Is Your Tax Code Wrong? How to Check and Fix It Before You Overpay

If you have ever opened your payslip and felt a sudden jolt of confusion because your net pay was lower than expected, you aren't alone. For many small business owners and directors in the UK, the tax code is a mysterious string of numbers and letters that dictates how much of their hard-earned money actually hits their bank account.

As we move into the 2026-27 tax year, staying on top of this has never been more critical. HMRC is currently undergoing a significant "shake-up" of how tax codes are managed. They are tightening the rules on what can be included in your code, which means many "extras" you previously relied on to lower your tax bill might have vanished overnight.

In this guide, we’ll break down what is happening with the 2026-27 tax code changes, how to spot an error, and what you need to do to fix it before you end up overpaying (or worse, receiving a surprise bill later). For a broader look at this year’s changes, see HMRC 2026-27 Tax Code Shake-Up: What You Need to Know.

What is a Tax Code and Why Does it Matter?

Your tax code is used by your employer or pension provider to work out how much Income Tax to take from your pay or pension. HMRC tells them which code to use.

For the vast majority of people in the UK, the standard tax code for the 2026-27 tax year remains 1257L. This code signifies that you are entitled to the standard Personal Allowance of £12,570: the amount of income you can earn each year before you start paying Income Tax.

However, as a small business owner or a company director, your tax situation is often more complex than a standard employee's. You might have multiple sources of income, benefits in kind (like a company car), or professional expenses that change how much tax you should be paying through PAYE. If your code is wrong, you could be paying hundreds of pounds too much every month, or you could be underpaying, leading to a massive, unexpected bill at the end of the year.

The 2026-27 HMRC Shake-Up: What Has Changed?

The biggest news for 2026-27 isn't an increase in the Personal Allowance: in fact, that remains frozen at £12,570. The real change is happening behind the scenes. According to recent reports from AccountingWEB and HMRC guidance, the tax office is "cleaning up" codes.

HMRC is now actively removing certain reliefs from PAYE codes if they believe the data is out-of-date or unsupported. If you’ve historically had professional fees, travel expenses, or Gift Aid relief built into your tax code, you need to check your latest notice immediately.

1. Removal of Employment Expenses

If you have work-related expenses over £120 built into your tax code, HMRC may have removed them for this tax year if:

• You haven't filed a Self Assessment return since 2021-22.

• The expenses in your code are higher than what you claimed on your last return.

• There has been a gap in your employment.

2. The Gift Aid "Clear Out"

Many directors use Gift Aid to claim higher-rate tax relief. Previously, HMRC often rolled this relief forward in your tax code year after year. For 2026-27, they are stripping this out if the amount hasn't changed for three years and you haven't filed a Self Assessment return in that time.

3. Dividend Tax Increases

While not directly part of your PAYE tax code, it is important to note that dividend tax rates have increased by 2 percentage points for 2026-27. Basic-rate dividend tax is now 10.75%, and higher-rate is 35.75%. This makes it even more important that your salary-based tax code is accurate, as your overall tax burden is likely rising elsewhere.

How to Check if Your Tax Code is Correct

Checking your tax code should be a monthly habit, but it is essential at the start of a new tax year. You can find your code on your latest payslip, your P60, or by logging into your HMRC Personal Tax Account.

Decoding the Letters

The letters in your code tell HMRC your situation:

• L: You’re entitled to the standard tax-free Personal Allowance.

• M: Marriage Allowance (you've received a transfer of allowance from your partner).

• N: Marriage Allowance (you've transferred some of your allowance to your partner).

• T: Your tax code includes other calculations to work out your Personal Allowance.

• BR / D0 / D1: All your income from this job is being taxed at the basic, higher, or additional rate (usually used for second jobs).

• K: This means you have "untaxed income" that is more than your Personal Allowance (common if you have a high-value company car or are paying back old tax debts).

Common Errors for SME Owners and Directors

Directors of small companies are particularly prone to tax code errors. Here are the most common pitfalls:

1. Multiple Income Streams

If you are a director of one company but also have a part-time role elsewhere, or perhaps you receive a pension, HMRC might split your Personal Allowance across both. This often results in you "using up" your tax-free allowance on a small salary while being overtaxed on your main income.

2. Benefits in Kind (BiK)

If your company provides you with a car, health insurance, or other perks, these must be "coded in." If you stop using a company car but HMRC doesn't update your code, you will continue to pay tax on a benefit you no longer receive.

3. Underpaid Tax from Previous Years

If you owed tax from a previous year, HMRC might reduce your current year's Personal Allowance to collect the debt. While this is a standard practice, the "K code" used to do this can sometimes be calculated incorrectly, leading to you paying back more than you actually owe.

What to Do if You’ve Overpaid (or Underpaid)

If you realize your code is wrong, don't panic, but do act quickly.

If you’ve overpaid: You are entitled to a refund. In many cases, if the error is fixed during the tax year, your next payslip will automatically adjust, and you'll receive a "tax rebate" in your take-home pay. If the tax year has already ended, HMRC will usually send you a P800 calculation and a cheque (or an electronic refund).

If you’ve underpaid: This is the one people fear most. If you realize you aren't paying enough tax, tell HMRC immediately. It is much easier to split the repayment over several months of future pay than to face a massive lump-sum demand later.

How to Fix It

1. Online: Use the 'Check your Income Tax' service on the GOV.UK website. This is usually the fastest way to update your estimated income or tell HMRC about changes to your benefits.

2. Phone: You can call HMRC’s Income Tax helpline. Be prepared for a wait, and have your National Insurance number and latest payslip ready.

3. Through an Accountant: For SME owners, this is the safest route. An accountant can look at your total income (salary, dividends, and benefits) to ensure your code is perfectly aligned with your actual liability. If you are still choosing who to work with, Accounting Services UK: The Complete Guide to Finding the Right Accountant for Your Business can help you compare your options.

How to Avoid a Surprise Tax Bill

With the introduction of Making Tax Digital (MTD) for many sole traders and landlords starting in April 2026, the tax landscape is becoming more automated. However, automation doesn't always mean accuracy.

To avoid surprises:

• Review your coding notice (P2) every March/April. Don't just file it away.

• Report changes in benefits immediately. If you hand back the company car keys in June, don't wait until next April to tell HMRC.

• Keep your Self Assessment updated. If you are no longer making the Gift Aid donations you used to, update your return so your code doesn't rely on "ghost" reliefs.

Why Professional Help is Essential

Managing a business is hard enough without having to become an expert in the ever-shifting sands of UK tax legislation. HMRC’s current "clean up" of tax codes means that many business owners who were on "autopilot" will find themselves with incorrect codes this year.

At Accountant Search, we specialize in matching SME owners with the perfect accounting partner. Whether you need help navigating the 2026-27 tax code changes, preparing for MTD, or optimizing your director’s salary and dividend split, a professional accountant can save you thousands in overpaid tax and hours of stress.

Don't wait for a letter from HMRC to find out you've been overpaying. Take control of your tax code today and ensure your business: and your personal finances: are on the right track. If you need help taking the next step, read How to Find an Accountant in the UK: A Step-by-Step Guide for Business Owners.

---

Author: Richard Richard is a senior contributor at Accountant Search, specializing in tax policy and financial planning for UK small businesses. With years of experience in the accounting sector, he focuses on making complex tax changes simple for entrepreneurs.