An estimated one in three UK employees has the wrong tax code at some point in their working life. A wrong tax code means you are overpaying or underpaying income tax through PAYE -- and because employers apply codes automatically without any verification step, errors can persist for years unnoticed. HMRC can correct your code and refund overpayments going back four full tax years. At current point that means you can reclaim overpaid tax back to the 2021-22 tax year. The process is free, takes approximately five to ten minutes online, and requires no agent, accountant or claims company. (Source: HMRC PAYE employer guidance; HMRC income tax statistics 2026)
This guide explains how to read every common tax code, what causes codes to go wrong, how to check yours in five minutes, how to correct it, and how to claim back any overpaid tax from previous years.
How to Read Your Tax Code
Your tax code appears on your payslip, your P60 (issued at the end of each tax year), your P45 (issued when you leave a job), and in your online Personal Tax Account. The numbers in the code represent your personal tax-free income divided by ten. So 1257L means a 12,570 pounds tax-free allowance. The letter tells your employer the basis on which to apply the allowance. L is the standard letter for the full personal allowance. Different letters indicate different circumstances, and these are where errors most commonly occur.
Seven Reasons Tax Codes Go Wrong
The most common cause is starting a new job without a P45. Without a P45 from your previous employer, HMRC does not know your earnings and tax paid to date in the current tax year. It defaults to an emergency code which taxes each pay period independently, ignoring cumulative earnings from earlier in the year. If you started a new job in the middle of the tax year after a period of unemployment, the emergency code is almost certain to over-tax you. The correction happens when HMRC receives information from your new employer and updates the code, but by then several payslips may have deducted too much.
Multiple income sources are the second cause. Your personal allowance must be assigned to one income source. If HMRC assigns it to the wrong one -- attaching your allowance to a small pension while your main employment income is taxed at BR -- you will significantly overpay. The correct assignment is for the personal allowance to apply to your largest income source. Check this specifically in your Personal Tax Account under the "income" section. (Source: HMRC, tax codes for multiple incomes)
Company benefits in kind are the third cause. Your employer submits a P11D declaring the taxable value of benefits provided -- company car, private medical insurance, gym membership, accommodation, interest-free loans above 10,000 pounds. HMRC uses P11D values to reduce your personal allowance in the tax code, collecting the resulting tax through payroll. If the P11D contains an error (wrong car fuel type, wrong list price, stale vehicle details from a previous tax year), the wrong amount is deducted from your allowance. P11D errors are particularly common when employees change cars mid-year and the P11D is not updated promptly. (Source: HMRC P11D guidance)
The State Pension creates a fourth category of error, especially in the first year of drawing it while still in employment. The State Pension is paid gross -- HMRC must collect the income tax due on it through your employment tax code by reducing your personal allowance by the estimated annual State Pension amount. If the State Pension started mid-year, HMRC may annualise an incorrect figure. If the full new State Pension is 11,502.40 pounds per year in 2026-27 (Source: DWP, new State Pension April 2026) and your personal allowance is 12,570 pounds, the State Pension almost entirely uses up your allowance, leaving only 1,067.60 pounds of allowance to apply against employment income. This is correct when set up properly but often incorrect in the transition year.
Under-claimed reliefs are the fifth cause -- but in the opposite direction. Working from home relief, Gift Aid donations, personal pension contributions made outside of your employer's scheme, professional subscriptions, and uniform washing allowances can all increase your personal allowance if claimed. If you have not told HMRC about these, your code does not include them and you are overpaying. These reliefs are cumulative -- claiming them all at once can result in a meaningful refund. (Source: HMRC, income tax reliefs and allowances)
Scottish residency changes are the sixth cause. Scottish taxpayers are subject to different income tax rates set by the Scottish Parliament. If you move into or out of Scotland during the tax year, or if HMRC has the wrong address on file, your code may apply the wrong rates. Scottish codes are prefixed with S (e.g. S1257L). Scottish rates in 2026-27 differ from UK rates at the starter rate band (19% on 12,571 to 14,876 pounds) and the intermediate rate band (21% on 14,877 to 26,561 pounds), among others. (Source: Scottish Government, income tax rates 2026-27)
K codes are the seventh and most complex cause of errors. A K code means you owe more tax on adjustments than your personal allowance can accommodate. The K code adds the adjustment amount to your taxable income rather than deducting it. K codes arise from large benefits in kind, underpaid tax being collected from previous years, or State Pension income exceeding the personal allowance for older employees. K code figures are frequently wrong because they accumulate errors from prior years' benefit assessments. There is also a regulatory limit: your employer cannot collect more than 50% of your gross pay in any pay period due to a K code, which can leave a growing accumulated debt if the underlying error is not corrected. (Source: HMRC, K codes technical guidance)
How to Check Your Tax Code -- The Five-Minute Process
Log into your Personal Tax Account at gov.uk/personal-tax-account using your Government Gateway user ID and password. If you do not have a Government Gateway account, creating one takes approximately 15 minutes and requires your National Insurance number and a method of identity verification (photo ID or recent payslip details). Once logged in, navigate to Pay As You Earn and select your income source. Your current tax code is displayed, and -- crucially -- a detailed breakdown shows exactly why the code is what it is: personal allowance, additions for reliefs, deductions for benefits, adjustments for prior year underpayments. Read every line and ask whether each item is correct.
