Changing Tax Code With HMRC: Fast 2025 Step-by-Step Guide

Paying too much or too little tax can be frustrating, especially when it’s due to a simple error in your tax code. If your payslip looks smaller than expected, or you’ve been placed on an emergency tax code without warning, you’re not alone. Many UK taxpayers face this issue each year. Understanding how to change your tax code with HMRC is crucial to ensure you pay the right amount and avoid overpaying or underpaying.

In this guide, we’ll walk you through the entire process step by step, explain why codes change, and provide practical tips to resolve issues quickly in 2025. Unlike many generic guides, this resource includes the latest HMRC updates, real-world examples, and a clear framework you can follow immediately.

What a Tax Code Really Means in 2025

Your tax code is a combination of numbers and letters that tells your employer or pension provider how much tax to deduct from your income.

Numbers indicate your tax-free allowance. For example, 1257L means you have £12,570 tax-free for the year.
Letters indicate your situation:
L = standard allowance
M = Marriage Allowance transferred
T = complex calculation required
K = your allowances are less than your deductions

Knowing your tax code helps ensure the correct amount of tax-free income and avoids overpaying taxes. You can find your code on your payslip, P60, P45, or HMRC coding notice.

When You Need to Change Your Tax Code With HMRC

Several situations may require a change:

Starting a new job, you may be put on an emergency tax code.
Multiple jobs or pensions simultaneously.
Changes in benefits or taxable perks (like a company car).
Overpaid or underpaid tax from the previous year.
Employer errors in payroll.

Signs your tax code might be wrong:

Your take-home pay suddenly decreases without explanation.
You receive a coding notice from HMRC showing unexpected letters/numbers.
Your tax-free allowance seems lower than expected.

The HMRC Tax Code Fix Flow (Unique Framework)

We created a simple framework to resolve tax code issues efficiently:

1 — Find Your Current Tax Code

Check your payslip, P60, or Personal Tax Account. Verify your NI number and ensure your employer is using the correct details.

2 — Diagnose What’s Wrong

Compare your code to your expected Personal Allowance and circumstances. Look for emergency codes (W1/M1) or unexpected letters.

3 — Update Details in Your Personal Tax Account

Most changes can now be submitted online. Update:

Employment income
Benefits in kind
Multiple jobs or pensions

4 — Submit Supporting Information

Provide P45, P60, or other relevant documents to HMRC or your employer.

5 — Wait for HMRC Calculation

HMRC typically recalculates your tax code within 24–72 hours.

6 — Check the Updated Code on Your Payslip

Ensure your new code is reflected in your next payslip to confirm correct tax deductions.

How to Change Your Tax Code With HMRC (Step-by-Step)

1 — Change Tax Code Online (Fastest)

Log in to your HMRC Personal Tax Account.
Select ‘Manage Your Tax Code’.
Update income, benefits, or multiple jobs.
Submit and wait for confirmation.

2 — Call HMRC (When Needed)

For complex cases or corrections not reflected online.
Contact HMRC at 0300 200 3300 (from the UK).
Have your National Insurance number, P45, P60, and recent payslips ready.

3 — Update Through Employer or Pension Provider

Notify your payroll department.
Provide HMRC correspondence or supporting documents.
Employer submits the change to HMRC.

Pro Tip: Online submissions are generally faster and leave a digital record for reference.

How Long Does HMRC Take to Change a Tax Code?

Most updates take 1–3 days, but delays can occur if:

HMRC needs further proof of income.
Payroll hasn’t been updated yet.
There are multiple employers/pensions involved.

In 2025, HMRC’s digital automation speeds up adjustments for routine changes, making online updates quicker than ever.

Case Study — How Sarah Fixed an Incorrect Tax Code in 24 Hours

Sarah started a second job and noticed her code changed to 1257L M1. She:

Checked her Personal Tax Account.
Identified her emergency tax code.
Submitted updated employment details online.
Uploaded her P45 from the new job.

By the next day, HMRC had corrected her code. Her next payslip reflected the correct tax-free allowance.

Key takeaway: Using the online HMRC portal is both secure and fast.

Common Mistakes That Slow Down Tax Code Updates

Using an old National Insurance number.
Not submitting full income or benefit information.
Assuming the employer will automatically fix it without contacting HMRC.
Ignoring coding notices from HMRC.
Confusing emergency codes with permanent errors.

Avoiding these mistakes ensures a smoother, faster correction.

Emergency Tax Codes Explained (1257L W1/M1)

Emergency codes apply when HMRC lacks complete information:

W1 / M1: Week/Month basis — tax-free allowance is not spread evenly over the year.
Common triggers: new job, missing P45, multiple jobs.
You can remove it by updating your Personal Tax Account or providing HMRC with the required documents.

Different Ways to Change Your Tax Code

Method	Speed	Accuracy	When Recommended
Online (Personal Tax Account)	24–72 hrs	High	Most updates, new jobs, and benefits
Phone Call to HMRC	1–5 days	Moderate	Complex issues, disputes
Employer/Pension	3–7 days	Moderate	Only if HMRC updates are not possible online

Tools & Resources for Checking Your Tax Code

HMRC Tax Code Checker: Verify your current code online.
Personal Tax Account: Central hub for managing tax details.
P45 / P60 / Payslips: Required documentation.
Benefits Checker: Confirm taxable benefits are included.

What to Do If HMRC Doesn’t Update Your Tax Code

Follow up using the Personal Tax Account message system.
Provide additional proof (P45, P60, payslips).
Contact payroll to confirm your employer submitted details.
Escalate to HMRC if the code remains incorrect after 7–10 days.

FAQs

Q1. How can I get HMRC to change my tax code?

You can update your tax code by logging into your HMRC Personal Tax Account, submitting your current employment or income details, or calling HMRC with your National Insurance number and recent payslips. Keeping your information up to date ensures you pay the correct tax.

Q2. How fast can HMRC change a tax code?

Most changes submitted online are processed within 24–72 hours, depending on the complexity of your employment or income situation.

3. Do I need to call HMRC to correct my tax code?

Not necessarily. Using the Personal Tax Account online is usually faster and sufficient for most tax code corrections. A call is only needed if there are complications or missing information.

Q4. Why has my tax code changed unexpectedly?

Unexpected tax code changes can occur due to:

Starting a new job or pension
Receiving taxable benefits
Payroll errors
Updates in HMRC records

Q5. What documents do I need to change my tax code?

To update your HMRC tax code, you typically need:

P45 from your previous employer
P60 for the current tax year
Recent payslips
National Insurance number
Details of any taxable benefits

Q6. How do I fix an emergency tax code?

If you’re on an emergency tax code, submit your current employment details and P45 through your HMRC Personal Tax Account to get the correct tax code applied.

Q7. Can my employer change my tax code?

Yes. Employers submit tax code changes to HMRC on your behalf, but HMRC confirms the final tax code. Always verify through your Personal Tax Account.

Q8. How do I check if HMRC updated my tax code?

Check your latest payslip or log in to your HMRC Personal Tax Account. The updated tax code should appear in the next payroll cycle.

Conclusion

Fixing an incorrect tax code is essential to avoid overpaying taxes or unexpected deductions. By following our step-by-step HMRC Tax Code Fix Flow, checking your documents, and using the Personal Tax Account, you can ensure your tax code is correct and up to date in 2025. Remember, acting quickly helps prevent long-term issues and ensures you get the tax-free allowance you’re entitled to.