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Every year, thousands of people open a letter from HMRC, spot the letter K at the start of their tax code, and immediately assume the worst. In over two decades of advising clients on PAYE matters, we have seen that reaction many times and we understand it. A K tax code looks unusual. It sits outside the normal pattern. And because HMRC does not routinely explain it in plain English, people understandably fill the gap with worry.
So let us be direct: a K tax code is not a punishment, not a mistake, and not a sign that HMRC has flagged you for investigation. It is a technical mechanism one HMRC uses when the standard PAYE system cannot accommodate your particular tax situation using the usual allowance approach. Once you know how it works, most of the anxiety dissolves.
IBISS & CO This guide covers everything you genuinely need to know: what a K code means in practice, why HMRC issues one, how to calculate the effect on your pay, what to do if your code looks wrong, and when it is worth involving a professional. We have written it for real people, not tax accountants though we hope accountants find it useful too.
What Is a K Tax Code?
A K tax code is issued by HMRC when the tax adjustments they need to make most commonly for company benefits, prior-year underpaid tax, or State Pension income are larger than your Personal Allowance of £12,570 for 2026/27. Because the standard allowance system cannot accommodate this, HMRC reverses the approach: instead of giving you a tax-free allowance, your employer adds a notional amount to your taxable income. The number after the K is that amount divided by ten. K400 means £4,000 is added to your taxable income. You pay more tax each pay period as a result but the amount is spread across the year, not collected as a lump sum.
What Does a K Tax Code Actually Mean?
To understand the K code properly, you need a short primer on how PAYE tax codes work in general because the K code is essentially the mirror image of a standard code.
Most employed people in the UK have a numeric code followed by L the most common being 1257L. That number (1257) tells your employer how much tax-free income you are entitled to: multiply it by ten, and you get your Personal Allowance of £12,570. Your employer deducts tax only on earnings above that figure.
The K code works in the opposite direction. Instead of protecting a slice of your income from tax, it instructs your employer to add a specified amount to your gross pay before calculating tax. Your tax bill is therefore based on a figure higher than your actual salary. You are not paying tax on money you have not earned, you are paying tax on a notional amount that represents the value of benefits or obligations HMRC cannot collect any other way.
A Simple Way to Think About It
- Standard code 1257L: Your employer says: This person has £12,570 tax free only tax what is above that.
- K code (e.g. K400): Your employer says: Add £4,000 to this person's taxable income before calculating any tax.
- Net result: Your tax is calculated on more than your actual salary. The difference accounts for whatever HMRC needs to collect but cannot collect through a normal allowance reduction alone.
How the K Code Fits Into the Wider Tax Code System
A K tax code is different from standard UK tax codes because it tells your employer to add an amount to your taxable income before calculating PAYE tax. This usually happens when HMRC believes your untaxed income, benefits, State Pension, or previous tax owed is greater than your available Personal Allowance. Our payroll guide explains how PAYE tax codes work in full detail.
| Tax Code | What HMRC Is Telling Your Employer | Typical Reason |
|---|---|---|
| 1257L | Give this employee the standard tax-free Personal Allowance. | Standard employment with no major adjustments. |
| BR | Tax all income from this source at 20%. | Second job or pension where the allowance is used elsewhere. |
| D0 | Tax all income from this source at 40%. | Higher-rate taxpayer with another income source. |
| D1 | Tax all income from this source at 45%. | Additional-rate taxpayer with another income source. |
| 0T | No Personal Allowance is available for this income source. | Allowance used elsewhere, no starter details, or income taxed without a tax-free allowance. |
| NT | No tax should be deducted from this income source. | Specific HMRC instruction or exemption applies. |
| K code | Add an amount to taxable income before calculating PAYE tax. | Company benefits, State Pension, previous underpaid tax, or other untaxed income. |
Note: Scottish and Welsh tax codes can use prefixes such as S or C, and rates may differ depending on the taxpayer’s residence.
One practical point worth knowing: your K code will often be accompanied by a suffix W1 (week 1) or M1 (month 1). This means HMRC has asked your employer to apply the code on a non-cumulative basis, treating each pay period independently rather than looking at your year-to-date position. HMRC uses this when issuing a new or amended code mid-year to avoid a large catch-up deduction. It is administrative housekeeping, not a cause for alarm.
Why Would HMRC Issue a K Tax Code?