Key checks to make on this page: Is the personal allowance 12,570 pounds? Are any benefits in kind listed accurate for your current situation? Is there an underpayment from a prior year being collected -- and is the amount correct? Are working from home or uniform reliefs included if you are entitled to them? Is the income source the code applies to your largest income source? (Source: HMRC, Personal Tax Account guidance)
Tip Check your tax code every April when the new tax year begins. HMRC issues revised codes in February and March for the coming year and this is when most new errors are introduced. Also check immediately whenever you start or leave a job, begin receiving a pension, have a change in company car or other benefits, or start making personal pension contributions. |
Correcting a Wrong Code -- and Claiming Previous Overpayments
To correct a current-year error, use the "Tell HMRC about a change" function in your Personal Tax Account. Select the relevant income source and report what is incorrect. HMRC typically issues a corrected code within a few working days. Your employer applies the new code from the next payroll run. If you have overpaid in the current tax year due to the wrong code, your employer will automatically refund the overpayment in the same payroll run where the new code is applied -- this happens because the cumulative tax calculation is recalculated from the start of the year, showing the difference between what you paid and what you should have paid.
For overpayments in previous tax years, HMRC will issue a P800 tax calculation once the error is confirmed. A P800 showing a refund amount gives you a link in the letter to claim the money online through your Personal Tax Account. The refund reaches your bank account within five working days of the claim. If you believe you have overpaid but have not received a P800, contact HMRC directly on 0300 200 3300 or request a review through your Personal Tax Account under the "Check how much income tax you paid last year" section. You can claim back overpayments for up to four tax years. (Source: HMRC, claiming a tax refund)
Important Never pay a claims management company to reclaim overpaid tax. These companies charge 25-40% of your refund for a ten-minute process you can do for free at gov.uk/personal-tax-account. HMRC has issued multiple consumer warnings about these firms. In some cases they submit claims without proper authorisation and retain refunds that should go to the taxpayer. Your money is yours to claim directly and it costs nothing. |
Emergency Tax in a New Job -- How to Fix It Quickly
When you start a new job, your employer needs a tax code from HMRC. If you provide your P45 from your previous employer, HMRC has the information it needs and usually issues a correct code quickly. If you have no P45 -- for example, you left your last job several months ago, were previously self-employed, or are starting your first job -- your employer applies an emergency code (typically 1257 W1 or M1) while waiting for HMRC to confirm the correct position.
The W1 and M1 suffixes mean the code is applied on a Week 1 or Month 1 basis -- each pay period is taxed independently as if it is the first period of the tax year. In practice this means your personal allowance is applied at 1/52 or 1/12 per period rather than cumulatively. If you earned nothing or earned less earlier in the year, the cumulative approach would give you more tax-free income in later months; the emergency code takes that away. The fix is to complete a New Employee Starter Checklist with your employer (replacing the old P46) so HMRC can confirm your year-to-date position and issue a cumulative code. Any overpayment from the emergency period is automatically corrected in the same payroll run when the cumulative code is applied. (Source: HMRC, starter checklist and emergency codes)
Disclaimer: This article is for information only and does not constitute financial or legal advice. Consult a qualified adviser for guidance tailored to your situation. |
Frequently Asked Questions
My code has letters I don't recognise -- what do they mean?
The most common unusual letters are T (your circumstances need a specific HMRC review and the standard L adjustment cannot be made without it -- often seen alongside allowance tapers for high earners), and 0T (all income taxed with no personal allowance -- applied when HMRC has no information about you or your allowance has been completely used up). If you see a T or 0T code and do not understand why, contact HMRC directly as these codes can indicate significant issues with your records.
I have two jobs -- how should my code be set up?
Your full personal allowance should apply to your main (highest-paying) job. Your second job should be coded BR (all earnings at 20% basic rate) unless your total earnings across both jobs push some of the second income into the higher rate band, in which case part should be coded D0. The most common error is the personal allowance being applied to the second job instead of the main job, or BR being applied to the main income source. If your main job is coded BR and your second job has the personal allowance, you are almost certainly overpaying. (Source: HMRC, multiple jobs and tax codes)
Can I request a specific code?
You cannot demand a specific code but you can give HMRC information that should result in a particular code. Claiming Gift Aid relief, working from home allowance, or professional subscriptions through your Personal Tax Account causes HMRC to increase your personal allowance and update the code accordingly. You are providing information -- HMRC sets the code. If HMRC does not issue the expected code after you provide information, call them and ask for an explanation.
How far back can I claim overpaid tax?
Four complete tax years. As of May 2026 this means 2021-22, 2022-23, 2023-24, and 2024-25. Claims for 2020-21 and earlier are now out of time. The four-year limit is strictly applied -- HMRC will not issue refunds for earlier years regardless of the circumstances. If you think you have a significant historic overpayment, act now before the 2021-22 window closes in April 2026 -- actually, as of May 2026 that window has just closed; you can only go back to 2022-23 at the very earliest now. (Source: HMRC, time limits for tax refund claims)
Sources
- HMRC Tax Codes: gov.uk/tax-codes
- HMRC Personal Tax Account: gov.uk/personal-tax-account
- HMRC Claim a Refund: gov.uk/claim-tax-refund
- Scottish Income Tax 2026-27: gov.scot
