Can You Change Your Tax Code After Filing Your Taxes?

Your tax code determines how much tax is deducted from your income. If it’s incorrect, you could end up paying too much or too little. Many people don’t realize their tax code can change due to various factors, sometimes requiring adjustments even after filing a return.

If you discover an issue with your tax code after submitting your taxes, there are steps you can take to correct it. Addressing errors promptly can help you avoid penalties or unexpected bills.

Reasons a Code Might Need Modifying

Changes in income, benefits, or personal circumstances can all affect your tax code. A salary increase might push you into a higher tax bracket, requiring an adjustment to ensure the correct amount is withheld. If you take on a second job or freelance work, your tax code may need to reflect this additional income to prevent underpayment.

Employment benefits also influence tax codes. Perks such as a company car or private healthcare are factored into your tax code calculation by HMRC. If these benefits change or are removed, your code should be updated.

Marriage or separation can also lead to modifications. The Marriage Allowance allows one spouse to transfer a portion of their personal allowance to the other, affecting tax codes. If your relationship status changes, failing to update this could result in paying more tax than necessary or receiving an unexpected bill.

How to Check for Accuracy

Tax codes in the UK typically consist of numbers and letters, such as 1257L, the standard code for most employees in the 2024/25 tax year. The number represents the tax-free personal allowance divided by ten, while the letter indicates specific tax conditions, such as additional income sources or special deductions. If your code includes an unexpected letter or a lower number than expected, there may be an issue.

Compare your tax code against your latest tax documents, such as your payslip, P60, or P45. Cross-referencing this with your personal allowance and any deductions applied by HMRC can help identify discrepancies. If you receive untaxed income from investments or rental properties, HMRC may adjust your tax code to collect tax throughout the year. If these figures are outdated or incorrect, your code may not reflect your actual tax liability.

Checking HMRC’s online portal provides another way to confirm accuracy. Logging into your Personal Tax Account allows you to see how your code was calculated, including adjustments for benefits, deductions, or previous underpayments. If the figures seem incorrect, reviewing your most recent Self Assessment tax return or PAYE coding notice can clarify whether the adjustments are justified.

Correcting an Incorrect Code

To fix an incorrect tax code, notify HMRC as soon as possible. Employers and pension providers use the code assigned to them, but they rely on HMRC for updates, so errors must be corrected at the source. The fastest way to report an issue is through HMRC’s online services or by calling their helpline. You may need to provide details about your income, deductions, or changes in employment. If the discrepancy stems from outdated or incorrect information, HMRC will update the code and inform your employer, often applying the correction to your next payslip.

In some cases, HMRC may require supporting documents. If your tax code is wrong due to an employment-related expense, such as professional subscriptions or uniform costs, HMRC may ask for receipts or proof of payment before making changes. If you’ve overpaid tax, HMRC will usually issue a refund automatically once the correction is processed. If your code caused an underpayment, HMRC may adjust future tax deductions rather than requesting an immediate lump sum payment.

Potential Effects on Your Return

An incorrect tax code can affect your final tax return, particularly when reconciling what was paid versus what was owed. If the wrong amount was withheld throughout the year, this discrepancy will be reflected when filing your return, potentially leading to an unexpected tax bill or refund. If your tax code resulted in excessive deductions, HMRC will typically refund the overpaid amount after processing your return. If too little tax was deducted, you may need to settle the shortfall, sometimes with added interest if the underpayment spans multiple tax years.

Errors in tax codes can also impact eligibility for certain reliefs and allowances. Overpayments might reduce the amount of tax relief available on pension contributions, as HMRC assumes a higher level of tax liability when calculating relief at source. An incorrect code could also affect entitlement to tax credits or benefits that are income-dependent, such as Child Benefit, where the High Income Child Benefit Charge applies when earnings exceed £50,000. If your tax code misrepresents your true income, your liability under these schemes could be miscalculated, leading to unexpected repayments or missed entitlements.

Adjustments After the Change

Once your tax code has been corrected, monitor how the adjustment affects your future tax payments. Changes to your code can alter your take-home pay, impact ongoing deductions, and require additional steps to ensure everything is properly accounted for in the next tax year.

If your tax code was incorrect and led to an overpayment, HMRC typically issues a refund automatically, either through your payroll if the adjustment occurs within the same tax year or as a direct payment if the correction is made later. If an underpayment is identified, HMRC may spread the repayment across future pay periods by adjusting your tax code to collect the owed amount gradually. This is often done through a restriction in your personal allowance, meaning you’ll pay slightly more tax each month until the balance is settled. For larger underpayments, HMRC may issue a formal payment request, requiring a lump sum settlement or offering a structured repayment plan.

To prevent future miscalculations, review your tax position for the following year. If your circumstances have changed—such as starting a new job, claiming new deductions, or receiving taxable benefits—ensuring your tax code reflects these updates can help avoid errors. Keeping track of coding notices from HMRC and regularly checking your payslips for unexpected changes can help catch mistakes early. If you’re self-employed or have multiple income sources, filing a Self Assessment tax return may provide additional oversight, ensuring your total tax liability is accurately reported.