There is no single reason HMRC issues a K code; several circumstances can produce one, and sometimes it is a combination of factors. Understanding your specific trigger is essential before you decide whether to challenge the code or accept it.
Company Benefits and Benefits in Kind (BIK)
This is the most common cause, and it is one IBISS & CO team deal with regularly. When your employer provides a taxable perk a company car, private medical insurance, living accommodation, a fuel card, or a beneficial loan HMRC assigns it a monetary value for tax purposes. That value is called a Benefit in Kind.
Your employer reports those values on form P11D each year, and HMRC uses them to adjust your tax code. Find out how P11D reporting is changing. If your total BIK value, when added to other adjustments, exceeds your Personal Allowance of £12,570, HMRC cannot reduce your allowance any further. so they issue a K code instead.
One point many people miss: HMRC calculates the BIK value, not your employer. A company car’s BIK value is based on its CO2 emissions and list price not what you actually pay for it. If your employer has reported the correct figures but HMRC has applied the wrong calculation, the resulting K code figure may be inaccurate even though no one has made a deliberate error.
Common Benefits in Kind That Trigger K Codes
- Company cars particularly diesel or higher-emission vehicles, which attract higher BIK rates
- Fuel provided for private use of a company car treated as a separate, substantial BIK
- Employer-provided accommodation, especially if the property is valued above £75,000
- Private medical or dental insurance the premium value is fully taxable
- Beneficial loans (interest-free or low-interest loans from your employer above £10,000)
- Living expenses paid by employer (subscriptions, gym memberships, etc.)
Underpaid Tax from a Previous Year
If HMRC determines that you paid too little income tax in a prior year through any combination of circumstances including an incorrect code, a mid-year pay change, or multiple sources of income they are entitled to collect the shortfall through your future PAYE code rather than demanding an immediate payment.
HMRC should notify you of this via a P800 tax calculation before amending your code. The P800 sets out what HMRC believes you owed and why. If the amount is under £3,000, HMRC collects it through a PAYE code adjustment. If it exceeds that threshold, they may request direct payment or raise a Self Assessment charge instead.
This is an area where errors genuinely do occur. In our experience, P800 calculations sometimes reflect data that has since been corrected, or they misattribute income to the wrong year. If you receive a P800 and anything seems off even slightly it is worth investigating before the adjustment finds its way into your code.
If the underpayment relates to a previous Self Assessment return, our Self Assessment tax return support can help check whether HMRC has already collected the amount or whether your PAYE code needs correcting.
State Pension That Cannot Be Collected at Source
The full new State Pension for 2026/27 is £11,502 per year. The Personal Allowance is £12,570 which means the State Pension alone leaves only £1,068 of allowance to spare. If you also have any other income (a workplace pension, part-time employment, rental income), the State Pension can effectively exhaust or exceed your Personal Allowance when all income sources are considered together.
The difficulty is that HMRC cannot deduct tax from the State Pension directly it is paid gross by the Department for Work and Pensions. So HMRC collects any tax due on the State Pension by adjusting your employment or private pension tax code. If your situation means HMRC needs to collect more tax than your allowance can absorb, a K code follows. This is increasingly common for people working past State Pension age. Our pensions guide explains how to reduce the tax you pay on your State Pension.
Outstanding Tax Debt via PAYE Coding
If you owe HMRC money from a Self Assessment return, or from a tax debt that has not been resolved, HMRC can in some cases collect it through a PAYE code adjustment rather than requiring immediate repayment. This creates a K code effect — your tax deductions increase each pay period until the debt is cleared.
HMRC must issue you with a formal notice (a P9 coding notice to your employer, and a separate notification to you) before doing this. If you have not received that notification but notice your code has changed, contact HMRC immediately.
If the amount relates to an old tax return or unpaid bill, our help with Self Assessment tax debt can check whether HMRC is collecting the right figure through your PAYE code.
How a K Tax Code Is Calculated 2026/27 Worked Example
Calculations tell the story more clearly than any explanation. Here is a realistic example using 2026/27 figures, worked through in full so you can apply the same logic to your own situation.
