If you’ve ever opened your payslip and noticed your take-home pay has dropped with no obvious explanation, a benefit in kind may be the culprit. A Reddit user on r/UKPersonalFinance described the moment perfectly: “Got my payslip today and my net pay is down about £80. Checked my tax code and it’s gone from 1257L to 557L. Rang HMRC and they said it’s because of the company car I got in March. Nobody told me my tax code would change.”
They’re not alone — this is one of the most common surprises in UK payroll. When you receive a taxable benefit from your employer and you don’t file a Self Assessment return, HMRC doesn’t send you a bill. Instead, they collect the tax by adjusting your PAYE tax code — a process known as “coding out.” Your tax-free Personal Allowance is reduced by the value of the benefit, so the extra tax is spread across the year and deducted from your salary each month.
Here’s the plain-English explanation of how coding out works, how your tax code is calculated, what a P2 coding notice tells you, and what to do when HMRC keeps taxing you for a benefit you no longer have. For the broader framework of how P11D forms and benefits in kind fit together, see our P11D explained guide.
What Is Coding Out?
Coding out is HMRC’s process of collecting tax on benefits in kind by adjusting your PAYE tax code rather than sending you a separate bill. If you receive taxable benefits (like a company car or private medical insurance) and don’t file a Self Assessment return, HMRC reduces your tax-free Personal Allowance by the cash equivalent of the benefit, so the extra tax is collected through your salary each month. You see the result as a lower tax code number and a drop in take-home pay.
1. How HMRC Collects BIK Tax Through Your Tax Code
There are two ways HMRC collects tax on benefits in kind, and you don’t get to choose which one applies — it depends on whether you file a Self Assessment return.
If you’re in Self Assessment (you’re self-employed, your income is over £100,000, you have untaxed income, etc.), you declare the cash equivalent of each benefit on your tax return in the “Employment benefits and expenses” section. The tax is then calculated as part of your overall Self Assessment bill and paid by 31 January. Your tax code is not adjusted for these benefits — that would be double-charging.
If you’re not in Self Assessment — which is most employees — HMRC collects the tax by adjusting your tax code. This is called “coding out.” They take the cash equivalent of your benefits, add it to your estimated income, and reduce your tax-free Personal Allowance so the extra tax is collected through PAYE across the year. You don’t get a separate bill; you simply see your take-home pay fall.
According to HMRC’s guidance on tax on company benefits, if you don’t complete a tax return, HMRC will change your tax code to collect the tax you owe on your benefits. You’ll receive a P2 coding notice telling you the new code, and your employer will apply it to your payroll.
2. The Mechanics: How Your Code Is Reduced
Understanding how your tax code is reduced requires knowing what the numbers actually mean. A standard tax code like 1257L means you have a tax-free Personal Allowance of £12,570 for the year. The code is simply the allowance with the last digit removed and a letter appended — 1257 = £12,570.
When HMRC codes out a benefit, they reduce your allowance by the cash equivalent of the benefit (not the tax owed). This is the crucial bit that confuses people: the allowance is reduced by the full benefit value, but the tax collected is that value × your marginal rate.
Here’s why that works. If you have £1,000 of taxable benefits and you’re a 20% taxpayer:
- Tax owed on the benefit: £1,000 × 20% = £200
- HMRC reduces your allowance by £1,000 (the cash equivalent)
- Your new allowance: £12,570 − £1,000 = £11,570
- Your new tax code: 1157L
- The £200 of extra tax is collected because £1,000 more of your salary is now taxable at 20%
So a £1,000 benefit at 20% drops your code from 1257L to 1157L and costs you £200/year (£16.67/month). For a 40% taxpayer with the same benefit, the code still drops to 1157L — but the tax collected is £400/year (£33.33/month), because the extra £1,000 of taxable income is now being taxed at 40% rather than 20%.
For the full breakdown of how tax codes work and what each letter means, see our tax code guide.
3. The P2 Coding Notice: What to Look For
When HMRC changes your tax code, they send you a P2 coding notice — either by post or as a notification in your Personal Tax Account. Your employer gets a copy too, which is how payroll knows to apply the new code.
The P2 shows:
- Your Personal Allowance (e.g., £12,570)
- Any deductions — this is where your benefit in kind charges appear, listed by type (company car, private medical insurance, etc.) with the cash equivalent for each
- The resulting tax code (e.g., 557L)
- The tax year the code applies to
- Your employer’s name — the code is specific to that employment
This is the document to check when your pay changes unexpectedly. If you see a benefit listed that you no longer receive — a car you handed back, insurance you cancelled — that’s a stale benefit and you need to tell HMRC (see section 7 below).
