Why your tax code changed

This site is an independent UK tax-code explainer. We outline the mechanics of the system, but we do not hold your personal data.

Who changes it, and under which rule

When your tax code changes unexpectedly, it is because HMRC has received new information. The system automatically recalculates your instruction based on a specific set of statutory factors. HMRC must determine the code under reg 13, and they do not alter it arbitrarily.

The factors are listed in reg 14(1). HMRC must have regard to: reliefs the employee is entitled to; other PAYE income; tax overpaid in earlier years not yet repaid; tax remaining unpaid from earlier years; over-repaid tax; other non-PAYE income unless the employee objects; and “such other adjustments as may be necessary”. If any of these elements change on your file, the code updates to reflect your new reality.

What moves a code, roughly in order of how often it is the answer

A benefit in kind starting or stopping

A benefit in kind starting or stopping is a common trigger. If you receive a company medical plan, this benefit consumes a portion of your tax-free allowance. HMRC reduces your free pay to account for the tax due on the benefit. On the payslip, the allowance portion of the code will drop. If the benefit stops, the allowance rises again.

A second job, or drawing a pension

Taking on an additional job or drawing a pension changes the code. You possess a single personal allowance. If it is already allocated to your main employment, an additional income source will receive a code that grants no further free pay. This might be a basic rate code, which reg 7(3)(a) creates, or a nil tax code under reg 33(1). On the nil tax code, the employer “must not deduct or repay any tax”. Note that a nil code is NOT the same as a zero per cent rate; it simply means no deduction is applied via that specific payroll.

A pensioner drawing a second pension is in exactly this position, and the rule does not change for being a pension rather than a job: one Personal Allowance, already allocated somewhere, and a code on the other source that grants no more of it. Every code page on this site is worded “this job or pension” for that reason.

Tax owed from an earlier year

Tax owed from an earlier year being collected through the code is another common factor. HMRC will reduce your allowance to collect the historical debt gradually over the year. Your free pay drops to reflect this collection mechanism, allowing the debt to be settled without a separate payment.

Moving across a UK border

A change of main home across a UK border changes the prefix. If you relocate to a different tax jurisdiction within the UK, HMRC updates the code to include an S or C prefix, applying the respective regional rules for Scotland or Wales.

An allowance starting or ending

Allowances starting or ending will alter the code. If you successfully claim for work expenses or marriage allowance, your free pay increases, and the letters on your code may change to reflect the new relief.

When the change reaches your payslip

When HMRC processes the new data, they generate a completely fresh reg 14(1) instruction and issue the amended code to your employer. Reg 20(3) then settles when you actually see it: on making any subsequent relevant payment, the employer must deduct or repay tax by reference to the amended code. So the change lands on the next payment after the amended code is issued — and a payment already made is not reworked by it, because the regulation reaches the payments that come after.

Source: Reg 20(3): “On making any subsequent relevant payment to the employee, the employer must deduct or repay tax by reference to the amended code.” Reg 20(2) treats a code contained in a document HMRC sends to the employer as received by them “for the purposes of these Regulations”.

How your employer gets hold of the code is not one fixed channel, and it is worth saying so because the difference is often blamed for a delay. HMRC alerts the employer, and the new code can be read in PAYE Online under “tax code notices”, in the PAYE Desktop Viewer application, or in payroll software — HMRC’s own wording is “if it has this feature”. The notice itself is the form usually called a P6. What HMRC asks of the employer is timing, not tooling: update the payroll record “as soon as possible, and before you next pay your employee”. And a code that arrives too late to use in the tax year should, on HMRC’s instruction, be used in the new one instead.

Source: HMRC / GOV.UK, “Understanding your employees’ tax codes: Changes during the tax year” (administrative publication, not statute): “Access the new tax code in PAYE Online (in ‘tax code notices’), the PAYE Desktop Viewer application, or in your payroll software (if it has this feature). … As soon as possible, and before you next pay your employee, update their payroll record with their new tax code. … A tax code notice is sometimes called a P6 form. … If you receive an employee’s new tax code too late to use in the tax year, you should use it in the new tax year.”

What each change looks like on the payslip varies widely. The letters might change, prefixes might appear, or the numerical portion might rise or fall. The tax deducted on the payslip will adjust automatically on the next payday based on the new instruction.

It is important to remember that the payroll software operates automatically based on these instructions. The software does not know your personal circumstances; it only knows the data HMRC has transmitted. If the data transmitted is outdated, the deduction will be mathematically accurate according to the instruction, but factually wrong for your life. This is why maintaining accurate records with the authorities is the only way to ensure the system operates as intended.

Because the recalculation relies entirely on the reg 14(1) factors held on your file, the only place a person’s actual code can be viewed is HMRC’s authenticated Personal Tax Account. This site cannot know it, and does not try to. You must check your account directly.

“PAYE code” and “tax code” are the same instruction

A payslip, a coding notice or a payroll system may call this a PAYE code rather than a tax code. They are two names for one thing: the instruction HMRC determines under reg 13 and sends to your employer, which reg 14(1) tells HMRC how to work out. Nothing on this site distinguishes them, and a change to your PAYE code is the change this page describes.

If the new code is one you do not recognise, the code pages decode it individually — the two a change most often lands on are BR, the basic rate code, and K, the one that adds notional income instead of giving an allowance. If you think the new code does not fit your circumstances at all, if your tax code is wrong sets out the route to changing it.

Common questions