Getting a letter or notification that your tax code has changed can be unsettling — especially if your take-home pay suddenly drops. Here’s why HMRC changes tax codes and what you can do about it.
What is a Tax Code?
Your tax code tells your employer how much tax-free income you’re entitled to. The standard code for 2026/27 is 1257L, meaning you get £12,570 tax-free before paying any income tax.
The breakdown:
- Numbers = your tax-free allowance (multiply by 10)
- Letters = your situation and which rates apply
| Code | Meaning |
|---|---|
| 1257L | Standard Personal Allowance (£12,570) |
| 1257M | You receive Marriage Allowance transfer |
| 1257N | You transferred Marriage Allowance to partner |
| BR | All income taxed at 20%, no allowance |
| D0 | All income taxed at 40%, no allowance |
| D1 | All income taxed at 45%, no allowance |
| K | Deductions exceed allowance — added to taxable income |
| 0T | No allowance applied |
| S1257L | Scottish rates with standard allowance |
| C1257L | Welsh rates with standard allowance |
12 Common Reasons Your Tax Code Changed
1. You Started a New Job
Starting a new job after being unemployed or changing employers triggers a new tax code. If your new employer doesn’t have your P45, they may use an emergency code (often 1257L M1/W1 or BR) until HMRC confirms your details.
What to do: Give your P45 to your new employer. If you don’t have one, complete a Starter Checklist so they apply the correct code.
2. You Work Multiple Jobs
With two or more jobs, HMRC splits your Personal Allowance or allocates it entirely to one job. Your second job typically uses code BR (20% on all earnings) or D0 (40% on all).
What to do: Check HMRC has allocated your allowance optimally. You can request your full £12,570 goes to your higher-paying job.
3. You Received a Pay Rise
A significant pay rise — especially one pushing you above £100,000 — can change your code. Above £100,000, your Personal Allowance reduces by £1 for every £2 earned, until it disappears at £125,140.
What to do: If you’ve crossed £100,000, expect a reduced allowance. The code will show a lower number than 1257.
4. You Have a Company Car or Taxable Benefits
Company cars, private medical insurance, and other benefits in kind (BiK) are taxed through your code. HMRC estimates their value and reduces your tax-free allowance to collect the tax due.
Example: A company car with £4,000 annual BiK value reduces your allowance by £4,000. Your code changes from 1257L to 857L.
What to do: Check the P11D value matches your actual benefits. Report any changes (like returning a car) to avoid overpaying.
5. You Underpaid Tax Last Year
If HMRC calculates you didn’t pay enough tax in a previous year, they’ll recover it by reducing your code. This spreads repayment over the year without sending a bill.
Example: You underpaid £400. HMRC reduces your allowance by £2,000 (£400 ÷ 20% basic rate), changing 1257L to 1057L.
What to do: Check why you underpaid. Common reasons include wrong code, untaxed income, or state pension. You can request to pay the bill directly instead of through your code.
6. You Overpaid Tax Last Year
Conversely, if HMRC owes you money, they might increase your code temporarily rather than sending a refund cheque.
What to do: You can request a direct refund instead by contacting HMRC.
7. You Claimed Marriage Allowance
If your spouse earns under £12,570 and you earn under £50,270, they can transfer £1,260 of their allowance to you. Your code changes to include an M suffix (e.g., 1383M).
What to do: Ensure both partners’ allowances are correctly adjusted. The lower earner’s code should end in N.
8. You’re Receiving the State Pension
Once you receive State Pension, HMRC adjusts your code to collect tax if your total income (pension + employment) exceeds £12,570. The State Pension itself has no tax deducted at source.
What to do: Check your code accounts for your expected State Pension amount. If your State Pension amount changed, your code should update.
9. You Have Interest or Dividend Income
Savings interest above your Personal Savings Allowance or dividends above the £500 Dividend Allowance are taxed. If you received significant amounts last year, HMRC may adjust your code.
What to do: Review the adjustments match your actual investment income. Report accurate estimates for the current year.
10. You’re Paying High Income Child Benefit Charge
If you or your partner earn over £60,000 and claim Child Benefit, HICBC applies. Some people have this collected through their tax code rather than filing Self Assessment.
What to do: Check the estimated Child Benefit recovery matches your actual claim. Contact HMRC if it’s wrong.
11. You Have Rental or Self-Employment Income
If HMRC knows you have untaxed income (reported on a tax return or detected through other sources), they may adjust your PAYE code to pre-collect some tax.
What to do: Verify the estimated income is accurate. If it’s significantly wrong, contact HMRC to update.
