If HMRC has included an outstanding debt in your tax code, it means your tax-free Personal Allowance has been reduced so that extra income tax comes out of each paycheque until the debt is cleared, usually across twelve months. This is called “coding out.” Instead of sending you a separate bill, HMRC recovers what you owe from an earlier year by shrinking the allowance your employer or pension provider uses to work out your PAYE deductions. The standard Personal Allowance is £12,570, and the coded-in debt reduces that figure.1GOV.UK. Income Tax Rates and Personal Allowances
Which Debts HMRC Can Collect Through Your Code
Not every tax bill can be coded out. The Income Tax (Pay As You Earn) Regulations 2003 set out which debts qualify.2Legislation.gov.uk. The Income Tax (Pay As You Earn) Regulations 2003 The main categories are:
- Underpaid income tax from an earlier year, usually because your employer or pension provider deducted too little.
- Tax credit overpayments recoverable under the Tax Credits Act 2002.
- The High Income Child Benefit Charge, which HMRC can collect through your code rather than requiring a Self Assessment payment.
- Tax your employer failed to deduct due to a payroll error.
- Class 2 National Insurance for self-employed workers and some overpaid state benefits.
If the entry on your P2 Notice of Coding doesn’t fit one of these categories, that’s worth querying before anything else.
How the Reduction Turns Into Extra Tax
The figure taken off your allowance isn’t the debt itself. HMRC reduces the allowance by whatever amount, when taxed at your marginal rate, produces the debt over the year.
Say you owe £1,200 in underpaid tax and you’re a basic-rate taxpayer. HMRC drops your allowance by £6,000, because £6,000 taxed at 20% equals £1,200. A standard 1257L code becomes roughly 657L. Spread across twelve months, that’s about £100 less in your pocket each month. Your employer applies whatever code HMRC issues; they don’t see the reason behind it and don’t need to.
Why You Might See a K Code
When the deductions loaded onto your code add up to more than the whole Personal Allowance, HMRC issues a K code. Instead of a negative allowance, the excess is treated as extra taxable pay. This tends to happen when a coded-in debt sits alongside other adjustments like a company car benefit or state pension income.3GOV.UK. Tax Codes – If You Have a K in Your Tax Code
A K code bites harder because it enlarges the income being taxed, but the 50% payslip protection described below still applies.
The Cap on How Much Can Be Coded Out
HMRC can’t load unlimited debt onto your code. A graduated scale sets the annual maximum based on your PAYE earnings:4GOV.UK. PAYE Manual – PAYE14010 – Coding Out Outstanding Debts
- Up to £29,999: £3,000
- £30,000 to £39,999: £5,000
- £40,000 to £49,999: £7,000
- £50,000 to £59,999: £9,000
- £60,000 to £69,999: £11,000
- £70,000 to £79,999: £13,000
- £80,000 to £89,999: £15,000
- £90,000 and above: £17,000
If your debt is larger than the cap for your income band, only the permitted portion goes into your code. HMRC then contacts you separately about the rest, usually with a request to pay directly or set up a Time to Pay arrangement. Someone earning £25,000 who owes £4,500 will see £3,000 coded out and hear from HMRC about the remaining £1,500.
The 50% Payslip Safeguard
Since April 2015, no more than 50% of your gross pay can be deducted in any single pay period, whatever your tax code says. This applies across the board, not just to K codes.5GOV.UK. PAYE Manual – PAYE11050 – Deductions If applying your code on the normal cumulative basis would breach that limit, your employer switches to a week 1 or month 1 basis for that period and recovers any shortfall later if possible.6HM Revenue and Customs. Explanatory Memorandum to the Income Tax (Pay As You Earn) (Amendment) Regulations 2014
You always take home at least half your gross pay.
Checking Where the Debt Figure Came From
Three documents matter here.
The P2 Notice of Coding is HMRC’s breakdown of your current code. A coded-in debt shows as a specific line reducing your allowances, often labelled “underpaid tax for earlier years” or similar.7GOV.UK. PAYE Manual – PAYE11030 – P2 Notice of Coding
The P800 Tax Calculation or PA302 Simple Assessment tells you how HMRC arrived at the debt. A P800 is issued automatically when their records show you paid the wrong tax. A PA302 does the same job for people outside Self Assessment, such as those with taxable state pension income.8GOV.UK. Simple Assessment – Ending the Tax Return Both show your total income, the tax paid, and the shortfall.
The P60 from your employer is the reference point. Compare its income and tax figures against what the P800 or PA302 used. Mismatches are common after a job change or when a taxable benefit was reported to HMRC late, and an error in the source figures flows straight through into the coded-out amount.
Disputing or Changing the Coded-Out Debt
If the figure looks wrong, the quickest route is your Personal Tax Account on GOV.UK. Under “Check your Income Tax” you can see how the code is built up, update income details, and challenge any figure you believe is incorrect. Revised codes usually follow within a few weeks.
If you’d rather speak to someone, the HMRC Income Tax helpline is 0300 200 3300. Have your National Insurance number and P2 to hand. An advisor can walk through the calculation and correct errors on the call.
If the Debt Is Right but the Deduction Hurts
Where the debt itself is correct but the monthly reduction causes hardship, ask HMRC about a Time to Pay arrangement. This pays the debt off through separate monthly instalments over a longer stretch, often at smaller amounts than the coding-out deduction. For debts up to £30,000 you may be able to set the plan up online.9GOV.UK. If You Cannot Pay Your Tax Bill on Time
Once a dispute or a Time to Pay arrangement changes things, HMRC sends a revised P2 to you and to your employer, and payroll updates from the next pay period. Keep the revised P2. If your pay hasn’t changed after a cycle or two, take the new code to your payroll team directly.
A Note for Self Assessment Filers
If you complete a Self Assessment return and want an underpayment collected through your code rather than paid as a lump sum by 31 January, you need to file online by 30 December after the tax year ends (31 October for paper returns). Miss that date and coding out isn’t available for that year’s debt.10GOV.UK. Pay Your Self Assessment Tax Bill – Through Your Tax Code The return also carries a box to opt out of coding out if you’d rather clear the debt in one go.
A Simple Assessment (PA302) works differently: it is not automatically coded out. You must pay it by 31 January following the tax year, or within three months of the letter if issued after 31 October, or contact HMRC to arrange something else.11GOV.UK. Pay Your Simple Assessment Tax Bill – Overview