An underpayment is the opposite of a refund: HMRC worked out you paid too little, and now wants the difference. Here is how it happens and how it is collected.
An underpayment letter from HMRC is the mirror image of a refund: instead of money coming back, you owe a little more. It rarely means you did anything wrong, but it does mean the PAYE system's estimates fell short, and HMRC will reclaim the difference one way or another.
PAYE tries to deduct exactly the right tax across the year, but it relies on estimates and the assumption that your circumstances stay the same. When they do not, an underpayment can build up. Typical causes include:
For PAYE taxpayers, HMRC usually issues a P800 tax calculation after the year ends, confirming the shortfall and how it will be collected. Where the amount is modest, it is collected by adjusting a future year's tax code, spreading the cost across 12 paydays. Larger underpayments that cannot be covered by reducing your allowance to zero are collected through a K code.
A K tax code is unusual because it adds to your taxable income rather than giving an allowance. It is used when deductions, such as an underpayment being collected or a large benefit in kind, exceed your Personal Allowance. The number after the K, multiplied by ten, is added to your taxable pay for the year.
Suppose Owen has a £40,000 salary and a company car with a taxable benefit of £6,000, but his tax code only accounted for part of the car benefit. By year-end he has underpaid £1,200 of tax.
HMRC collects this by coding out the £1,200 across the next tax year:
| Step | Effect |
|---|---|
| Underpayment to collect | £1,200 |
| Tax rate | 20% |
| Extra taxable income needed | £1,200 ÷ 20% = £6,000 |
| Personal Allowance reduced by | £6,000 |
| New code | roughly 657L (£12,570 − £6,000 = £6,570) |
Owen's take-home pay falls by about £100 a month for a year until the £1,200 is recovered. If the adjustment were larger than his whole allowance, his code would flip to a K code instead. Confirm any coding change by choosing to check your tax code.
The best defence is checking your tax code whenever your circumstances change: a new job, a second income, a company benefit, a pay rise or the start of the State Pension. Catching a wrong code early stops an underpayment building in the first place, because the code can be corrected before too many paydays pass. If you receive a P800 showing an underpayment, check the figures rather than assuming they are right; HMRC's estimates can be wrong in both directions, and an incorrect benefit value or a missing source of income on the calculation can inflate the amount it says you owe. Where the figures are genuinely correct, coding the debt out over a year is usually the gentlest way to settle it, since it spreads the cost rather than demanding a lump sum.
An underpayment is rarely a mistake you made. It is the gap between what PAYE guessed and what you actually owed, and HMRC always closes it.
Sole traders rarely receive a P800, because their tax is settled through Self Assessment rather than PAYE. For them, an underpayment surfaces as a larger balancing payment on 31 January, payable in full by the deadline. Unlike a coded-out PAYE underpayment, a Self Assessment shortfall attracts interest from 1 February and a 5% late-payment penalty if still unpaid 30 days after the deadline. From April 2026, Making Tax Digital quarterly updates will give sole traders a clearer running picture, helping them set aside enough to avoid an underpayment at year-end.
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