You can view your current code and coding history in your Personal Tax Account under “Check your Income Tax.” For a step-by-step walkthrough, see our guide on how to check your tax code.
4. Company Cars and Form P46 (Car)
Company cars trigger an automatic tax code change — you don’t have to wait until the end of the tax year. When you get a new or replacement company car, your employer reports it to HMRC on form P46 (Car) within 28 days, as set out in HMRC’s company car reporting guidance. HMRC then updates your tax code mid-year to start collecting the car benefit through PAYE immediately.
This is why the Reddit user at the start of this article saw their code change in March — their employer filed the P46 (Car) and HMRC issued a new code straight away. The cash equivalent for a company car is calculated as the car’s list price × the BIK percentage rate (based on CO2 emissions), and that full amount is deducted from your allowance.
For example, a car with a £7,000 cash equivalent drops your code from 1257L to 557L — a reduction of £7,000 in your tax-free allowance. At 20% tax, that’s £1,400/year (£116.67/month) collected through your salary. At 40%, it’s £2,800/year (£233.33/month).
If you change cars during the year, another P46 (Car) is filed and your code is adjusted again. If you give the car back, your employer reports that too and HMRC should remove the charge — though this is a common source of stale codes (see section 7).
5. When BIK Tax Goes Through Self Assessment Instead
If you file a Self Assessment tax return, your benefits in kind are not coded out — they’re declared on your return and the tax is collected through Self Assessment. This avoids you being taxed twice on the same benefit.
You’re in Self Assessment if any of the following apply, per HMRC’s Self Assessment criteria:
- You’re self-employed as a sole trader with income over £1,000
- Your income is £100,000 or more
- You have untaxed income (rental income, savings interest over the allowance, foreign income)
- You’re a partner in a business partnership
- You have Capital Gains Tax to pay
- HMRC specifically asked you to file
If you’re in Self Assessment, you declare the cash equivalent of each benefit from your P11D in the “Employment benefits and expenses” section — Box 1 for company cars, Box 3 for private medical insurance, and so on. The tax is then part of your overall bill, due by 31 January after the tax year. For the full process, see our P11D explained guide.
One edge case: if you’re newly in Self Assessment but HMRC has already coded out a benefit from earlier in the year, you may find the benefit appears both in your tax code and on your return. HMRC usually reconciles this automatically, but it’s worth checking your final Self Assessment calculation to make sure you haven’t been double-charged.
6. Worked Examples: How Your Take-Home Pay Changes
Let’s run through three concrete scenarios so you can see exactly how a benefit affects your tax code and your monthly pay.
Example 1: Private medical insurance, basic-rate taxpayer
- Benefit: £1,200/year private medical insurance
- Cash equivalent: £1,200
- Tax rate: 20%
- Allowance reduction: £1,200
- Tax code: 1257L → 1137L
- Extra tax: £1,200 × 20% = £240/year (£20/month)
Example 2: Company car, higher-rate taxpayer
- Benefit: £7,000 company car cash equivalent
- Tax rate: 40%
- Allowance reduction: £7,000
- Tax code: 1257L → 557L
- Extra tax: £7,000 × 40% = £2,800/year (£233.33/month)
Example 3: Multiple benefits, additional-rate taxpayer
- Company car: £5,000 cash equivalent
- Private medical insurance: £1,800
- Total benefits: £6,800
- Tax rate: 45%
- Allowance reduction: £6,800
- Tax code: 1257L → 577L
- Extra tax: £6,800 × 45% = £3,060/year (£255/month)
The pattern is consistent: your code drops by the total cash equivalent of all your benefits, and the tax collected depends on your marginal rate. The more benefits and the higher your tax band, the bigger the hit to your take-home pay. Use our benefit in kind tax calculator to work out exactly how much tax you’ll pay on your company car, private medical insurance, or low-interest loan. For the tax bands and allowances behind these calculations, see our Personal Tax Allowance 2026 guide.
7. Stale Benefits: When HMRC Keeps Taxing You for Something You No Longer Have
This is the single most common problem with coding out — and it’s not really HMRC’s fault. The system relies on employers reporting changes promptly, and on HMRC processing those changes before the next tax year’s codes are issued. When a benefit ends but the code isn’t updated, you keep paying tax on a benefit you no longer receive.
