A wrong tax code can cost you hundreds of pounds a year. Here is how to check if yours is wrong, how much you may be owed back, and the fastest way to fix it.
A wrong tax code means HMRC is taking too much or too little tax from your wages. It happens more often than most people think, and the financial impact adds up fast. If your code is wrong and you do nothing, you could overpay by hundreds or even thousands of pounds before the end of the tax year. The good news: spotting a wrong code is straightforward, fixing it takes minutes, and you can reclaim overpaid tax going back four years.
Not sure whether your code is correct? These are the five clearest warning signs. If any of them apply to you, act now rather than waiting for HMRC to catch the error at year end.
1. You have an emergency code (W1, M1, or X) for more than two months on your main job
Emergency codes are meant to be temporary, lasting only until HMRC processes your details after starting a new job. If you still see W1, M1, or X on your payslip after two full pay periods, something has gone wrong. Your employer may not have submitted your Starter Checklist, or HMRC may not have processed it. The cost: at a £30,000 salary, an emergency code can cause overpayments of £50 to £150 per month depending on your earnings pattern. Over a full year, that compounds to £600 or more in tax you should not have paid.
2. BR or D0 is applied to your only job
BR means Basic Rate -- 20% tax on every pound with no Personal Allowance. D0 means 40% on every pound. Both are correct for a second job or pension, but if either appears on your main (or only) employment, you are losing your entire £12,570 tax-free allowance. On a £25,000 salary, a wrong BR code costs you £2,514 per year. On a £45,000 salary, it costs the same £2,514. That is money taken from your pay that you are legally entitled to keep.
3. Your code number does not match your expected Personal Allowance
The number in your tax code multiplied by 10 equals your Personal Allowance. The standard code is 1257L, which gives you £12,570 tax-free. If your code shows a lower number, for example 1100L instead of 1257L, HMRC has reduced your allowance by £1,570. That reduction should only happen if you have benefits in kind, underpaid tax from a previous year being collected, or other specific adjustments. If none of those apply, the lower number is wrong and you are overpaying by roughly £314 per year (£1,570 times 20%).
4. You started a new job but your employer does not have your P45
Without a P45 from your previous employer, your new employer has no record of your tax code or how much tax you have already paid this year. They will either apply an emergency code or default to BR, both of which usually result in overpayment. This is the single most common cause of wrong tax codes in the UK. If you started a new job and did not hand over your P45 within the first week, check your payslip immediately.
5. Your code includes deductions you no longer have
HMRC adjusts your tax code to collect tax on benefits in kind -- company cars, private medical insurance, interest-free loans. If you returned a company car six months ago, repaid a student loan, or stopped receiving a taxable benefit, but HMRC has not updated your code, you are still being taxed as though you have those benefits. A company car benefit of £5,000 built into your code costs you £1,000 per year in extra tax (at 20%) or £2,000 (at 40%) that you should not be paying.
The amount you have overpaid depends on what your code should be versus what it currently is. The most common scenario is having BR (no Personal Allowance) when you should have 1257L (full £12,570 allowance). Here is the impact at three income levels:
| Annual salary | Tax with 1257L (correct) | Tax with BR (wrong) | Annual overpayment | Monthly overpayment |
|---|---|---|---|---|
| £25,000 | £2,486 | £5,000 | £2,514 | £210 |
| £45,000 | £6,486 | £9,000 | £2,514 | £210 |
| £75,000 | £14,486 | £17,000 | £2,514 | £210 |
The overpayment is the same across all three salary levels because the lost Personal Allowance is always £12,570, and the tax on that amount at the basic rate is always £2,514. Higher earners on a wrong BR code may lose even more if they should be on a code that accounts for their higher rate band correctly.
If you are a sole trader filing through Self Assessment, a wrong PAYE code on your employment income throws off your entire tax calculation. TapTax can help you track what you actually owe across all income sources, so you are never caught out by HMRC discrepancies. Check your real tax position with TapTax.
The average wrong tax code goes uncorrected for 3 to 4 months. At \u00a3210 per month in overpaid tax, that is over \u00a3800 sitting with HMRC that belongs in your pocket.
These are the tax code errors HMRC helpline staff deal with most frequently:
BR on your main job. You should have 1257L but HMRC thinks you have two employers, so your only job gets BR with no allowance. Cause: your previous employer did not notify HMRC through RTI that you left, or your P45 was not processed.
Wrong number (e.g. 1100L instead of 1257L). Your code includes deductions for benefits in kind, underpaid tax, or estimated untaxed income that do not apply to you. A wrong number reduces your allowance and increases every pay packet's tax deduction.
Emergency W1 or M1 stuck on your code. You started a new job, got put on 1257L W1 (or M1), and it never changed to cumulative. Each pay period is taxed in isolation, and you lose the benefit of any weeks or months where you earned less than the weekly or monthly allowance fraction. The longer this runs, the bigger the overpayment.
