Check My Tax Code: What HMRC Gets Wrong About You

Your tax code is wrong more often than HMRC would like you to believe. In fact, one in three UK taxpayers is estimated to be on the wrong tax code at some point in their working life, and the errors almost never fall in your favour.

If you are a sole trader with employment income on the side, or you've recently gone fully self-employed, your tax code is particularly vulnerable to HMRC getting its assumptions spectacularly wrong. Here is how to check your tax code, understand what it actually means, and fix it before it costs you money you won't easily get back.

Why Your Tax Code Matters More as a Sole Trader

If you are purely self-employed, your tax code affects you less directly. You pay income tax through Self Assessment, not PAYE, so there is no employer deducting tax at source. But the moment you also have employment income, a second job, rental income, or a pension, HMRC will issue a tax code and use it to collect tax on your non-self-employment income.

The problem is that HMRC does not always have accurate, up-to-date information about your self-employment earnings. It estimates. And those estimates, baked into your tax code through what is called a "coding out" adjustment, can quietly drain money from your wages every month before you've had a chance to dispute them.

For a sole trader who also earns from employment, say a plumber picking up shifts through an agency between contracts, or a freelance designer who kept a part-time employed role, that £343 overpayment is real money sitting in HMRC's account instead of yours.

How to Check Your Tax Code Right Now

There are three ways to check your tax code, and the fastest takes about two minutes.

Via Your Personal Tax Account

Go to gov.uk and log in to your Personal Tax Account using your Government Gateway credentials. Under the "Pay As You Earn" section, you will see your current tax code and a breakdown of how it was calculated. This is the most useful route because it shows the component parts: your Personal Allowance, any deductions for company benefits, and any adjustments HMRC has made to collect underpaid tax from previous years.

Via the HMRC App

The HMRC app (available on iOS and Android) shows your tax code under the "Tax" section of your account. It is less detailed than the full Personal Tax Account but useful for a quick check.

On Your Payslip or P60

Your employer includes your tax code on every payslip. If you have not received a P2 notice of coding from HMRC this year, your payslip is the fastest offline check. Your P60, issued after 5 April each year, also shows the tax code applied across the year.

What the Letters and Numbers Actually Mean

Most people ignore their tax code because it looks like bureaucratic noise. It is not. Each part carries a specific meaning.

The number tells your employer your tax-free income for the year. Take the number, multiply by ten, and that is your annual allowance. The standard code for 2025/26 is 1257L, reflecting the £12,570 Personal Allowance.

The letter is where things get interesting, and where HMRC's assumptions become visible:

• L: You get the standard Personal Allowance. Nothing unusual.
• M: You have received a transfer of 10% of your partner's Personal Allowance under the Marriage Allowance scheme.
• N: You have transferred 10% of your Personal Allowance to your partner.
• T: Your tax code includes other calculations HMRC needs to review. Often triggered by higher earners above £100,000 where the Personal Allowance tapers.
• 0T: No Personal Allowance at all. This can happen when HMRC has no information about you, when you have used your full allowance, or when you have started a new job and not yet submitted a P45.
• BR: All income taxed at the basic rate (20%). Typically applied to a second job or pension.
• D0: All income taxed at the higher rate (40%). Applied to second income sources where HMRC assumes your main income already uses the basic rate band.
• D1: All income taxed at the additional rate (45%).
• K: This is the one that should make you pay attention. A K code means HMRC is adding income to your taxable pay rather than subtracting an allowance. It happens when deductions (like underpaid tax being collected, or taxable benefits) exceed your Personal Allowance.
• W1 or M1: Emergency tax codes applied on a "week 1" or "month 1" basis. HMRC is taxing each pay period independently rather than cumulatively. You are almost certainly overpaying.

For sole traders with mixed income, the codes most likely to cause problems are K codes (where HMRC is collecting self-assessment debt through your wages), BR and D0 (applied to secondary income sources), and emergency codes applied when you switch employers or start a new contract.

The K Code Problem for Sole Traders

The K code deserves its own section because it is the code most likely to blindside a sole trader who also has employment income.

Here is the scenario. You earned £55,000 last year: £35,000 from employment and £20,000 from self-employment. Your Self Assessment bill for the self-employment income comes to, say, £4,000 after allowable expenses. You pay it late, or you set up a time-to-pay arrangement. HMRC decides, rather than waiting for your next Self Assessment payment, to recover the underpayment through your PAYE tax code instead.

The result is a K code on your employment income. Your employer is now deducting extra tax from your monthly wages to cover what HMRC believes you owe from your self-employment. You might not even notice it happening until your payslip looks inexplicably thin.

HMRC can only collect up to 50% of your PAYE income this way, so it will not leave you with nothing. But it can meaningfully cut your take-home pay without any direct notification beyond a P2 notice of coding that is easy to miss.

If you have recently filed a Self Assessment return and your employment income suddenly drops, check for a K code immediately.