A Reddit user on r/HMRC described the classic scenario: “I handed my company car back in November when I changed jobs, but my tax code still has the car benefit in it six months later. I’ve overpaid about £900. How do I get this fixed?”
Stale benefits happen for several reasons:
- Your employer didn’t file the change — they should report a ceased car on P46 (Car), but some payroll teams are slow
- HMRC issued the new year’s code before processing the change — codes for the next tax year are often generated in January, so a change in February may not be reflected until the following year
- You changed employers — your old employer’s P11D still shows the benefit, and HMRC carries it forward
- The benefit was a one-off — e.g., a season ticket loan that’s been repaid but still appears on your code
The fix is straightforward: log into your Personal Tax Account, go to “Check your Income Tax,” and update your employment details or benefits. HMRC will issue a corrected code, and your next payslip will adjust — including a refund of any tax you’ve overpaid. You can also call HMRC on 0300 200 3300. For the full walkthrough, see our guide on how to check your tax code.
8. How to Check and Fix Your BIK Tax Code
Checking your tax code takes five minutes and can save you hundreds of pounds. Here’s the process.
Step 1: Find your current code
Your code is on your payslip, your P45 (if you’ve recently changed jobs), or your P2 coding notice. You can also see it in your Personal Tax Account under “Check your Income Tax.”
Step 2: Compare it to what you expect
A standard code with no benefits is 1257L (for 2025/26 and 2026/27). If your code is lower — say 1057L — that’s a £2,200 reduction, which means HMRC thinks you have £2,200 of taxable benefits. Check whether that matches reality.
Step 3: Check for the S prefix
If your code starts with an S (e.g., S1257L), you’re being taxed as a Scottish resident. If you live in England, Wales, or Northern Ireland, this is wrong and you’re paying Scottish rates — which are different. This usually happens because HMRC has an old address on file. See our guide on the Scottish tax code wrong address problem for how to fix it.
Step 4: Update HMRC if something’s wrong
Log into your Personal Tax Account and update your benefits, employment details, or address. HMRC will issue a new code, usually within a few weeks. Your employer will apply it to your next payroll run, and any overpaid tax will be refunded through your salary.
If you can’t use the online service, call HMRC on 0300 200 3300. Have your National Insurance number ready.
9. The K Code: When Benefits Exceed Your Allowance
Sometimes your benefits are so large that they exceed your entire Personal Allowance. When that happens, HMRC can’t reduce your allowance any further — it’s already at zero — so they issue a K code instead.
A K code works in reverse: instead of giving you tax-free allowance, it adds negative allowance to your income. The number on a K code represents additional taxable income, not tax-free allowance. A code of K100 means £1,000 is added to your taxable income on top of your salary.
K codes typically arise when:
- You have a high-value company car with a large cash equivalent
- You have multiple benefits that together exceed £12,570
- You have underpaid tax from a previous year being collected through your code
For example, if your company car has a cash equivalent of £15,000, that’s £2,430 more than your £12,570 allowance. HMRC issues a K code of roughly K243, adding £2,430 to your taxable income. The extra tax is then collected via PAYE.
There’s a safeguard: HMRC limits the amount of tax that can be collected through a K code to 50% of your income before tax, to prevent your entire pay being swallowed. If the underpayment is larger than that, the excess is carried forward to the next tax year. For more on this, see HMRC’s guidance on K codes.
10. The Bottom Line
- If you don’t file a Self Assessment, HMRC collects your BIK tax by adjusting your tax code — a process called coding out.
- Your Personal Allowance is reduced by the cash equivalent of your benefits, not by the tax owed. A £1,000 benefit at 20% drops your code from 1257L to 1157L and costs £200/year.
- Company cars trigger an automatic code change via form P46 (Car) — you don’t wait until the end of the year.
- If you’re in Self Assessment, benefits go on your return, not your tax code — you can’t choose which method is used.
- Check your P2 coding notice for benefits you no longer have — stale codes are the most common problem, and they cost you money every month until you fix them.
- A K code means your benefits exceed your allowance — HMRC adds the excess to your taxable income instead of reducing your allowance.
- Fix a wrong code in your Personal Tax Account or by calling HMRC on 0300 200 3300. Refunds come through your next payslip.
For the full framework of P11D forms and how benefits in kind are calculated, see our P11D explained guide. For how tax codes work in detail, see our tax code guide. For step-by-step instructions on checking your code, see how to check your tax code. For tax bands and allowances, see our Personal Tax Allowance 2026 guide. If your code has an S prefix and you don’t live in Scotland, see our guide on the Scottish tax code wrong address problem.