tax?
When HMRC estimates an underpayment, it normally adjusts your tax code to collect it. After the year end, if you owe tax through PAYE, HMRC usually collects it through your code "in equal instalments for 12 months from the start of the following tax year". That happens automatically if you pay tax through an employer or pension, have enough income above your allowance, and owe less than £3,000. HMRC's PAYE Manual confirms that underpayments of £3,000 or more cannot be collected this way. You may receive a Simple Assessment instead, with its own payment deadline. You can also choose to pay before the next tax year starts.
When HMRC's mistake means you do not have to pay
Extra-Statutory Concession A19 lets HMRC give up Income Tax arrears where all of these apply:
- HMRC did not use information it was given. For example, by you, your employer or DWP about a change of job or a pension;
- HMRC told you late. It told you about the tax more than 12 months after the end of the tax year in which it received the information;
- You reasonably believed your affairs were in order. And HMRC agrees.
Arrears from the last tax year are written off only in exceptional circumstances. Apply by phone or in writing, ideally before paying. Explain which arrears you dispute, what information HMRC should have acted on and when it was provided, and why you thought your tax was right. If HMRC refuses, you can ask for a review.
Why codes go wrong in the first place
- New job without a P45. Or the wrong statement ticked on the starter checklist. See our starter checklist guide;
- Two jobs or a job plus a pension. The allowance is put against the wrong source;
- Benefits in kind. Changes not reported to HMRC, or coded on an old estimate;
- Earlier underpayments. Being collected through the code, sometimes producing a K code. With a K code, deductions cannot exceed half of pre-tax pay;
- State Pension, Marriage Allowance or the High Income Child Benefit Charge. All of these are adjusted through the code.
HMRC's guidance is on gov.uk: if you think your tax code is wrong and if HMRC did not act on information.
Acumon reviews PAYE coding notices and P800s, handles ESC A19 claims and corrects codes for directors and higher earners through its income tax returns and tax compliance services. If HMRC has written about arrears more than a year after a job change you reported, check whether A19 applies before you pay.