Common Mistakes HMRC Makes With Sole Trader Tax Codes

Using Outdated Income Estimates

HMRC uses your previous year's Self Assessment figures to estimate your current-year self-employment profit. If your income has changed significantly, the code will be wrong. A plumber who earned £60,000 in one year and £35,000 the next (perhaps due to illness or a slow period) will have a tax code based on the higher figure, resulting in excess deductions from any PAYE income.

Double-Counting Adjustments

If you have asked HMRC to collect underpaid tax through your code and also submitted a Self Assessment return covering the same period, there is a real risk of double-collection. It happens. The reconciliation usually catches it eventually, but "eventually" can mean 12 to 18 months after the money has left your account.

Removing the Personal Allowance Incorrectly

HMRC sometimes removes the Personal Allowance entirely (issuing an 0T code) based on incorrect assumptions that your total income exceeds £125,140 the point at which the allowance disappears entirely. For sole traders whose income fluctuates year to year, this can be triggered by a single unusually good year and not corrected when income falls back.

Applying Emergency Codes When Changing Contracts

If you move between employed contracts, especially common for contractors and freelancers who occasionally operate under PAYE, HMRC may not receive P45 information quickly enough and will default to an emergency code. Emergency codes typically mean you lose your full Personal Allowance for the period.

How to Challenge a Wrong Tax Code

You do not have to accept the code HMRC has issued. You have the right to challenge it, and the process is more straightforward than most government interactions.

Online: Log in to your Personal Tax Account. Use the "check your income tax" service to update HMRC on your expected income for the current year. The system will recalculate your code and issue an updated P2 notice to your employer.

By phone: Call HMRC's income tax helpline on 0300 200 3300. Lines are open Monday to Friday, 8am to 6pm. Have your National Insurance number, employer details, and a rough estimate of your expected income ready. Wait times are notoriously long; the online route is faster.

By post: Write to HMRC's Pay As You Earn office. This is the slowest option by a significant margin and is only worth pursuing if the online and phone routes have failed.

Once HMRC corrects your code, your employer applies the new code from the next payroll run. If you have overpaid tax in the current year, HMRC will refund it either automatically through a P800 calculation after the tax year ends, or you can request an in-year refund via your Personal Tax Account.

What This Means for MTD and Self Assessment

From April 2026, Making Tax Digital for Income Tax requires sole traders earning above £50,000 to submit quarterly updates to HMRC rather than a single annual Self Assessment return. The threshold drops to £30,000 in April 2027.

This changes the dynamic around tax codes in a meaningful way. Currently, HMRC estimates your self-employment income based on your last filed return, which can be over a year out of date. Under MTD, HMRC will receive quarterly updates on your income and expenses. In theory, this should make tax code adjustments more accurate because HMRC will have near-real-time information.

In practice, it introduces new risks. If HMRC uses your quarterly MTD submissions to adjust your tax code mid-year, and then your income falls in the second half of the year, you could end up having overpaid tax for months before the annual reconciliation catches up. The five-submission structure of MTD means HMRC will have more data points than ever. Whether it uses them to issue more accurate codes, or simply to generate more complicated ones, remains to be seen.

For sole traders who also have employment income, monitoring your tax code will become an active, year-round task rather than a once-a-year check. Pairing MTD compliance software with a habit of quarterly tax code reviews is the sensible approach.

If you want to understand how your self-employment income interacts with your tax bill more broadly, the Self Employed Tax Estimator 2026 is a useful starting point before you engage with HMRC directly.

One Concrete Scenario: The Electrician With Two Income Streams

Mark is an electrician. He earns £28,000 from self-employment and £18,000 from a part-time employed role at a facilities management company. His total income is £46,000.

Last year, his self-employment profit was £32,000. HMRC used that figure when setting this year's tax code for his employment income. It applied a coding adjustment to collect an estimated £800 in self-employment tax through his wages, giving him a reduced allowance in his PAYE code.

This year, Mark's self-employment income dropped by £4,000 because of a slow January and February. His Self Assessment liability will be lower than HMRC assumed. But by the time the tax year ends and HMRC reconciles his P800, he will have had £800 deducted from his wages unnecessarily for twelve months.

If Mark had logged into his Personal Tax Account in April and updated his expected self-employment income, HMRC would have revised his code. He would have had roughly £67 more per month in his pocket throughout the year.

It is not a life-changing sum. But it is his money, not HMRC's.

Check Your Tax Code Today, Not in April

That is the point this piece started with: HMRC gets your tax code wrong more often than it admits, and the errors almost never fall in your favour. As a sole trader with any employment income, your code is built on assumptions about income that may bear no resemblance to this year's reality.

Log in to your Personal Tax Account today. Find your tax code. Look at the breakdown. If the self-employment adjustment looks wrong, update your expected income. It takes ten minutes and could put hundreds of pounds back where they belong.

For the broader picture of how self-employment tax, MTD compliance, and digital record-keeping fit together, TapTax is built specifically for sole traders who want to stay on top of their obligations without drowning in admin.