WI158 is an internal HMRC reference that appears on a P2 Notice of Coding. It is not your tax code, and there is no publicly published HMRC definition of it individually: it works as an administrative marker inside the PAYE system, identifying which internal procedure generated the notice you received.1GOV.UK. PAYE Manual – PAYE11030 – P2 Notice of Coding The document itself is what matters. Your P2 shows the tax code HMRC has told your employer or pension provider to use, the allowances behind it, and every deduction that reduces your tax-free pay. If a notice carrying the WI158 reference has landed with you, the useful thing to do is check whether the numbers on it are right.
What the P2 Notice Is Telling You
The P2 is a personalised letter breaking down everything feeding into your tax code for the tax year, which runs 6 April to 5 April. It lists your employments and pensions, your National Insurance number, and an arithmetical calculation of the code.1GOV.UK. PAYE Manual – PAYE11030 – P2 Notice of Coding The calculation starts with your Personal Allowance entitlement and subtracts anything that shrinks your tax-free amount: company benefits, untaxed income, or previous underpayments being collected. The result is the tax code your employer uses to work out how much of your pay to shield from tax.
The notice also explains how much you can earn before tax applies and how much of your income falls into each band. If your code carries a K prefix, meaning deductions exceed allowances, the P2 sets out separately why. HMRC reissues a P2 whenever the code changes, not only at the start of the tax year.
Reading the Tax Code Itself
A tax code is a number followed by a letter. The number is your tax-free allowance with the last digit dropped: 1257L means £12,570 of tax-free income and the standard Personal Allowance. For 2026-27, the standard Personal Allowance stays at £12,570, and is scheduled to remain there until at least April 2031.2GOV.UK. Income Tax Rates and Personal Allowances
The letter tells HMRC and your employer which rules to apply.
- L — the standard Personal Allowance. The most common code.3GOV.UK. Tax Codes – What Your Tax Code Means
- M — you receive 10% of your partner’s Personal Allowance through Marriage Allowance.
- N — you have transferred 10% of your Personal Allowance to your partner.
- K — deductions and untaxed income exceed your allowances, so the code adds taxable pay instead of shielding it.
- S — Scottish taxpayer, taxed at Scottish rates.
- C — Welsh taxpayer.
- BR — all income from this source taxed at 20% with no allowances. Typical for a second job.
- D0 — all income from this source taxed at 40% with no allowances.
- NT — no tax deducted, used only in exceptional circumstances.
- T — HMRC needs to review the code annually rather than rolling it forward.
Scottish rates set by the Scottish Parliament differ from those in the rest of the UK, and Scottish taxpayers earning above £29,526 pay more income tax than someone on the same salary in England.4Scottish Government. Scottish Income Tax 2026 to 2027 Technical Factsheet If you have moved between Scotland and the rest of the UK, check that the S or L prefix on your code matches where you now live.
Emergency Codes
If your code ends in W1, M1, or X, it is an emergency code. Some payslips show this as “NONCUM.” Emergency codes mean HMRC does not have enough information to calculate a full-year code, so each pay period is taxed in isolation rather than cumulatively. This usually happens when you start a new job without a P45. The practical result is normally overpaying tax. Contact HMRC or update your details through your Personal Tax Account to move onto the correct cumulative code; any overpayment should be refunded through later payslips once the right code is in place.5GOV.UK. Emergency Tax Codes
What Is Reducing Your Tax-Free Amount
The deductions listed on your P2 are where mistakes tend to hide. Stale figures carried forward from a previous year, benefits you no longer receive, or income estimates based on outdated data can all quietly cost you hundreds of pounds across a full tax year.
Common items that reduce the allowance include company cars, fuel allowances, and private medical insurance paid by your employer.6GOV.UK. Expenses and Benefits for Employers HMRC also factors in untaxed savings interest above the Personal Savings Allowance (£1,000 for basic-rate taxpayers, £500 for higher-rate) and dividend income above the dividend allowance.7GOV.UK. Tax on Savings Interest – How Much Tax You Pay
If you earned too little tax in an earlier year, HMRC often collects the shortfall by shrinking your current code rather than asking for a lump sum. This route is only available if you owe less than £3,000, you already pay through PAYE, and you filed your return on time. The adjustment cannot push total deductions above 50% of your PAYE income or more than double your normal tax bill.8GOV.UK. Pay Your Self Assessment Tax Bill – Through Your Tax Code
The High Income Child Benefit Charge is another regular P2 line. If you or your partner claim Child Benefit and either has adjusted net income above £60,000, you repay 1% of the benefit for every £200 of income over that threshold, with the whole benefit clawed back at £80,000. The charge can be collected through your code, showing up as a deduction on the notice.9GOV.UK. High Income Child Benefit Charge
Earnings above £100,000 taper the Personal Allowance itself: you lose £1 of allowance for every £2 of income over the threshold, and the allowance is gone entirely at £125,140.2GOV.UK. Income Tax Rates and Personal Allowances That taper produces an effective 60% rate on income between those two figures, and catches many people after a pay rise or bonus.
Checking the P2 Against Your Records
Compare every line on the notice against documents you already have. Your P60, which your employer must give you by 31 May, summarises the last complete tax year’s pay and tax.10GOV.UK. P60 If you changed jobs, your P45 from the previous employer shows earnings and tax up to your leaving date.11GOV.UK. Your P45, P60 and P11D Form Your P11D lists the taxable value of each workplace benefit your employer reported, broken down by category — company cars, fuel, medical insurance, accommodation, loans and so on.12GOV.UK. P11D Set the P11D values next to the deductions on your P2 and look for figures that no longer match reality: a car you have handed back, cover you no longer receive, an estimate based on a benefit that has changed.
Correcting the Code
The fastest route is the “Check your Income Tax” service inside your Personal Tax Account on GOV.UK. You can see the breakdown of your current code, update job and pension income, and tell HMRC about anything that affects the code.13GOV.UK. Check Your Income Tax for the Current Year After you submit changes, HMRC issues a revised P2 to you and sends a coding notification to your employer, who applies the new code in the next payroll run.14GOV.UK. Understanding Your Employees Tax Codes – Changes
If you would rather speak to someone, the HMRC income tax helpline is 0300 200 3300, or +44 135 535 9022 from outside the UK, open Monday to Friday, 8am to 6pm.15GOV.UK. Income Tax Enquiries Have your National Insurance number and a recent payslip in front of you. The agent can adjust the code on the call and a revised P2 follows.
If the Code Turns Out to Be Wrong
Receiving an incorrect code is not itself a penalty offence: HMRC does not fine you for a coding error. If the wrong code leaves you underpaying, however, you owe the difference whoever caused the mistake, and HMRC charges late payment interest on unpaid income tax at 7.75% as of January 2026, running from the date the tax was originally due.16GOV.UK. HMRC Interest Rates for Late and Early Payments Separately, if you exceed your savings allowance and have not heard from HMRC by 31 March of the following tax year, you must contact them; not doing so can lead to a penalty.7GOV.UK. Tax on Savings Interest – How Much Tax You Pay
Going the other way, if the code has left you overpaying, you have four years from the end of the tax year in which the overpayment occurred to reclaim it. Miss the window and the money is gone. For 2025-26, the deadline is 5 April 2030.
Once HMRC issues a corrected code, allow one to three payroll cycles for it to appear on your payslip. Where your employer applies the new code on a cumulative basis, which is the default, any earlier overpayment or underpayment from the same tax year is corrected in your next pay run rather than needing a separate refund.