What Is a Tax Code and What Happens When It's Wrong?

A four-digit number and a letter. That is a tax code. It arrives from HMRC, it sits in the payroll software, and it determines — every single month, for every employee it applies to — how much of their pay disappears to the government before they see it.

Most employees have the standard code. One code, applied the same way, nothing unusual. Those employees are not the problem.

The problem is everyone else. The new starter who joined last month without a P45. The employee with two jobs running at the same time. The director who had a company car added to their benefit package. The worker who underpaid tax two years ago and has been on a K code ever since without understanding why their take-home keeps dropping.

The standard tax code for 2026/27 is 1257L. The number reflects the personal allowance — £12,570 — divided by ten. The L means standard. Together they tell the payroll: deduct no tax on the first £12,570 of annual earnings, then 20% on the next band, then higher rates as earnings rise. Simple when it's 1257L. More complicated when it isn't.

How the Calculation Works

The personal allowance operates cumulatively through the tax year. Each month, the payroll calculates how much of the annual allowance has been used and applies the remainder against that month's earnings.

In April — the first month of the tax year — the employee has their full £12,570 personal allowance available. Divide by twelve: roughly £1,047 of tax-free pay in April. The same in May. The same in June. By the end of the year, the full allowance has been used.

This cumulative method means that if an employee was over-taxed in one month — perhaps because a payroll error applied too high a deduction — the correction happens automatically in the next month, when the cumulative calculation shows that too much has been collected year-to-date. The system self-corrects. Slowly, and not always visibly to the employee, but it does correct.

The emergency code — week 1 / month 1 — does not use this cumulative method. It applies only that period's slice of allowance, ignoring what has happened before. This is why emergency tax typically over-deducts: it treats every pay period as if it's the first of the year, instead of accumulating and adjusting.

The New Starter Problem

New starters are where tax code errors begin.

When an employee joins with a P45 from their previous employer, the P45 tells the payroll their tax code, their earnings to date in the tax year, and the tax paid. The payroll uses this information to set the cumulative position correctly. New employee, existing tax year, correct code, running total accurate from day one.

When an employee joins without a P45 — they lost it, they've been self-employed, they're joining the workforce for the first time — the employer uses a starter declaration form. The form asks three questions about the employee's employment situation, and the answers determine which starting code to apply:

Statement A — this is their only job and they've not been employed or received benefits since 6 April: code 1257L cumulative

Statement B — this is their only job but they've had another job or taxable benefits earlier in the year: code 1257L week 1 / month 1

Statement C — they have another job or receive a pension: code BR (basic rate, 20% flat, no personal allowance)

The employer relies on the employee completing the form honestly. When they don't — when they tick the wrong box, or when the employer doesn't issue the form at all and makes an assumption — the wrong code runs until HMRC issues a correct one, which can take weeks.

The Codes That Signal a Problem

Some tax codes are benign variations. Others signal that something in the employee's tax position requires attention.

BR on a primary job means the full personal allowance has been allocated elsewhere — perhaps the employee told HMRC their primary income is a pension, or the code was applied in error and hasn't been corrected. An employee on BR on their main employment is overpaying tax by the equivalent of their personal allowance.

0T means no personal allowance at all — either the full allowance has been used elsewhere, or HMRC has no information about the employee and has applied a holding code. It produces higher deductions than BR.

K codes mean the employee has deductions exceeding their personal allowance — typically from benefits in kind like a company car, or from underpaid tax in a prior year being collected through PAYE. The K prefix adds the excess back to taxable income. An employee on K500 has an additional £5,000 added to their taxable earnings on top of their gross pay.

NT — no tax — means nothing is deducted. This should be rare. If an employee is on NT code and it hasn't been authorised by HMRC, someone has made an error.

What the Employer's Responsibility Is

The employer's job is to apply the code HMRC has issued, update it when HMRC issues a new one, and flag to the employee when an unusual code appears so they can query it with HMRC if needed.

The employer does not determine the correct tax code. That is HMRC's function. But the employer carries the risk when the wrong code runs — if too little tax is deducted due to an employer error (applying the wrong code, failing to update a code), HMRC can pursue the employer for the underpaid amount.

When HMRC issues a new coding notice for an employee, it arrives through the payroll software. The employer applies it from the date specified — usually the start of the next pay period. Not applying an updated code, or applying it late, is a payroll error with potential HMRC consequences.

bookd. monitors tax code updates as part of the monthly payroll cycle — applying new codes when issued, flagging unusual codes to clients, and ensuring new starters have a starter declaration completed before the first pay run. A wrong code running for three months is three months of the wrong amount leaving someone's pay. It gets fixed. It also generates admin, queries, and occasionally a very unhappy employee. The prevention is simpler than the cure.