Find the cause fast — the P2 letter
HMRC issues a P2 "Notice of Coding" letter whenever your tax code changes mid-year. The letter explains: - The new code - The breakdown of why (allowance, deductions, additions) - The effective date
Check your post (and your spam/junk inbox if you receive HMRC by email). The reason is right there.
If you don't have the P2 letter, log into your HMRC Personal Tax Account at gov.uk/personal-tax-account — the current code's calculation is visible there.
If neither helps, work through the 12 common causes below.
The 12 common reasons
1. You started a new job
Most common cause. Your previous tax code might have been 1257L; in the new job, HMRC defaults to a non-cumulative emergency code (W1, M1, X) until your P45 is processed. Resolves within 1-3 pay periods.
Read about tax codes after starting a new job →
2. You started receiving a benefit in kind (BiK)
Common triggers: company car, private medical insurance, gym membership, beneficial loan. Your employer submits a P11D form to HMRC; HMRC reduces your personal allowance by the value of the benefit, dropping your code number (e.g. 1257L → 957L for a £3,000 BiK).
3. You stopped receiving a benefit in kind
Reverse of above. You returned the company car or cancelled the private medical. Your allowance restores, and your tax code number increases (e.g. 957L → 1257L back).
Sometimes there's a delay — HMRC's record updates after the next P11D submission, which can be months later.
4. You applied for Marriage Allowance (and received it)
Your spouse transferred 10% of their personal allowance to you. Code becomes 1383M (£12,570 + £1,260). Read about tax code M →
5. You transferred Marriage Allowance (giver)
You gave 10% of your allowance to your spouse. Code becomes 1131N (£12,570 − £1,260). Read about tax code N →
6. Marriage Allowance ended
If either partner's circumstances changed (income exceeded £50,270, divorce), Marriage Allowance ends. Both partners' codes revert. From 1383M / 1131N → both 1257L.
7. HMRC is collecting owed tax from a previous year
If a previous tax year showed you underpaid (via P800 reconciliation), HMRC may recover the shortfall through your current year's PAYE by reducing your code's allowance.
For owed tax of £3,000, your allowance reduces by £3,000 → code drops by 300 → 1257L becomes 957L (or appropriate equivalent).
Read about K codes → — in extreme cases the allowance goes negative and a K-prefix code applies.
8. You moved to or from Scotland
Scottish residents get an S-prefix (S1257L). Moving to Scotland triggers a code change to S1257L; moving from Scotland reverts to 1257L. HMRC catches this from your Personal Tax Account record updates or P85 / address changes.
9. You moved to or from Wales
Less common but parallel: C-prefix for Welsh residents (C1257L). Currently the same band structure as rUK but a separate code.
10. You started or stopped a second income source
A second job or pension. HMRC issues an additional code for the second source (typically BR, D0, or 0T) and may adjust the main code if allowances were being shared.
11. A P11D was submitted retrospectively
Your employer's annual P11D return lists all benefits in kind. HMRC processes these in summer, and codes may change in July-October to reflect benefits you had during the previous tax year.
12. Payroll or HMRC clerical error
Less common but real. Wrong code applied by payroll team, or HMRC's record contains incorrect information. Spot via P2 letter (the breakdown won't match your actual circumstances). Correctable via Personal Tax Account.
What to do for each scenario
| Cause | Action |
|---|---|
| New job | Wait 1-3 pay periods; provide P45 if not yet done |
| BiK started | Verify the BiK value in P2 matches your actual benefit |
| BiK ended | If P11D not yet processed, contact HMRC to expedite |
| Marriage Allowance gained | Confirm via Personal Tax Account |
| Marriage Allowance ended | Confirm both partners' codes reverted |
| Owed tax recovery | Verify amount owed; option to pay lump sum instead |
| Region change | Update HMRC address; confirm new S/C prefix correct |
| Second income | Verify second source is correctly classified |
| P11D processing | Check BiK values match P11D; query if wrong |
| Clerical error | Update via Personal Tax Account or call HMRC |
Lower numeric code = more tax
If your new code's number is lower than your old code's (e.g. 1257L → 957L), your tax-free allowance has reduced. You'll pay more tax going forward, and the cumulative calculation may produce a one-time catch-up deduction in the next pay run.
If your new code's number is higher (e.g. 957L → 1257L), allowance has increased. You'll pay less tax, and any over-deducted tax year-to-date is refunded via PAYE.
K-prefix codes add to your taxable income rather than subtracting an allowance — see tax code K →.
Mid-year vs year-start code changes
Two distinct types:
Year-start (April 6)
HMRC issues a new code at the start of each tax year. Most employees' codes don't change much from year to year — the number reflects the standard allowance (£12,570 frozen until 2031) plus any usual adjustments.
Mid-year
Triggered by an event (job change, BiK change, Marriage Allowance, owed tax recovery, region change). The P2 letter and your Personal Tax Account both show why.
If you spot a mid-year change with no obvious trigger, investigate immediately — clerical errors are most likely to slip through here.
Comparing the new code to expected
Pre-check what you think your code should be:
- Standard one-job, no BiK: 1257L
- Standard with private medical (£800/year): 1177L (£12,570 − £800 = £11,770)
- Standard with company car (£5,000 BiK): 757L
- Received Marriage Allowance: 1383M
- Gave Marriage Allowance: 1131N
- Second job (basic rate): BR (or 1257L if it's the main source)
If your actual code matches expected → fine. If not → investigate.
Verifying via Personal Tax Account
The cleanest verification:
- Log into gov.uk/personal-tax-account
- View "Current tax code"
- See the breakdown:
- Standard personal allowance: £12,570
- Less: any BiK values (each listed)
- Less: any tax owed
- Less: any other deductions
- = Final allowance
- Compare each line against your reality
If the breakdown shows BiK values that don't match what you actually receive (e.g. "company car £4,000" when you don't have one), the underlying data is wrong. Update HMRC.
Scotland and Wales
Code changes still occur the same way; just the prefix (S or C) is added. Moving cross-border triggers the prefix change. Note: NI is UK-wide so doesn't change with the code.
When to escalate
| Situation | Action |
|---|---|
| Code change with explanation that matches your circumstances | Accept |
| Code change with no P2 letter and you can't see the reason | Personal Tax Account |
| Code change implies a benefit you don't have | Update HMRC, request correction |
| Code change implies owed tax you don't recognise | Request HMRC explanation in writing |
| Recurring code changes within a few months | Call HMRC 0300 200 3300; may need formal review |
Practical checklist
- Find the P2 letter explaining the change
- Cross-check with Personal Tax Account — the breakdown should be visible
- Identify the cause from the 12 common reasons above
- Verify the values match reality (BiK amount, owed tax amount, etc.)
- If correct, no action — the code stands
- If wrong, update HMRC via Personal Tax Account or call 0300 200 3300
- Run the new code through the tax code checker → to see take-home impact
In short
Tax code changes are usually triggered by a documented cause: new job, BiK change, Marriage Allowance, owed tax recovery, region change. HMRC issues a P2 letter explaining each change. The Personal Tax Account shows the calculation behind any code. Verify against the 12 common reasons; if anything doesn't match, contact HMRC for correction. For broader context see tax codes hub → and how to correct a tax code →.