Last updated: 14 October 2026
How do you challenge a K tax code?
Check what HMRC has included in it, then tell HMRC if anything is wrong. A K code means your deductions (such as a company car, unpaid tax or untaxed income) are higher than your tax-free allowances, so HMRC adds an amount to your taxable pay. The breakdown is in your personal tax account or the HMRC app. If HMRC has used the wrong figures, you can update them online or by phone, and your employer or pension provider will get a corrected code.
Why you might have a K code
| Reason | Example |
| Benefits in kind | Company car or medical insurance worth more than your allowance |
| Underpaid tax | Collecting last year’s underpayment through your code |
| State Pension | Taxed through a private pension code, as the State Pension is paid gross |
| Untaxed income | Rental income or interest HMRC estimates you’ll receive |
| High income | Personal Allowance reduced to nil above £125,140 |
How a K code works
- K475 means £4,750 is added to your pay for tax purposes
- There’s a safeguard: tax deducted under a K code can’t exceed 50% of your pay in that period
- Codes are reviewed each tax year, and can change mid-year
Steps to challenge it
- Sign in to your personal tax account and view your tax code breakdown
2. Check each item against your P11D, pension statements and income
3. Update estimated income, such as savings interest or rental profit
4. Tell HMRC when a benefit, like a company car, has ended
5. If HMRC is collecting an underpayment you don’t agree with, ask for a review
Example
Sophie gets a K code of K233 in 2026/27. The breakdown shows a company car benefit of £8,400 and estimated untaxed interest of £6,500. She handed back the car in March 2026, and the interest estimate is based on an account she closed. She updates both items online. Her code changes to 1257L, and her employer refunds the overpaid tax through payroll.
State Pension and K codes
If your State Pension, currently £241.30 a week, is more than your Personal Allowance once other deductions are included, HMRC may give your private pension a K code to collect the tax. This is often correct, but check the State Pension figure used.
If you still disagree
Call HMRC with your National Insurance number and explain. If an underpayment arose because of HMRC’s or your employer’s mistake, you may be able to ask HMRC to write it off under Extra-Statutory Concession A19.
Common mistakes
- Ignoring a coding notice because the code letter looks unfamiliar
- Not telling HMRC when a company car or medical cover ends
- Leaving an old savings interest estimate in place after closing an account
- Assuming the 50% limit means you will not owe more tax later
Frequently asked questions
Is a K code always wrong?
No, it’s often correct.
Can my employer change my K code?
No, only HMRC can.
Will I get a refund?
Yes, through payroll or after the year ends.
How long does a change take?
Usually one or two pay periods.
Related: the 647L tax code question, tax code changes for savings interest and paying tax on the State Pension.
Talk to Accotax
Want this handled for you? Our team at Accotax can help with tax code checks and refunds. See our tax refund accountants service, or contact us for a free, no-obligation quote.
This page is general guidance based on HMRC rules for the 2026/27 tax year, checked against gov.uk on 14 October 2026. It is not personal advice, so speak to an accountant about your own situation.
