Tax Code Errors – Common Causes

Reviewed by Ed WilkinsonLast updated: March 10, 2026 4:33 pm

Image credit: Markus Spiske @Pexels

Tax code errors can cause problems for employees and employers alike, leading to the wrong amount of tax being taken from pay. Understanding the common causes helps you prevent issues and make sure you pay the correct tax. This article explains why tax code mistakes happen and what you can do about them.

Understanding Tax Codes

A tax code is used by employers to work out how much Income Tax to deduct from your pay. In the UK, these codes consist of numbers and one or more letters. The numbers reflect your Personal Allowance,  the amount you can earn each tax year without paying Income Tax, while the letter(s) indicate your circumstances, including any adjustments that might affect the calculation. Errors in a tax code can lead to underpayments or overpayments.

Composition of Tax Codes

The numerical part of a tax code represents how much tax-free income you are entitled to in that tax year. The letter(s), usually at the end of the code, tell your employer about any special considerations. For example, tax code 1257L remains the standard code for the 2026/27 tax year. It reflects the £12,570 Personal Allowance¹, which is currently frozen until April 2028 unless the government changes the rules.

Examples of Tax Code Letters

• L – Indicates that the taxpayer is entitled to the standard tax-free Personal Allowance (currently £12,570).
• M and N – Used when up to 10% of a spouse’s or civil partner’s Personal Allowance is transferred under the Marriage Allowance². ‘M’ indicates you received the transfer, while ‘N’ indicates you have transferred it.
• T – Generally used when other calculations are required to determine the Personal Allowance, for example, where the allowance is tapered because income is over £100,000, or where HMRC needs to review other adjustments.
• BR, D0, D1 – These codes are used when all the income from this source is taxed at the basic, higher or additional rate, respectively, without any Personal Allowance. Rates and letter prefixes can differ for Scottish (S) and Welsh (C) taxpayers.

Understanding these components helps employees and employers verify that the tax code applied is correct for the individual’s circumstances, thereby preventing potential complications. Regular checks and updates of your tax code are crucial, especially after significant life events such as marriage, changing jobs, starting or ending a benefit in kind, or where an emergency code (for example, 1257L W1/M1) has been applied.

Adjusting Your Tax Code

If there is a discrepancy in your tax code, act promptly to avoid a build-up of under- or overpaid tax. To put it right, you should:

1. Review the tax code on your payslip and compare it with the information in your Personal Tax Account (HMRC online account) or the HMRC app.
2. If discrepancies are found, contact HMRC for clarification and ask for an adjustment. Employers cannot change your code without HMRC instructions.
3. Keep HMRC informed about any changes in your circumstances that might affect your tax code, such as new or ceased jobs, changes in taxable benefits, or additional sources of income.

Taking these steps helps reduce the risk of paying too much or too little tax and supports compliance with HMRC rules.

Common Causes of Tax Code Errors

Tax code errors can arise from a variety of sources, often linked to changes in an individual’s personal or financial circumstances. Below are some of the most common causes:

• Changing jobs: Not providing a P45 to a new employer, or errors on it, can lead to an emergency code (often 1257L W1/M1) being used temporarily until HMRC receives the correct details. This is particularly relevant in a jobs market where short-term and variable-hours roles are common.
• Having multiple jobs: Each job will have a tax code. Your Personal Allowance is usually allocated to one main job or pension; other jobs are often taxed at BR/D0/D1. Mistakes can occur if the allowance is allocated incorrectly across employees.
• Changes in benefits: Any changes in company benefits, such as a company car or private medical insurance, can affect your tax code because their taxable value (benefit in kind) is included in PAYE. HMRC updates your code when these values change.
• Incorrect information: Errors in taxpayer data, whether from employer records or HMRC’s own databases, can lead to the wrong tax code being applied. Even small errors can propagate through the system unnoticed.
• Outdated information: Not updating a change in circumstances, like marriage or starting to receive a pension, can result in outdated tax codes. It is crucial to review and update your HMRC status during major life events.
• Technical errors: Software or administrative errors within HR departments or HMRC can also lead to incorrect tax codes. Regular audits and updates of payroll systems are recommended to minimise these risks.

