How do I tell HMRC my Tax Code is Wrong?

Various circumstances can lead to being assigned an incorrect tax code, which may result in paying either too much or too little tax.

Having the correct tax code is essential for ensuring accurate PAYE tax deductions from your income.

Your tax code, which your employer (or pension provider) uses, can be issued directly by HMRC or determined by the employer following HMRC’s standard guidelines.

Regardless of the source, it’s vital to check that your employer is using the appropriate tax code for your situation, and if you discover any errors, you must contact HMRC.

When reaching out to HMRC, ensure you provide complete and precise details to help them determine your correct tax code.

Our short guide highlights the best ways to contact HMRC, the details you’ll need and what to expect when your tax code is changed.

Ways to tell HMRC that your tax code is incorrect

As soon as you realise there has been a mistake with your PAYE tax code or you have had a change in circumstances you must get in touch with HMRC online, by phone or letter.

Online:

For most people, the quickest and simplest way to sort out a tax code is by using the GOV.UK online service. It’s a direct line to HMRC that lets you skip the phone queues and avoid waiting for the post.

To get started, you’ll need to head over to the ‘Check your Income Tax’ service. This requires a Government Gateway user ID and password.

If you haven’t got one, don’t worry – you can set one up as you go, but be prepared for a few identity verification checks.

Phone:

To talk about your tax code the main number you’ll need is 0300 200 3300. Wait times can be a bit of a lottery.

Calling first thing in the morning between 8 am and 9 am, or later in the day, often means you’ll get through quicker. Midday and Mondays are almost always the busiest times.

By letter:

If your query isn’t urgent or you simply prefer having a paper trail, sending a letter is a perfectly fine way to go.

Your letter must clearly state your personal details, including your full name, address, and national insurance number.

After that, explain exactly why you think your tax code is wrong. Be specific – mention things like a change in company benefits or that you’ve started a second job.

Accuracy is key to a correct tax code

Be honest and precise with your figures. If you’ve started a second job or a side hustle, you must declare that income.

HMRC’s systems are more connected than ever, and failing to report changes can lead to underpayment issues down the line.

After submitting your changes, the system will confirm it has received your request, and you can usually track its progress through your online account. HMRC will then review the information you’ve provided.

What do I need to tell HMRC that my tax code is wrong?

Before getting in touch it’s recommended to gather together as many of the details (that are relevant to you) listed below:

• Full name (including middle names).
• Date of birth.
• Email address.
• National insurance number.
• Employer/pension provider tax reference number.
• Employee works number or pension number (private and/or company).
• Estimate of total income for current tax year.
• Company benefits (e.g. health care, car fuel allowance).
• Any other income (e.g. savings interest, property rental).
• Weekly state benefits or state pension payments.

What happens after I have told HMRC about my incorrect tax code?

When you have asked for a review of your tax code HMRC will check your tax record and make appropriate adjustments.

If a change is needed, they’ll issue a new tax code. They will then electronically send your employer (or pension provider) a new tax code to be used the next time you are paid.

If you have overpaid income tax in the current tax year because of an incorrect code you should receive this back automatically in your next pay (after your new tax code has been operated).

When will your payslip change?

At the same time HMRC sends you your tax code notification, they’ll also send your new code directly to your employer.

But don’t expect it to appear on your very next payslip. The timing really depends on your employer’s specific payroll cycle.

Most companies run their payroll monthly, so it can often take between four to six weeks for the updated code to actually be applied.

If you’re paid weekly, you might see the change a bit sooner. The main thing is to keep a close eye on your payslips in the weeks following your request.

Overpayments and underpayments of tax

After the dust settles and your new code is in place, there are really only two possible outcomes if your old code was incorrect.

Knowing which one applies to you is key to getting your finances back on track.

• You’ve overpaid tax: Good news! If your new code shows you’ve been paying too much tax, HMRC will sort out a refund automatically.

For the current tax year this usually happens through your salary. Your employer will adjust your tax deductions, meaning you’ll pay less tax in the following months until the overpayment is balanced out. In some cases, HMRC might send you a P800 tax calculation followed by a cheque in the post.

• You’ve underpaid tax: If it turns out you haven’t paid enough tax, HMRC will need to collect the shortfall. They don’t usually demand a lump sum.

Instead, they typically adjust your tax code for the next tax year, spreading the collection over 12 months. This is designed to avoid a big, sudden drop in your take-home pay.

Why your tax code might be wrong

Before diving into how to fix it, it’s worth understanding why your tax code might be wrong in the first place.

An error can leave you overpaying tax and needing a refund, or worse, underpaying and facing an unexpected bill from HMRC down the line.

Most of the time, the culprits are simple life changes. These events can alter your financial circumstances, and HMRC’s systems don’t always catch up in real-time.

Common Reasons for an incorrect code

Several real-world scenarios can easily throw your tax code off balance. Starting a new job is a classic trigger, especially if your P45 from your last employer is delayed.

Without it, you’ll often be put on an emergency tax code until things are straightened out.

Tip

If you don’t have a P45 ensure you complete a starter checklist which is a form your employer can give you.

Other frequent causes include:

• Having multiple jobs or pensions: Each income source needs to be tallied up correctly to ensure your total tax-free personal allowance is allocated properly and you’re not getting it twice.
• Receiving new company benefits: Things like a company car or private medical insurance are considered taxable benefits in kind, and your tax code must be adjusted to account for their value.
• A significant change in earnings: A promotion, a hefty bonus, or even a change in hours can push you into a different earnings bracket, requiring a quick code update.

Common tax code mistakes to look out for

It’s surprisingly easy for tax code errors to creep in, but with a bit of proactive management, most of them are entirely preventable.

Staying on top of things doesn’t just save you from a future financial headache; it makes sure you’re paying the right amount of tax from day one.

One of the most common slip-ups is people not telling HMRC about a second job or a new side hustle. It’s an easy mistake to make – you assume your main employer’s PAYE system has it all covered.

But any extra income needs to be declared, otherwise you aren’t paying tax on that money, and that will almost certainly lead to an underpayment bill down the line.

Don’t assume it’s all automatic

Another classic error is assuming your employer automatically tells HMRC about every little change in your circumstances.

While they handle your main salary, they often don’t have the full picture of your financial life.

Here are a few actionable steps to avoid common pitfalls:

• Report taxable benefits immediately: As soon as you receive a new benefit like private medical insurance or a company car, log into your online tax account and add it. Don’t wait for your P11D form.
• Update your estimated income after a pay rise: A hefty salary increase can easily push you into a higher tax bracket. Your most actionable step is to update your estimated earnings for the tax year via the GOV.UK portal to get your code corrected swiftly.
• Actively claim tax reliefs you’re entitled to: If you work from home or have to wear a specific uniform for your job, you could be due reliefs that lower your tax bill. You must actively claim these; they are not automatic.

Tip

Being proactive is key: When something changes login to your online tax account and update your estimated annual income or other information. Don’t wait for HMRC to play catch-up.