Is my tax code wrong? A step-by-step UK checklist

In plain English

Tax codes go wrong for predictable reasons: an outdated employer feed, a missed benefit-in-kind change, a second job, or a delay between HMRC issuing a new code and payroll applying it. The first job is to confirm whether the code itself is wrong, or whether payroll is simply lagging behind. These have different fixes.

Take-home pay is the usual symptom. If your net pay drops sharply without a pay cut, or your code suddenly contains BR, 0T or K, that may indicate something has changed in HMRC's view of your income. If take-home rises unexpectedly, the new code may include allowances you have not yet earned the right to - which can lead to an underpayment notice later.

It helps to remember that PAYE is self-correcting on a cumulative basis. Even when a wrong code is applied for a month or two, the next correct code typically rebalances tax-to-date in the following payroll. Year-end reconciliation by HMRC catches the rest. The exception is non-cumulative codes, where corrections may need a manual claim.

Based on HMRC figures for 2026/27, the standard personal allowance is £12,570, the basic rate of 20% applies up to £50,270, the higher rate of 40% applies up to £125,140, and the additional rate of 45% applies above. National Insurance of 8% applies between £12,570 and £50,270, and 2% above. Knowing these figures lets you sense-check the deductions on your payslip against the code you have been given. If the numbers do not appear consistent with these thresholds, that may indicate a code mismatch worth investigating - though it does not prove an error in itself.

Tax codes are issued by HMRC and applied by employers; the two are not the same. A "wrong" code can therefore be diagnosed in two layers. The first layer is whether the code on your payslip matches the latest P2 notice HMRC sent you. The second layer is whether HMRC's underlying assumptions - your benefits in kind, untaxed income, residency, multiple jobs - are themselves accurate. Confusing these two layers is a common reason people contact the wrong organisation first and lose time. The checklist below addresses both.

Symptoms of a possible wrong tax code

• Net pay changed sharply without a pay rise or pay cut.
• Your code now includes BR, 0T, D0, D1, K or NT unexpectedly.
• Your code shows W1, M1 or X (non-cumulative basis) for more than two pay periods.
• The code on your payslip differs from your latest HMRC P2 notice.
• You have a benefit in kind (car, medical) that started or ended recently and the code has not changed.

Common causes

• Starting a new job without a P45.
• Adding or losing a company benefit.
• Untaxed income reported separately to HMRC.
• Multiple PAYE sources - second job or pension.
• Marriage Allowance claim or cancellation.
• Moving in or out of Scotland or Wales.

Worked example (2026/27 figures)

Priya earns £30,000 with code 1257L. In May, payroll applies BR by mistake after her Marriage Allowance was incorrectly cancelled. The May tax becomes £30,000 ÷ 12 × 20% ≈ £500, instead of (£30,000 − £12,570) × 20% ÷ 12 ≈ £290. After Priya updates her Personal Tax Account, HMRC reissues 1257L cumulatively in June, and the June payroll appears consistent with refunding the over-collected ~£210. This does not prove the original deduction was an HMRC error rather than a payroll timing issue - both look identical on the payslip until reconciled.

Consider a second scenario. Marcus earns £62,000 with code 1257L cumulative. From October he gains a company car worth £5,400 a year in benefit-in-kind value. HMRC adjusts his code to 717L (£12,570 − £5,400 = £7,170) to collect the extra tax through PAYE. His take-home drops by roughly £5,400 × 40% ÷ 12 ≈ £180 a month - not because of an error, but because the higher-rate tax on the benefit is now being collected through payroll. Marcus's first reaction may be that the code looks wrong; in fact it appears consistent with the new circumstances. The P2 notice would explain the adjustment, and a review of recent P11D events would confirm the cause.

A third scenario: Kerry has two jobs, the main paying £40,000 (1257L) and a second paying £8,000 (BR). Kerry believes BR is wrong because it taxes everything at 20%. In fact, BR appears correct here because the personal allowance is fully used at the main job; £8,000 × 20% = £1,600 in tax on the second job. If Kerry instead asked HMRC to split the allowance - for example 800L on the second job and 457L on the main - the totals across the year would broadly match, but month-to-month deductions would shift. This does not prove either approach is wrong; it is a choice about how the allowance is allocated.

Common mistakes people make

• Calling HMRC before checking the P2 notice - the code may already be correct.
• Asking payroll to change the code - they cannot.
• Ignoring the W1/M1/X marker for months.
• Not factoring in benefits in kind shown on a P11D.
• Assuming a refund must be claimed separately when PAYE will handle it.
• Treating BR on a second job as automatically wrong; in many cases it appears consistent with the personal allowance being used elsewhere.
• Cancelling the Marriage Allowance accidentally and not realising the spouse's code will revert.
• Not updating HMRC promptly when a company benefit (car, medical) starts or ends, leaving the code stuck on outdated assumptions.
• Comparing two payslips from the same year without accounting for cumulative catch-up adjustments after a code change.
• Assuming a small monthly variation indicates a wrong code; cumulative PAYE produces small fluctuations as the year progresses, especially after a bonus or overtime.

When this might apply to you

1. You changed jobs in the past three months.
2. You added private medical insurance or a company car.
3. You received a bonus that pushed your annual income into a higher band.
4. Your spouse cancelled or claimed Marriage Allowance.
5. You moved house across the Scotland/England/Wales border.
6. You started receiving a workplace or state pension alongside employment income.
7. Your earnings approached or crossed the £100,000 personal allowance taper threshold.
8. You switched between full-time and part-time hours, or moved onto a job-share arrangement, mid tax year.

What to do step by step

1. Sign in to your HMRC Personal Tax Account and find your latest tax code notice (the P2).
2. Compare it with the code on your most recent payslip, including any W1, M1 or X marker.
3. Read the P2 breakdown - it shows allowances added and deductions taken (benefits, owed tax, untaxed income).
4. If the codes differ, ask payroll which code they currently hold from HMRC and when the next payroll cut-off falls.
5. If HMRC's code is wrong, update your details in the Personal Tax Account or call HMRC; have your NI number and recent payslip to hand.
6. Check your benefits-in-kind position with HR to confirm the latest P11D values are reflected in HMRC's calculation.
7. Re-check your next payslip - corrections normally appear within one pay cycle once HMRC issues a new code notice.
8. Keep records of dates, codes seen, and any reference numbers from HMRC calls in case a follow-up is needed.
9. Use our free payslip check to validate the figures provided against 2026/27 thresholds.

When to contact HMRC, payroll, or a professional

Contact payroll first when the payslip code differs from the HMRC notice. Contact HMRC when the issue is in the underlying code (income, benefits, residency). Consider a tax professional for complex cases - multiple employments, untaxed self-employment income, or large bonuses that cross thresholds.

Frequently asked questions

Related guides and tools

• UK tax codes explained
• Emergency tax explained
• Payslip line by line
• Tax code checker
• Take-home pay calculator
• Run a free payslip check

Educational content only; not regulated tax advice.