You can still owe tax on PAYE income because the system only collects the right amount if every input HMRC uses is correct and up to date. Your tax code, your employer’s information, and any income outside your main job all have to line up. When they don’t, the shortfall builds quietly across the year and lands as a bill after 5 April. The personal allowance is frozen at £12,570 through at least 2026/27, and anything above that has to be taxed at the right rate for the books to balance.1UK Parliament. Direct Taxes: Rates and Allowances for 2026/27
Your Tax Code Was Wrong
Everything in PAYE flows from your tax code. The number tells your employer how much of your pay to shield from tax; the standard code 1257L shields the first £12,570.2GOV.UK. Understanding Your Employees’ Tax Codes: What the Numbers Mean If HMRC has the wrong information, every payslip that follows deducts too little tax, and you finish the year short.
Common triggers: HMRC doesn’t know about a second income, still thinks you get a benefit you’ve lost, or is working from stale figures. Your employer applies whatever code they’re given. By the time HMRC compares payroll data against your real liability after 5 April, months of underpayment have accumulated.
You can check your current code through HMRC’s Personal Tax Account and tell HMRC directly if something looks wrong.3GOV.UK. Check Your Income Tax for the Current Year Catching an error in October is much cheaper than discovering it the following summer.
Emergency Codes Are a Common Culprit
When you start a new job and your new employer doesn’t have your previous tax details, HMRC puts you on an emergency code, marked with a W1, M1, or X suffix.4GOV.UK. Emergency Tax Codes Emergency codes look at each pay period on its own instead of your total earnings for the year. If you had a gap in work or earned differently earlier in the year, the code can’t reflect that, and the numbers won’t reconcile at year-end.
You Have More Than One Income
Two jobs, or a salary plus a private pension, is one of the most common reasons for a PAYE bill. HMRC usually assigns your full personal allowance to your main income and taxes the second at a flat rate: a BR code applies 20%, D0 applies 40%, D1 applies 45%.5GOV.UK. Tax Codes: What the Letters Mean
Problems start when both employers apply the personal allowance at the same time. It happens easily with seasonal work, side roles, or a new job starting before the old one closes on HMRC’s records. Both payrolls shield £12,570, so you effectively receive the allowance twice. When HMRC combines the two income streams, the gap is obvious and you owe the tax on whatever was wrongly shielded.
If your combined income pushes above £50,270, part of it should have been taxed at 40% rather than 20%, and neither employer will necessarily have accounted for that. This is where bills get larger.
Income PAYE Doesn’t See in Real Time
Some taxable income never passes through your employer’s payroll, so PAYE has to collect the tax by adjusting your code after the fact. If the estimates are off, you end up underpaid.
The State Pension
The state pension is taxable, but the Department for Work and Pensions pays it gross, with no tax deducted at source.6GOV.UK. Pay Your Simple Assessment Tax Bill: Overview If you also have a private or workplace pension, HMRC normally adjusts the code on that pension to collect the tax on both. If HMRC’s figures are wrong, or your state pension rises before the code catches up, you finish the year short. Retirees with only a state pension have no PAYE code to adjust at all, so HMRC collects any tax owed through a Simple Assessment.
Savings and Investment Interest
Banks and building societies pay interest without deducting tax. Basic-rate taxpayers get a £1,000 personal savings allowance, higher-rate taxpayers get £500, and anyone earning above £125,140 gets nothing.7GOV.UK. Income Tax Rates and Personal Allowances Interest above your allowance is taxable, and HMRC collects it by adjusting the following year’s code based on an estimate. With today’s rates, a modest pot at 4–5% can easily clear £1,000. If the estimate lags behind your actual interest, you owe the difference.
Benefits in Kind From Your Employer
Non-cash perks like a company car, private medical insurance, or an interest-free loan are taxable. Employers typically report them on a P11D after the tax year ends, so the tax often isn’t collected through pay in real time.8GOV.UK. Expenses and Benefits for Employers: Reporting and Paying HMRC should then reduce your following year’s code to recover it. Employers can instead payroll the benefits and tax them as you go, which avoids a year-end surprise.9GOV.UK. Tax Employees’ Benefits and Expenses Through Your Payroll Many don’t. If a new benefit appears mid-year, the code often doesn’t catch up before 5 April.
Benefits can also push you into a higher band. If your salary sits just below £50,270 and a company car adds a few thousand on top, part of your income crosses into the 40% band, and standard code deductions miss that without a specific adjustment.7GOV.UK. Income Tax Rates and Personal Allowances
Your Income Crossed a Threshold Your Code Didn’t Expect
Your tax code is set on the assumption of a particular income level. When actual earnings deviate, the code doesn’t automatically adjust.
The sharpest example is the personal allowance taper. Once your adjusted net income passes £100,000, your allowance shrinks by £1 for every £2 above the threshold, and disappears entirely at £125,140.7GOV.UK. Income Tax Rates and Personal Allowances That creates an effective marginal rate of 60% in the taper band. If a bonus, share option, or unexpected earnings push you across £100,000, the lost allowance goes uncollected until HMRC reconciles the year.
Changing jobs mid-year has a similar effect. You should get a P45 showing earnings and tax paid so your new employer starts from the correct cumulative position.10GOV.UK. Your P45, P60 and P11D Form If the P45 is late or missing, the new employer defaults to an emergency code and the cumulative picture is lost. Lump-sum leaving payments cause their own distortion: payroll software pro-rates a large one-off amount as if you’ll receive it every month, taxing it at the wrong rate, and the correction shows up later as a bill.
How HMRC Tells You, and How They Collect
After 5 April, HMRC matches what your employers reported against what you actually owed. If the numbers disagree, you’ll receive a P800 tax calculation, usually between June and the following March.11GOV.UK. Tax Overpayments and Underpayments It shows the shortfall and how HMRC arrived at it.
For underpayments below £3,000, HMRC prefers to collect gradually by adjusting your next year’s code. Your take-home drops slightly until the balance clears. For £3,000 or more, code collection isn’t available and HMRC issues a Simple Assessment instead, with a set deadline to pay the full amount.12GOV.UK. PAYE12070 – Coding Out Underpayments
Simple Assessments are also used where there’s no PAYE code to adjust, such as for retirees on the state pension alone. If the letter arrives before 31 October, payment is due by the following 31 January. If it arrives on or after 31 October, you have three months from the date of the letter.6GOV.UK. Pay Your Simple Assessment Tax Bill: Overview
Checking, Disputing, and Paying the Bill
Before paying, check the calculation against your own records. Compare each figure to your P60s, P45s, P11Ds, and bank statements. HMRC sometimes uses estimated figures without flagging them, or bundles multiple income sources into a single line that hides a mistake. If something doesn’t match, call HMRC’s Income Tax helpline with the P800 in front of you.
If the shortfall resulted from HMRC’s own error rather than yours, you can ask for the debt to be written off under Extra-Statutory Concession A19. It applies when HMRC failed to act on information they already held and the underpayment relates to a tax year that ended more than 12 months earlier. If the fault was your employer’s, HMRC should pursue the employer, not you. Neither argument is guaranteed, but both are worth raising before you accept the bill.
If you agree you owe the money but can’t pay in one go, HMRC can spread payments through a Time to Pay arrangement, typically over up to three years, usually by direct debit. Ignoring the letter is the worst move. Once a Simple Assessment is in place it’s a legal debt, and HMRC can escalate to enforcement including debt collection.