Can I Change My Tax Code — UK Guide to HMRC Tax Codes

If your tax code is wrong, you’re paying too much or too little tax. Here’s how to check and get it fixed.

Understanding Your Tax Code

What a Tax Code Means

Your tax code tells your employer how much tax-free income you get before tax is deducted. The most common code is 1257L, which means:

• Multiply the numbers by 10 = £12,570 tax-free allowance
• L = standard personal allowance

Common Tax Codes

Where to Find Your Tax Code

• Payslip — usually near the top
• P45 — from your previous employer
• P60 — end-of-year summary
• HMRC Personal Tax Account — online at gov.uk
• Coding notice (P2) — letter from HMRC

Signs Your Tax Code Is Wrong

You Might Be Paying Too Much Tax If

• You’ve started a new job and are on BR or 0T (emergency tax)
• Your tax code doesn’t include your personal allowance
• You haven’t received your P45 from a previous employer and your new employer used a default code
• You have tax-deductible expenses (like uniform allowance) not reflected in your code
• You stopped receiving a taxable benefit but the code wasn’t updated

You Might Be Paying Too Little Tax If

• You have untaxed income not accounted for (savings interest, rental income)
• You receive taxable benefits in kind (car, health insurance) but they’re not in your code
• You have multiple jobs and your allowance is split incorrectly
• An old benefit adjustment wasn’t removed when it should have been

How to Check Your Tax Code

Step 1 — Log Into Your Personal Tax Account

Go to gov.uk and sign in to your Personal Tax Account. You can:

• See your current tax code(s)
• See what your code is based on (allowances, deductions)
• Check if HMRC has the right information about your income
• Report changes directly

Step 2 — Check the Breakdown

Your coding notice shows how your code is calculated:

Check each item — is the company car value correct? Do you still have medical insurance? Are all deductions legitimate?

How to Get Your Tax Code Changed

Online (Fastest)

1. Sign in to your Personal Tax Account on gov.uk
2. Go to “Check your Income Tax” or “Tax codes”
3. Select “Tell HMRC about a change” or check the details and flag what’s wrong
4. HMRC will review and update your code

By Phone

Call 0300 200 3300 (Income Tax helpline):

• Monday to Friday, 8am to 6pm
• Have your NI number ready
• Have a recent payslip to hand
• Be prepared to explain what you think is wrong

By Post (Slowest)

Write to: Pay As You Earn and Self Assessment, HM Revenue and Customs, BX9 1AS

Include your NI number, current tax code, employer details, and what you think is wrong.

Common Situations

Emergency Tax (New Job)

If you’ve started a new job without giving your employer a P45:

• You may be put on an emergency tax code (often shown as W1 or M1)
• You’ll be taxed on each pay period individually, without carrying forward unused allowance
• Once HMRC has your correct details, they’ll issue the right code
• Speed this up: Give your P45 to your new employer, or complete a Starter Checklist (form P46) so your employer can use the correct code

Multiple Jobs

If you have two or more jobs:

• Your personal allowance should be allocated to one job (usually the highest-paying)
• Other jobs are usually coded BR (20%) or D0 (40%)
• If your allowance is split incorrectly, you could be overtaxed or undertaxed
• You can ask HMRC to reallocate your allowance between jobs

Taxable Benefits in Kind

If you receive benefits like a company car, private health insurance, or interest-free loans:

• The taxable value is deducted from your personal allowance through your tax code
• If you stop receiving a benefit, tell HMRC so they remove it from your code
• If the benefit value changes, report it

Marriage Allowance

If you’re eligible for Marriage Allowance (one partner earns under £12,570), the transfer can be applied through your tax code. If it’s not showing, apply at gov.uk.

Getting a Tax Refund

If You’ve Overpaid This Tax Year

HMRC can:

• Issue a new tax code that refunds the overpayment through your remaining pay
• The adjusted code gives you a larger tax-free allowance to compensate

If You’ve Overpaid in Previous Years

• HMRC sends a P800 tax calculation (usually by post or online)
• You can claim a refund online through your Personal Tax Account
• Refunds are paid into your bank account within 5-14 days
• You can claim overpaid tax for the previous 4 tax years

If You’ve Underpaid

• HMRC may collect the underpayment through next year’s tax code
• The amount is spread over 12 months (code adjusted to give less allowance)
• For larger amounts, HMRC may ask for direct payment
• You cannot be charged interest on PAYE underpayments that are HMRC’s error (within limits)

Related Guides

• Income Tax Guide
• Self Assessment Guide
• Marriage Allowance Guide