A wrong tax code can silently drain hundreds from your pay each month. Here’s how to spot it, fix it, and get your money back.
What Is a Tax Code?
Your tax code tells your employer how much tax to deduct from your pay. It’s based on:
- Your personal allowance (£12,570 for most people in 2026/27)
- Adjustments for benefits, company cars, or unpaid tax from previous years
- Multiple employment income splits
The standard code 1257L means you get the full £12,570 allowance before tax applies.
Common Wrong Tax Codes
| Code | What It Means | Why It Might Be Wrong |
|---|---|---|
| BR | All income taxed at 20% | Applied to second jobs — wrong if it’s your only or main job |
| 0T | No personal allowance | Emergency code — usually temporary |
| D0 | All income taxed at 40% | For second income — check it’s not on your main job |
| W1 or M1 (suffix) | Week 1/Month 1 basis | Emergency — tax not calculated cumulatively |
| K (prefix) | Negative allowance — extra tax deducted | May include benefits in kind or tax owed from previous years |
| S or C (prefix) | Scottish or Welsh rates | Wrong if you live in England/NI, or vice versa |
How Much a Wrong Code Costs
| Wrong Code | Impact Per Month (approx.) | Annual Cost |
|---|---|---|
| BR instead of 1257L | ~£209 overpaid | ~£2,514 |
| 0T instead of 1257L | ~£209 overpaid | ~£2,514 |
| D0 instead of 1257L | ~£418 overpaid | ~£5,028 |
| Wrong Scottish rate (S) | Variable | Up to several hundred |
These figures assume you should have the standard personal allowance. Actual amounts depend on your salary.
Why Tax Codes Go Wrong
Starting a New Job
The most common cause. If you don’t give your new employer a P45 from your previous job, they must use an emergency code until HMRC issues the correct one.
Multiple Jobs
Each employer receives a separate tax code. HMRC should allocate your personal allowance to your main job and tax extra jobs at the basic rate. But sometimes they get the split wrong, or apply the allowance to the wrong employer.
Benefits in Kind
Company cars, private medical insurance, and other benefits reduce your personal allowance through your tax code. If you no longer receive a benefit but HMRC hasn’t been told, your code will be wrong.
Previous Year’s Underpayment
If you underpaid tax last year, HMRC collects the shortfall by adjusting your current tax code — reducing your allowance. This is correct but can be confusing if you weren’t expecting it.
Marriage Allowance Issues
Transferring the Marriage Allowance changes both partners’ tax codes. Problems arise when couples separate but don’t cancel the transfer, or when it’s applied to the wrong tax year.
How to Check Your Tax Code
On Your Payslip
Your tax code appears on every payslip. Compare it to what you expect based on your circumstances.
Through Your Personal Tax Account
- Go to gov.uk/personal-tax-account
- Sign in with Government Gateway
- Select “Check your Income Tax”
- See your current tax code and what it’s based on
- Check each item — personal allowance, deductions, income estimates
On Your P2 Notice of Coding
HMRC sends this each year (or when your code changes) showing exactly how your code was calculated. Check every line.
How to Fix a Wrong Tax Code
Step 1 — Identify What’s Wrong
Log into your Personal Tax Account and look at the breakdown. Common errors:
- Income estimate too high or too low
- Benefits you no longer receive still included
- Personal allowance not applied at all
- Previous year’s underpayment being collected that you’ve already paid
Step 2 — Update Your Details Online
You can change many things directly through your Personal Tax Account:
- Update your income estimates
- Remove benefits in kind you no longer receive
- Add or remove a second job
- Update your address
Step 3 — Contact HMRC
If you can’t fix it online:
- Phone: 0300 200 3300 (Monday to Friday, 8am to 6pm)
- Online chat: Available through the Personal Tax Account
- Post: HMRC, Pay As You Earn, PO Box 1970, Liverpool, L75 1WX
Have your National Insurance number and recent payslip ready.
Step 4 — Wait for the New Code
HMRC issues a new tax code to your employer, usually within 2-4 weeks. Your employer then adjusts your pay to correct the cumulative position — so if you’ve been overpaying, you should get a larger net payment.
Getting Overpaid Tax Back
If you’ve been on the wrong code:
| Scenario | How You Get Money Back |
|---|---|
| Code corrected mid-year | Employer adjusts on next payslip (cumulative correction) |
| Code corrected after tax year ends | HMRC sends P800 — claim online or wait for cheque |
| Previous tax years | Contact HMRC — refunds available for last 4 years |
Emergency Tax — What to Do
If you’ve just started a new job and are on emergency tax:
- Give your employer your P45 from your previous job
- If you don’t have a P45, complete the HMRC starter checklist your employer provides
- HMRC should issue a correct code within 2-6 weeks
- Any overpaid tax is refunded through your salary automatically
- If it’s not corrected within 2 months, contact HMRC directly