The Scenario: Sarah, Operations Director
- Salary: £48,000 per year
- Employer provides a company car a petrol SUV with a list price of £32,000 and CO2 emissions of 145g/km
- HMRC-calculated BIK rate for this vehicle in 2026/27: 30%
- Company car BIK value: £32,000 × 30% = £9,600
- Employer-paid private medical insurance: premium value £1,800
- Total Benefits in Kind: £9,600 + £1,800 = £11,400
- No other adjustments; no prior-year underpayments
Step by Step How HMRC Arrives at the K Code
| Step | Calculation | Result |
|---|---|---|
| 1 | 2026/27 Personal Allowance | £12,570 |
| 2 | Company car benefit in kind: £32,000 × 30% | £9,600 |
| 3 | Private medical insurance benefit in kind | £1,800 |
| 4 | Total deductions HMRC needs to collect tax on | £11,400 |
| 5 | £12,570 − £11,400 = allowance remaining | £1,170 remaining |
| Because some allowance remains, Sarah gets tax code 117L, not a K code. | ||
| Now adjust the example: Sarah’s medical insurance benefit rises to £3,500. | ||
| 6 | New total benefits: £9,600 + £3,500 | £13,100 |
| 7 | £13,100 exceeds the £12,570 Personal Allowance by | £530 |
| 8 | K code number = £530 ÷ 10 | K53 |
| 9 | Notional income added to Sarah’s salary, assuming salary of £30,000 | £30,000 + £530 = £30,530 |
| 10 | Basic-rate tax on the extra £530, assuming the amount falls within the basic-rate band | £106 extra tax per year |
| 11 | Spread across 12 monthly payslips | About £8.83 extra per month |
Note: This is a simplified example. The actual tax collected through PAYE can differ depending on salary level, tax band, Scottish or Welsh tax codes, and other adjustments in the employee’s HMRC tax code notice.
That final figure less than £9 per month is worth sitting with for a moment. It is a far cry from what many people fear when they first see a K code. The code looks alarming; the actual financial effect, when the BIK values are modest, is often very small.
Where the impact becomes more significant is when higher-value BIK triggers a premium company car, fuel for private use, or accumulated prior-year underpayments. In those cases, the monthly deduction increase can run into hundreds of pounds, which is why checking that the code is correct matters.
What K400 Actually Means on Your Payslip
- K400 Decoded K400 means HMRC is adding £4,000 (K number × 10) to your taxable income. If your salary is £35,000, your employer deducts tax as though you earn £39,000. The additional tax collected on that notional £4,000 at 20% for a basic-rate taxpayer is £800 per year or roughly £67 per month.
- Where might K400 arise? A company car with a BIK value of £16,570 (which when set against a £12,570 allowance leaves £4,000 to be taxed via a K code). Or a combination of a smaller car benefit, private medical insurance, and a prior-year underpayment that together exceed the allowance by £4,000.
Common K Tax Codes and What They Mean
Whatever number follows your K, the maths behind it is identical to Sarah’s example above. Multiply the number by 10, and that’s what gets added to your taxable income before your employer works out your tax. K55 adds £550. K320 adds £3,200. A bigger number just means a bigger benefit or debt sitting behind it, not a different kind of code.
That formula works for any K code you’ve got. Here are the ones people ask us about most:
K Code Tax Impact
Additional taxable income and estimated extra tax per year
| K Code | Added to taxable income |
Extra tax at 20% |
Extra tax at 40% |
|---|---|---|---|
| K2 | £20 | £4 | £8 |
| K4 | £40 | £8 | £16 |
| K6 | £60 | £12 | £24 |
| K9 | £90 | £18 | £36 |
| K44 | £440 | £88 | £176 |
| K64 | £640 | £128 | £256 |
| K82 | £820 | £164 | £328 |
| K444 | £4,440 | £888 | £1,776 |
| K666 | £6,660 | £1,332 | £2,664 |
| K789 | £7,890 | £1,578 | £3,156 |
These are yearly amounts, so divide by 12 if you’re paid monthly or by 52 if you’re paid weekly. Two things can change what actually comes off your payslip: the 50% safeguard below, and whether HMRC has you on a W1 or M1 basis instead of the usual cumulative one.
If your number doesn’t match the benefits or income you know about, don’t take it on faith. See Is a K Tax Code Bad? below for how to check it.
50% Safeguard What It Protects You From
There is a statutory limit on how much tax your employer can deduct under a K code in any single pay period: the deduction cannot exceed 50% of your gross pay for that period. This is not a rounding rule or an approximation, it is a legal cap set out in HMRC’s PAYE regulations.
So if your monthly gross pay is £4,000, the maximum K code tax deduction in that month is £2,000 regardless of what the code calculation would otherwise produce. Any uncollected tax carries forward to the next pay period. If you are on a low income and your K code is large relative to your pay, this protection may apply to you every month, with a running balance accumulating. That is not a problem as long as the underlying code is correct HMRC accounts for the carry-forward when reconciling at year end.