12. It’s the Start of a New Tax Year
Every April, HMRC issues new tax codes reflecting any budget changes or adjustments. The code number might stay the same if your allowance hasn’t changed, but HMRC still reconfirms it.
What to do: Review your April coding notice to catch any errors before they affect your pay.
How to Check if Your Tax Code is Correct
Step 1: Sign into Your Personal Tax Account
Go to gov.uk/personal-tax-account and sign in with Government Gateway. You’ll see:
- Your current tax code(s)
- Your estimated income for the year
- Any adjustments applied
Step 2: Check the Components
Review what HMRC thinks applies to you:
| Component | Check |
|---|---|
| Personal Allowance | Should be £12,570 unless income >£100k |
| Benefits in kind | Match your actual company benefits |
| Taxable state benefits | Correct amount for State Pension/ESA/JSA |
| Underpaid tax | Check you actually owe this from last year |
| Other income | Verify any savings/dividend/rental income estimates |
Step 3: Compare to Your Payslip
Your payslip shows tax code and calculations. If the code matches what HMRC shows but your take-home seems wrong, the issue may be with your employer’s calculations.
What to Do If Your Tax Code is Wrong
Option 1: Update Online
In your Personal Tax Account, you can:
- Report incorrect estimated income
- Tell HMRC about benefits changes
- Update employment details
- Request a tax code review
Changes typically process within a few days.
Option 2: Phone HMRC
Call 0300 200 3300 (Income Tax helpline). Lines are open Monday–Friday 8am–8pm, Saturday 8am–4pm.
Have ready:
- Your National Insurance number
- Recent payslip
- Your coding notice (P2)
- Details of what’s wrong
Option 3: Use the HMRC App
The official HMRC app lets you check your code and request updates. Download from app stores.
Option 4: Write to HMRC
For complex issues, write to:
Pay As You Earn and Self Assessment
HM Revenue and Customs
BX9 1AS
Include your National Insurance number and full details of the problem.
What Happens After You Report a Problem
- HMRC reviews your case — usually within 2-4 weeks
- They issue a new tax code — mailed to you as a P2 notice
- They notify your employer — electronically
- Your employer applies the new code — typically next payslip
- If you overpaid tax — you may receive a refund or increased allowance
- If you underpaid — you may owe tax, collected via code adjustment or bill
Emergency Tax Codes
If you’ve started a new job without a P45 or HMRC hasn’t confirmed your code, you may be on an emergency code:
| Code | Meaning |
|---|---|
| 1257L M1 | Month 1 emergency — tax calculated on this month only |
| 1257L W1 | Week 1 emergency — tax calculated weekly basis |
| BR | All income taxed at 20%, no allowance |
| 0T | No allowance, but split rate taxation |
Emergency codes often lead to overpaying tax initially. Once HMRC confirms your correct code, you’ll receive a refund through adjusted future payslips.
K Codes: When Your Tax Code is Negative
A K code means your deductions exceed your Personal Allowance. Instead of getting a tax-free amount, this value is added to your taxable income.
Example: Code K475 means HMRC adds £4,750 to your taxable income.
Common causes:
- Large company car benefit
- Underpaid tax being recovered
- State Pension plus employment income
K codes can feel aggressive, but there’s a cap: you can’t pay more than 50% of your earnings in tax through PAYE.
Tips for Managing Tax Code Changes
Keep Records
Store:
- P45 from previous employment
- P60 annual summaries
- P11D benefit statements
- Coding notices (P2)
- Payslips
These help verify HMRC’s calculations and support any disputes.
Act Quickly on Errors
Wrong codes compound over time. Catching an error in Month 1 is easier to fix than discovering it in Month 12 when you’ve over/underpaid substantially.
Review Annually in April
When the new tax year starts, check your coding notice matches your expectations. Report any discrepancies before they affect your pay.
Be Proactive About Life Changes
Inform HMRC when you:
- Return a company car
- Stop receiving benefits
- Change your work pattern
- Move to Scotland/Wales
Don’t wait for HMRC to catch up — they may use outdated data.
Key Takeaways
- Your tax code determines how much tax-free income you receive before paying tax
- HMRC changes codes automatically based on employer reports, benefits, and previous tax years
- The standard code for 2026/27 is 1257L (£12,570 tax-free)
- Check your Personal Tax Account to see exactly why your code changed
- If it’s wrong, contact HMRC immediately to avoid over/underpaying
- Emergency codes are common when starting jobs — they sort themselves out within a few months
- K codes mean your deductions exceed your allowance — tax is calculated differently
This guide reflects 2026/27 tax rules. Contact HMRC directly for issues with your specific tax code. This is not tax advice.