Your situation
Tax code changed: why, and what it does to your pay
A new code means HMRC thinks you should pay a different amount of tax. Find the trigger, then measure the change on your payslip.
1100L takes more over the year
£314.00
| 1257L | 1100L | |
|---|---|---|
| Tax-free amount | £12,570 | £11,000 |
| Tax, first month | £323.60 | £349.80 |
| Tax over the year | £3,884.00 | £4,198.00 |
| Take-home after tax | £28,116 | £27,802 |
Same pay every month, HMRC’s cumulative method, Income Tax only (no National Insurance).
HMRC changes a tax code when it believes you should be paying a different amount of tax, almost always because your income or circumstances have moved. GOV.UK lists the common triggers: a new job, a second job or pension, taxable state benefits, a change in your weekly State Pension, company benefits starting or stopping, savings interest above your Personal Savings Allowance, a Marriage Allowance claim, expenses with tax relief, the High Income Child Benefit Charge collected through pay, a Winter Fuel Payment or Pension Age Winter Heating Payment to repay, and tax owed after a wrong code. To see the reason for your own change, sign in to the Check your Income Tax online service. The effect is easy to measure: on £32,000 a year, moving from 1257L to 1100L, as when £1,570 of medical insurance is added, takes about £26.17 more each month, while receiving Marriage Allowance and moving to 1383M takes about £21.00 less.
What a new code number changes
Change in tax each month
+£26.17
| Allowance change | −£1,570 |
| Tax over the year with 1257L | £3,884.00 |
| Tax over the year with 1100L | £4,198.00 |
Rules read on HMRC’s texts by Radif Partners · Editorial policy · Method
The usual reasons for a new code
Your code tells payroll how much you can earn tax-free from one job or pension, so anything that changes that amount changes the code. HMRC's own list (GOV.UK) groups into three kinds of event:
- more income to tax: starting a new job, income from an additional job or pension, taxable state benefits, a rise in your weekly State Pension, savings interest above your Personal Savings Allowance;
- work benefits and reliefs: your employer telling HMRC that a benefit such as a car or medical insurance has started or stopped, or a claim for expenses that get tax relief;
- charges and corrections: Marriage Allowance claimed, the High Income Child Benefit Charge paid through your wages or pension, a Winter Fuel Payment or Pension Age Winter Heating Payment to pay back, or tax owed after you were on the wrong code.
A new job without your previous income details can also put you on an emergency code, which ends in W1, M1 or X. With Marriage Allowance, the letter changes as well as the number: M for the partner who receives the transfer, N for the one who gives it.
Finding the reason for your change
The Check your Income Tax service is where HMRC explains a change. Once signed in, it shows your code and Personal Allowance, the income it expects from each job and pension, the tax you can expect to pay this year, and whether the code has changed. HMRC usually writes to explain how it calculated a new code too; the P2 coding notice guide shows how to read that letter line by line.
If the reason is wrong, for example a benefit you never received or a job you left months ago, update the details in the same service. GOV.UK says a code that needs correcting is reissued to you and your employer within 15 working days. The wrong code guide covers the rest of that process.
What the new code does to your pay
The table shows the monthly effect of common changes on a £32,000 salary, using the 2026 to 2027 rates for England and Northern Ireland. The compare tool at the top runs any pair of codes on your own pay.
| Change | Typical cause | Tax each month |
|---|---|---|
| 1257L to 1100L | company benefit added | +£26.17 |
| 1257L to 1383M | Marriage Allowance received | −£21.00 |
| 1257L to 1131N | Marriage Allowance transferred | +£21.00 |
| 1257L to BR | source treated as a second job | +£209.67 |
| 1257L to K475 | deductions above the allowance | +£288.98 |
What a new code number changes
Change in tax each month
+£26.17
| Allowance change | −£1,570 |
| Tax over the year with 1257L | £3,884.00 |
| Tax over the year with 1100L | £4,198.00 |
The payslip where the change lands
Employers receive the new code from HMRC and should update the payroll record before your next pay (GOV.UK). Because most codes are cumulative, that first payslip does not just apply the new monthly rate: it recalculates the tax due on everything paid since 6 April and takes or returns the difference. A lower code in the middle of the year therefore costs more in its first month than in later ones. With the same £32,000 salary moving from 1257L to 1100L:
| New code first used in | Tax a month before | Tax that month | Tax a month after |
|---|---|---|---|
| July | £323.60 | £428.20 | £349.80 |
| October | £323.60 | £506.80 | £349.80 |
| January | £323.60 | £585.40 | £349.80 |
The later in the year the change arrives, the bigger that one-off catch-up. A rise in the code number works the other way, returning the excess in one go. Whatever the direction, payroll cannot take more than half of your pay before tax in a single period. If you receive the notice too late in the year for your employer to use it, the employer applies it in the new tax year instead.