Scottish S-prefix on a non-Scottish resident. If you live in England, Wales, or Northern Ireland but your code starts with S (for example, S1257L), you are being taxed at Scottish rates. The Scottish starter rate is 19% on the first £2,162, but the higher rate threshold kicks in at £43,662 rather than £50,270. If you are not a Scottish taxpayer, the S-prefix is wrong and could cost or save you money depending on your income level.
D0 on a second job that should be BR. If your second income falls entirely within the basic rate band, it should carry BR (20%), not D0 (40%). Getting D0 when you should have BR means you are paying double the correct tax rate on every pound from that source.
Fixing a wrong tax code is faster than most people expect. Follow these steps in order:
Step 1: Sign into your HMRC Personal Tax Account. Go to gov.uk/personal-tax-account and log in with your Government Gateway credentials. If you do not have an account, you can create one -- you will need your National Insurance number and a form of ID.
Step 2: Go to "Check your Income Tax." This section shows your current tax codes for every employer and pension provider. Review each one. Look for codes that do not match the signs listed above.
Step 3: Select "Manage your tax codes." Here you can tell HMRC about changes to your employment, benefits in kind, or other income. Update anything that is out of date. Remove old employers that HMRC still has on record. Report benefits in kind that have ended.
Step 4: If you prefer the phone, call 0300 200 3300. The Income Tax helpline is open Monday to Friday, 8am to 6pm. Have your National Insurance number, employer name, employer PAYE reference (from your payslip), and the reason for your call ready. The agent can often update your code during the call.
Step 5: Your employer receives the updated code automatically. Once HMRC processes the change, they send a P6 notice to your employer electronically. Your employer does not need to do anything -- their payroll system picks up the new code and applies it from the next pay run.
If you are also self-employed, managing your PAYE code alongside Self Assessment can be confusing. TapTax tracks both your employment tax and self-employment income in one place, helping you spot discrepancies before they become expensive. See how TapTax works.
You can reclaim overpaid tax going back up to four tax years. For the current 2025/26 tax year, that means you could reclaim overpayments from as far back as 2021/22.
Current tax year refund (automatic). When HMRC corrects your code mid-year, your employer recalculates your tax on a cumulative basis. The overpaid amount is refunded through your next pay packet. You do not need to file a separate claim. If you have been on a wrong code for six months and the overpayment is £1,200, expect an unusually large net pay in the month the correction takes effect.
Previous tax year refund via P800. After each tax year ends on 5 April, HMRC reconciles your records. If you overpaid, they issue a P800 tax calculation (usually between June and November). You can claim the refund online through your Personal Tax Account, and HMRC typically pays within 5 working days. If you do not claim online, HMRC posts a cheque within 60 days.
Self Assessment refund. If you file a Self Assessment tax return, any overpaid PAYE tax is factored into your return. The overpayment either reduces your Self Assessment bill or generates a refund. You can amend a Self Assessment return for a previous year to claim back overpaid tax if the original return did not account for the wrong code.
PAYE refund vs Self Assessment refund. PAYE employees who do not file Self Assessment get refunds through P800 or payroll adjustments. Self Assessment filers get refunds through their tax return. Do not try to claim through both routes -- HMRC cross-references the systems and will reject duplicate claims.
Writing to HMRC. If the online system does not cover your situation, you can write to: Pay As You Earn and Self Assessment, HM Revenue and Customs, BX9 1AS. Include your National Insurance number, the tax years affected, your employer details, and a clear explanation of the wrong code and the overpayment.
Once you have reported the problem, here is the typical timeline:
Week 1-2: HMRC reviews your request. Online submissions are processed faster than phone or letter requests. You may see the updated code in your Personal Tax Account within a few days.
Week 2-3: HMRC sends a P6 notice to your employer. This is an electronic instruction telling payroll to use your new tax code. Most employers process P6 notices in their next pay run.
Your next payslip: Check that the new code appears on your payslip. The code should match what HMRC confirmed. If your employer is using a cumulative code, your pay will include a refund of any overpaid tax from earlier in the year.
Confirming the correction was applied: Log back into your HMRC Personal Tax Account and check "Income Tax for the current year." Your new code should appear next to the relevant employer. If it still shows the old code after a full pay cycle, contact HMRC again -- the P6 may not have reached your employer.
If your employer has not applied the new code: Speak to your payroll department directly. Give them your new tax code and the date HMRC issued it. They can apply it manually while waiting for the electronic P6. In rare cases, payroll systems have a delay of one additional pay period before the change takes effect.
Next steps: How to check your tax code --> | How to change your tax code -->
Related tax codes: 1257L tax code | BR tax code | W1 tax code | M1 tax code | 0T tax code
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