Is a K Tax Code Bad? Our Honest Assessment
We are asked this question constantly and the truthful answer requires a little nuance.
A K code that accurately reflects your tax position is not bad. It simply means your employer is collecting the right amount of tax on your behalf, accounting for benefits or other income HMRC cannot handle any other way. You are not being penalised. You are not under investigation. You are just in a slightly more complex tax situation than the standard 1257L covers.
Where a K code becomes genuinely problematic is when it is based on wrong information. And in our experience, that happens more often than most people realise not through malice, but through the normal friction of an imperfect system.
Most Common Reasons K Codes Are Incorrect
- Outdated BIK values: Your employer reported last year’s company car value but the car has changed. HMRC carries the old figure forward unless someone corrects it.
- Benefits that have ceased: You returned a company car or left an employer providing medical insurance, but the P11D data was not updated in time.
- P11D errors by your employer: Employers occasionally report the wrong list price, the wrong CO2 band, or calculate the fuel BIK incorrectly. These errors flow directly into your tax code.
- Prior-year adjustments for tax already paid: HMRC carries forward an underpayment that was actually settled through a Self Assessment return but the code has not been updated to reflect that.
- State Pension miscalculation: HMRC overestimates your State Pension entitlement, or fails to account for the fact that not all of it was paid in the relevant year.
Treat Your K Code as Guilty Until Proven Innocent
That sounds provocative, but it is genuinely useful advice. Do not simply assume your K code is correct because HMRC issued it. Look at the breakdown in your Personal Tax Account. Compare the BIK values to your P11D. Cross-reference any prior-year adjustment against your previous returns. If everything matches, accept the code. If something does not add up, challenge it promptly.
How to Fix a K Tax Code A Practical Guide
If you have reason to believe your K code is wrong, the process for challenging it is more straightforward than most people expect. The key is to gather evidence before you make contact HMRC will need specific figures, not just a general sense that something feels off.
Step 1: Get the Full Breakdown From HMRC
Log in to your HMRC Personal Tax Account at gov.uk using your Government Gateway credentials. Navigate to Check your Income Tax for the current tax year (2026/27). HMRC will show you a line-by-line breakdown of every adjustment included in your code each benefit, each income source, and any prior-year element.
Screenshot or print this breakdown. It is the foundation of any challenge.
Step 2: Cross Reference Against Your Own Documents
Now compare the HMRC breakdown against your own paperwork:
- For company benefits: Ask your employer’s payroll or HR team for your P11D (or P11D(b) if you need the employer’s summary). Check every BIK value against what HMRC shows.
- For prior year underpayments: Locate the P800 tax calculation HMRC sent you. Compare the underpayment figure against your P60 and payslips for the relevant year.
- For State Pension: Check your State Pension statement from the DWP against the pension income figure HMRC has used.
- For ceased benefits: Get written confirmation from your employer of when the benefit ended. the date matters for working out which tax year the adjustment should apply to.
Step 3: Raise the Discrepancy With HMRC
If you find an error, you have three practical routes:
- Online (fastest): Within your Personal Tax Account, you can update certain information directly, particularly employment income and benefit values. Changes made here typically update your code within a few days.
- By phone: Call HMRC income tax helpline on 0300 200 3300, Monday to Friday, 8am to 6pm. Have your National Insurance number, the specific figures you are disputing, and your supporting documents to hand before you call.
- By letter: Write to PAYE & Self Assessment, HM Revenue & Customs, BX9 1AS. Include your NI number, the specific tax years involved, and copies of supporting documents. Slower, but creates a paper trail.
Step 4: Confirm the Updated Code on Your Next Payslip
Once HMRC processes your amendment, they issue a revised coding notice (form P2) to both you and your employer. Check your next payslip to confirm the new code has been applied. If your employer continues using the old code which occasionally happens due to payroll timing contact them directly and reference the P2 date.
Step 5: Recover Any Overpaid Tax
If you have been on an incorrect K code for any length of time, you may have overpaid tax. How you recover it depends on timing:
- Within the same tax year: Once the code is corrected, your employer’s payroll software will typically recalculate your cumulative position and refund the overpayment through your next pay.
- After the tax year ends: HMRC will issue a P800 showing the overpayment, or you can claim a refund via your Personal Tax Account. Refunds are usually processed within 5–10 working days if claimed online.