Potential Impacts of Incorrect Tax Codes

Using the wrong tax code can lead to either overpaying or underpaying Income Tax. Overpayment can affect personal cash flow until it is refunded, while underpayment can result in a bill later. HMRC will usually reconcile your position after the tax year (for example, via a P800 or Simple Assessment³) and may collect underpaid tax through a later tax code or Self Assessment. Ensuring your tax code accurately reflects your current situation in the 2026/27 tax year can help avoid unexpected financial stress.

Statistical Insight

HMRC issues millions of coding notices (P2) each year, and it is common for codes to be adjusted during the year. Regularly checking your tax code in your Personal Tax Account or the HMRC app can help identify and rectify issues promptly.

Preventative Measures

To prevent tax code errors, it is advisable to:

1. Regularly check updates from HMRC regarding tax code changes and understand how they apply to your circumstances.
2. Communicate any changes in your personal or financial situation to HMRC as soon as possible.
3. Ensure all your employment documentation is accurate and submitted on time.

By taking these steps, individuals can maintain better control over their financial affairs and avoid the pitfalls of incorrect tax coding.

How to Identify and Correct Tax Code Errors

Reviewing your tax code regularly is essential to ensure that it reflects your current situation. Here are the steps to take if you suspect an error:

1. Check your tax code on your payslip against your Personal Tax Account online or the HMRC app to ensure they match. If discrepancies are noted, gather relevant documents that could influence your tax code, such as P60s, P11Ds and details of taxable benefits.
2. Contact HMRC directly if you believe your tax code is wrong. They can adjust your tax code and inform your employer of the change. When contacting HMRC, it helps to have your National Insurance number and a recent payslip to hand.
3. Provide accurate and up-to-date information to HMRC, especially if your circumstances change. This includes changes in your income, marital status or receipt of new benefits. Prompt reporting can prevent future inaccuracies in your tax code.

Additional Considerations

While following the steps above, consider these additional factors:

• HMRC’s online services and the HMRC app allow you to view and, in some cases, update your tax code information. Using these tools can reduce the likelihood of errors.
• Be aware of common triggers for tax code changes, such as starting a new job or receiving a new benefit from your employer. These events often lead to adjustments in your tax code.

Prevention Tips

Preventing tax code errors primarily involves staying informed and proactive about your tax affairs:

• Maintain accurate and up-to-date records of your employment and benefits. This includes keeping a log of employment changes and related documentation that could affect your tax code.
• Communicate any changes in your circumstances to HMRC promptly. Quick communication helps ensure that tax code adjustments are reflected accurately.
• Understand the basics of how tax codes are structured and what your own code means. This knowledge can alert you to obvious discrepancies before they become problematic.
• Use HMRC tools, such as your Personal Tax Account and the HMRC app, to verify your tax code, particularly after any change in your employment or personal circumstances.

By embracing these practices, you can help ensure your tax code accurately represents your financial situation, avoiding unexpected tax bills or discrepancies.

A Financial Adviser Can Help

While many people can manage tax code matters themselves, a financial adviser can help you understand more complex situations and pull together the information HMRC needs. Only HMRC can issue or change a PAYE tax code.

Financial advisers are particularly useful in situations where multiple income streams, benefits in kind or other complicating factors make managing tax affairs more challenging.

If you’re unsure how changes in your life might impact your taxes or wider financial position, seeking professional support can be a prudent step.

At FinancialAdvisers.co.uk, we can connect you with experienced, FCA-authorised and regulated financial advisers in your area. The information in this article is general guidance and does not constitute personal tax advice.

This article contains external links for further information on topics. Whilst all attempts are made to verify the trustworthiness of these external sources, we accept no responsibilty for the upkeep or security of these external websites. Some of these links in particular are annotated below:

• ¹ https://www.gov.uk/income-tax-rates
• ² https://www.moneyhelper.org.uk/en/work/employment/marriage-and-married-couples-allowance
• ³ https://www.gov.uk/simple-assessment