When HMRC says underpaid tax has been included in your PAYE coding, it means a shortfall from an earlier tax year is being collected by reducing your tax-free personal allowance, so a bit more tax comes out of each pay packet across the year rather than in one lump sum. This is known as “coding out.” It applies to underpayments below £3,000 and shows up as a line on your P2 coding notice, cutting your tax-free amount by however much is needed to claw back the debt at your marginal rate.
Why the Underpayment Ended Up in Your Code
Most coded-out underpayments trace back to something HMRC did not know about, or did not know accurately, during the year the tax was due.
Benefits in kind are the classic cause. A company car, fuel for private use, or private medical insurance is taxable, but if the benefit was not built into your code at the time, the tax on it only surfaces after your employer files a P11D. That gap becomes an underpayment for the following year.
Multiple jobs are another common trigger. Each employer needs its own code, and if your personal allowance was split wrongly or applied in full by more than one employer, too little tax came out overall. HMRC picks this up when it reconciles year-end records from every employer.
State Pension is paid gross. It is taxable, but no tax is deducted at source, so HMRC has to collect the tax on it through your code on another PAYE source such as a private pension or a part-time job. If the State Pension uses up most of your allowance, the deduction on your remaining income can look surprisingly steep.
Savings interest above your Personal Savings Allowance, underestimated rental income, or losing an allowance you previously had (Marriage Allowance is a frequent example) can produce the same result.
How the Code Reduction Is Worked Out
HMRC converts the debt into a reduction of your tax-free amount, scaled to your tax rate. Owe £200 as a basic-rate (20%) taxpayer, and your allowance drops by £1,000, because 20% of that £1,000 is exactly £200. A higher-rate (40%) taxpayer owing the same £200 sees a £500 reduction instead.
Your P2 coding notice sets this out line by line: your personal allowance, any deductions for benefits, and any amount being recovered for a previous year. If the change is made partway through the year, it shows as an “In Year Adjustment,” and the code runs on a week 1 or month 1 basis so the extra tax is spread over the pay periods still to come rather than backdated.
K Codes
Sometimes the deductions in your code (benefits plus a coded-out underpayment, for example) add up to more than your personal allowance. When that happens, HMRC issues a K code. Instead of subtracting a tax-free amount, a K code effectively adds taxable income to your pay, so more tax comes out. It can look alarming on a payslip, but it just reflects that the deductions outweigh the allowance and the difference has to be collected through withholding.
Limits on What Can Be Coded Out
HMRC cannot use your code to collect any amount it likes. Three safeguards apply.
- The £3,000 ceiling. Underpayments of £3,000 or more cannot be recovered through your code. They must be collected through Self Assessment or paid directly.
- The 50% limit. Your employer or pension provider cannot deduct more than half of your pre-tax pay under any code, K codes included. If a coding adjustment would push deductions past that point, HMRC has to collect the rest another way.
- The doubling safeguard. HMRC will not code out an amount that would roughly double the tax normally taken from your pay. This is a non-statutory protection, so HMRC can override it if you or your agent ask, once the impact on take-home pay has been explained.
Even under the £3,000 line, coding out only works if your income from the PAYE source is high enough to absorb the deduction. Where it is not, HMRC will look for another route.
P800 or Simple Assessment
After the tax year ends, HMRC reconciles what you paid against what you owed. If you are employed or drawing a pension and there is a shortfall, you get a P800 tax calculation showing the amount and whether it will be recovered through your next code.
You will get a Simple Assessment instead of a P800 if you owe more than £3,000, if the tax cannot be automatically taken from your income, or if you need to pay tax on your State Pension and have no other PAYE income to soak up the adjustment. A Simple Assessment is a formal demand with its own deadline; it does not offer the option of spreading the debt through your code. Interest at 7.75% runs on any amount paid late. If you are already in Self Assessment, you get neither letter, because your bill is worked out through your return.
Self Assessment Filers: The 30 December Deadline
If you file a Self Assessment return and want an underpayment collected through PAYE rather than paid in full by 31 January, you have to file online by 30 December. Miss that date and coding out is off the table for that year’s bill. The debt still has to be under £3,000 and your income still has to support the deduction, so the deadline alone does not guarantee it.
Checking Your Code and Disputing the Figure
The fastest check is the “Check your Income Tax” service in your Personal Tax Account on GOV.UK. It shows your current code, your estimated income from each job and pension, the expected tax for the year, and a breakdown of the code itself, including any amount being recovered for a previous underpayment.
To verify the figures against your own records, pull together your P60 (total pay and tax for the year, issued by 31 May), any P45 from a job you left, your P11D if you had taxable benefits, and the P2 coding notice from HMRC. Compare what is on those documents with what HMRC has recorded. Discrepancies often come from an outdated company car, a benefit that stopped, or pension income logged at the wrong figure.
If the underpayment looks wrong, update your details through the online service or call HMRC. Common grounds for dispute include benefits you never actually received, income allocated to the wrong tax year, or an allowance that was not applied. If HMRC agrees, it issues a revised code to you and your employer, and the new code should show up on your next monthly payslip, or within a few weeks if you are paid weekly. Check the payslip once it arrives, as payroll systems sometimes lag behind HMRC.
Paying It Off Instead
You are not obliged to have the underpayment coded out even when it is correct. If your P800 says you owe tax, you can pay online before the start of the next tax year and stop the debt from being added to your code at all. That is often the cleaner option if the coded adjustment would squeeze your monthly pay noticeably and you have the money available.
If the reduced take-home pay would cause real difficulty, or the debt is too large to code out, contact HMRC before the position worsens. A Time to Pay arrangement can spread the debt across monthly Direct Debit instalments once HMRC has looked at your income, outgoings, and assets. Interest at 7.75% runs on anything paid after the normal due date, so paying sooner costs less either way.