- If HMRC is slow to act: You are entitled to claim statutory interest on tax overpaid as a result of HMRC error. This is not widely publicised, but it is a right under Section 102 of the Finance Act 2009.
K Tax Code Frequently Asked Questions
These are the questions clients ask us most often. We have written the answers as we would give them in a meeting direct, specific, and without unnecessary hedging.
Is a K tax code bad?
Not in itself. A K code means your deductions exceed your £12,570 Personal Allowance, so HMRC adds notional income to your taxable pay to collect the difference. If the code is accurate, you are paying the right amount just collected differently. The problem is when the code is wrong, which happens more often than people realise. Benefits never removed, prior-year debts already settled, or outdated BIK values can all inflate the figure. Checking takes 20 minutes via your HMRC Personal Tax Account.
What is a K400 tax code?
K400 means HMRC is adding £4,000 of notional income to your taxable pay before calculating your tax. For a basic-rate taxpayer that costs approximately £800 extra per year. For a higher-rate taxpayer, the same code costs around £1,600 per year.
It most commonly comes from a company car BIK combined with another benefit or from a prior-year underpayment being collected through PAYE. Log into your Personal Tax Account and view the code breakdown. HMRC lists every component that makes up the figure.
How do I fix a K tax code?
Log into your HMRC Personal Tax Account, view the 2026/27 breakdown, and identify the specific figure that looks wrong. If you find an error, update your details online or call HMRC on 0300 200 3300 with your National Insurance number and the exact discrepancy. Most amendments process within 2–5 working days.
For complex cases multiple years of adjustments or disputed benefit values a tax adviser can review your full history, correspond with HMRC directly, and calculate any refund owed including interest.
Can I have a K code and a normal code at the same time?
Yes. HMRC issues a separate code for each income source. Two employments, a pension alongside employment, or State Pension combined with other income each gets its own code. Having a K code on one source and BR or 0T on another is entirely normal.
The risk is reading codes in isolation. What matters is the combined tax effect across all your income sources, not each code individually.
How long does it take HMRC to update a K tax code?
Online amendments take 2–5 working days. Phone amendments take one to two weeks. Written correspondence can take four to eight weeks, particularly January to March during Self Assessment peak.
If four weeks have passed and your code has not changed, follow up. Every pay period under a wrong code means more tax deducted than you owe.
Not Sure About Your K Code? We Can Help.
Most K code issues are resolvable but the resolution requires the right information, presented clearly to HMRC. If your code looks wrong, if you have been on an incorrect code for more than one tax year, or if your situation involves multiple income sources and you are not confident the codes are interacting correctly, it is worth talking to someone who deals with this every day.
At IBISS & CO, our advisers handle HMRC tax code challenges regularly. We review your code breakdown, identify any discrepancies, handle correspondence with HMRC on your behalf, and calculate any refund you may be owed including interest where applicable. We also check whether your wider tax position is correct, not just the immediate code issue.
Unsure whether your K tax code is correct? Our guide to Self Assessment and PAYE together covers how the two interact, or speak to a tax adviser at IBISS & CO before extra tax is deducted from your pay.
You do not need to have a large or complicated tax affair to use our service. A straightforward code review and correction is exactly the kind of practical help we are here to provide.
Speak to a Tax Adviser About Your K Code
Whether you need a quick code check, a full review across multiple years, or professional representation with HMRC we are here.
- Free Initial Consultation
- No Obligation
- Qualified UK Tax Advisers
Important Information
This article is for general information only and does not constitute personal tax advice. All figures apply to the 2026/27 UK tax year and are based on HMRC guidance current at the date of publication. Tax law changes regularly. If you are unsure how the K tax code applies to your specific circumstances, please take professional advice before acting. IBISS & CO accepts no liability for decisions made in reliance on this article alone.
Sources: HMRC — Income Tax (PAYE) Regulations 2003 · GOV.UK — Tax Codes · HMRC — Check Your Income Tax · Finance Act 2009, s.102 (Interest on Overpayments)
Muhammad Sufyan Moavia
Reviewed by Muhammad Sufyan Moavia, Chartered Tax Adviser and Fellow Chartered Certified Accountant at IBISS & CO. He has 15 years experience in accounting and tax, advising individuals and business owners on personal tax, HMRC matters, IHT, CGT and tax planning.
Last review: September 2026
