Your tax code will change automatically in most situations. HMRC receives pay and pension data continuously from employers and from the Department for Work and Pensions, and it adjusts your code whenever that data shows a change is needed. The standard code for the 2026/27 tax year is 1257L, giving you £12,570 of tax-free income before deductions start. The catch is that HMRC can only act on what it is told, so a handful of changes will sit unnoticed until you report them yourself.
What HMRC Updates Without You Asking
Employers report your pay and deductions every payday through a system called Real Time Information.1HM Revenue & Customs. PAYE Manual – Background: Real Time Information (RTI): Submission Types That constant feed is what lets HMRC spot when your code needs to move. The routine triggers are:
- Starting a new job. Once your new employer submits your P45 details or a Starter Checklist, HMRC recalculates your code against your current income.2GOV.UK. Starter Checklist if You’re Starting a New Job
- Reaching State Pension. Because the State Pension is paid without tax deducted, HMRC reduces the tax-free allowance in your other income sources to collect the tax owed on it. The State Pension figure is pulled straight from DWP records during the annual coding review.3HM Revenue & Customs. PAYE Manual – Coding: General Principles: Annual Coding
- Changes to taxable state benefits. Employment and Support Allowance, Jobseeker’s Allowance, and similar payments are reported to HMRC by DWP, and your code follows.
- State Pension uprating each April. HMRC adjusts the deduction in your code to match the new weekly amount.4GOV.UK. Tax Codes: Why Your Tax Code Might Change
- Marriage Allowance. Once claimed, the transfer of £1,260 of Personal Allowance renews every year until you cancel it.5GOV.UK. Marriage Allowance
Every year before 6 April, HMRC reviews almost every active tax record to set the code for the coming year, looking at all income sources, allowances, deductions, and any underpaid tax carried forward. If your code changes, HMRC sends you a P2 coding notice and notifies your employer separately. Where DWP data is not yet available for someone on the State Pension or benefits, HMRC uprates the figure by an agreed percentage so the code stays roughly accurate.3HM Revenue & Customs. PAYE Manual – Coding: General Principles: Annual Coding
What HMRC Will Not Catch On Its Own
The automatic system relies on the information HMRC has. If something changes and nobody reports it, your code will not move. You need to tell HMRC yourself when:
- You start earning untaxed income, such as rent from a lodger or savings interest above your allowance.
- You lose eligibility for a benefit or allowance built into your current code.
- Your total income from all sources changes in a way HMRC would not see from payroll alone.
Employment benefits like a company car or private medical insurance are a slower case. Your employer reports them on a P11D filed after the tax year ends, so there can be a long lag before your code reflects the current position.6GOV.UK. Expenses and Benefits for Employers: Reporting and Paying If your benefits package has just changed, do not assume your code has caught up.
How to Tell if Your Code Is Wrong
A quick look at your payslip usually reveals the problem. Two common signs:
The suffix W1, M1, X, or the word NONCUM means you are on an emergency code.7GOV.UK. Tax Codes: Emergency Tax Codes Each pay period is being taxed in isolation, ignoring what you have earned and paid so far this year, which often means you are paying more than you should. Emergency codes usually kick in when a new employer does not yet have your previous income details, typically because no P45 was provided. HMRC normally resolves it within 35 days of your job start date once the necessary information has come through. If the emergency code was triggered by starting to receive company benefits or the State Pension, you may stay on it until the end of the tax year and only move to a normal code the following April.
The number in your code is the second thing to check. Multiplied by 10, it should equal your tax-free allowance for the year, so 1257L gives you £12,570.8GOV.UK. Understanding Your Employees’ Tax Codes: What the Numbers Mean If that number is well below the standard allowance and you cannot see why, or the letter does not match your situation (BR and D0 tax all income from that job at basic or higher rate with no allowance; K means untaxed income exceeds your allowance and extra tax is being collected through your wages), something is off.9GOV.UK. Tax Codes: What Your Tax Code Means
How to Check or Correct Your Code
The quickest route is the HMRC online service on GOV.UK. Sign in with your Government Gateway or GOV.UK One Login credentials (photo ID may be needed to verify your identity the first time), and you can check your current code, see estimated income from all jobs and pensions, and tell HMRC about changes that affect your code.10GOV.UK. Check Your Income Tax for the Current Year One limitation: if Self Assessment is the only way you pay income tax, you cannot use the online service to check your current-year code. You can also call the income tax helpline, which can look into your code and make adjustments over the phone.11GOV.UK. Income Tax: Enquiries
Have these to hand before you start:
- Your National Insurance number, which HMRC uses to find your record.12GOV.UK. Your National Insurance Number
- Recent payslips, showing your current code, taxable pay to date, and tax deducted this year.
- Your P60 from the last tax year, which confirms total earnings and tax paid and helps you check whether under- or overpaid tax has been carried forward.13GOV.UK. Your P45, P60 and P11D Form: P60
- Your P11D if you have one, showing benefits in kind such as a company car or medical insurance that reduce your allowance.14GOV.UK. Your P45, P60 and P11D Form: P11D
- An estimate of your total income from all jobs, pensions, and other sources.
How Quickly the Change Reaches Your Payslip
HMRC aims to notify you and your employer within 15 working days of processing a code change. Employers receive the new code as a paper P9(T) or a digital notification through PAYE Online,15GOV.UK. P9X – Tax Codes to Use From 6 April 2026 and you get a P2 notice of coding showing how your code was worked out.16HM Revenue & Customs. PAYE Manual – Coding: Codes: How They Are Used and Calculated: P2 Notice of Coding
Payroll then needs to pick it up. If you are paid monthly, expect the change on your next or the following month’s payslip. If you are paid weekly, it should appear by your third payslip after the change.17GOV.UK. Tax Codes: How to Update Your Tax Code When the new code is cumulative, payroll recalculates your year-to-date tax and adjusts that pay period’s deduction up or down to correct the running total, which is how automatic refunds of overpaid tax often reach you.
What Happens at Year-End if the Code Was Wrong
After the tax year ends on 5 April, HMRC checks whether you paid the right amount. If not, it sends either a P800 tax calculation letter or a Simple Assessment letter, going out between June and March of the following year.18GOV.UK. Tax Overpayments and Underpayments
If you have overpaid, a P800 will say whether you can claim the refund online using the reference from the letter and your National Insurance number. A bank transfer arrives within five working days; a cheque takes around six weeks. In some cases HMRC will post a cheque automatically within 14 days of the letter date.19GOV.UK. If Your Tax Calculation Letter (P800) Says You’re Due a Refund
If you have underpaid less than £3,000, HMRC normally collects it by adjusting your code for the following year, shrinking your tax-free allowance so the shortfall is spread across your future pay.20GOV.UK. Pay Your Self Assessment Tax Bill: Through Your Tax Code £3,000 or more cannot be collected this way. You will get a Simple Assessment letter with a deadline, typically 31 January following the tax year, or three months from the letter date if it arrives after 31 October.21GOV.UK. Pay Your Simple Assessment Tax Bill
Unpaid tax accrues interest. As of January 2026, HMRC charges 7.75% on late payments (the Bank of England base rate plus 4%), while overpayments HMRC owes you earn 2.75%.22GOV.UK. HMRC Interest Rates for Late and Early Payments That gap is a reason to check your code once a year rather than wait for a letter to arrive.