A 23T tax code means HMRC is giving you only £230 of tax-free income for the year, and the T signals that your personal allowance is being calculated in a way HMRC wants to control directly rather than adjust automatically. Because the standard personal allowance is £12,570, something substantial has cut into it: usually high-income tapering, workplace benefits, an underpayment being clawed back from a previous year, or a mix of these. A code this low can be legitimate, but it can also reflect out-of-date information sitting on your record, so it’s worth checking before you accept it.
What the Number and the Letter Are Telling You
Under PAYE, HMRC tells your employer how much tax-free income to apply to your wages through a tax code.1GOV.UK. PAYE and Payroll for Employers The number is your annual tax-free allowance with the last digit dropped, so you multiply by 10 to get the real figure. A 1257 code means £12,570 tax-free. A 23 code means £230.2GOV.UK. Understanding Your Employees Tax Codes – What the Numbers Mean
The letter tells the employer how to treat that allowance. The most common letter is L, which means the code carries the standard personal allowance and will be uplifted automatically whenever HMRC raises the national threshold. T works differently. It tells the employer not to apply any automatic adjustment; HMRC has to review the code and issue a new instruction before it changes.3HM Revenue & Customs. Coding – Codes – How They Are Used and Calculated – Suffix Codes – The Suffix HMRC’s own wording is that a T code “includes other calculations to work out your Personal Allowance.”4GOV.UK. Tax Codes – What Your Tax Code Means In plain terms, your situation doesn’t fit the standard boxes, so HMRC keeps it under manual control.
Why Your Allowance Has Dropped to £230
A £230 allowance doesn’t happen by accident. A few situations can cut it this far, and more than one can apply at the same time.
Your Income Is Near the £125,140 Cliff
Once your adjusted net income passes £100,000, your personal allowance is reduced by £1 for every £2 above that threshold, disappearing entirely at £125,140.5GOV.UK. Income Tax Rates and Personal Allowances A £230 allowance corresponds to an income of about £124,680: the £24,680 above £100,000, halved, gives £12,340 of tapering, leaving £230 of the £12,570 allowance intact. HMRC typically uses the T suffix wherever tapering applies, which is why high earners near the cliff often see a 23T rather than an L.
You Get Substantial Benefits in Kind
Company cars, private medical insurance, and interest-free loans are taxable, and HMRC collects the tax by shrinking your code rather than billing you separately.6GOV.UK. Expenses and Benefits for Employers Benefits worth £12,340 will cut a £12,570 allowance down to £230. If you earn well under £100,000, heavy benefits are the likeliest explanation. There’s a floor to this process: once deductions exceed the full allowance, HMRC switches to a K code, which adds notional income instead of reducing allowance. A 23T sits just above that switch.
You Underpaid Tax in a Previous Year
If you didn’t pay enough tax last year, HMRC will usually collect the shortfall by reducing this year’s code, spreading the amount across twelve months of pay.7GOV.UK. Tax Overpayments and Underpayments – If Your Tax Calculation Letter (P800) Says You Owe Tax A sizeable underpayment combined with modest benefits can produce a 23T for the recovery period, with the code returning to a higher number the following tax year once the debt clears.
You Have More Than One Job
When you hold multiple jobs, HMRC normally assigns the full personal allowance to your highest-paying one and gives the others a much lower code so tax isn’t under-collected across your total earnings.8HM Revenue & Customs. How Tax Works If You Have More Than One Job A secondary job carrying a small share of allowance, plus a benefit or a bit of underpayment, can land at 23T. This is also where errors creep in: if HMRC still thinks you hold a job you’ve left, your allowance may be split when it should sit entirely with your current employer.
If You Live in Scotland or Wales
Scottish residents see an S prefix (S23T) and Welsh residents a C prefix (C23T).9HM Revenue & Customs. Coding – General Principles – Scottish Income Tax / Welsh Income Tax The prefix doesn’t change your personal allowance or the £100,000 tapering threshold; both apply UK-wide. What changes is the rate table your employer uses when taxing your income above the allowance.
When a 23T Code Is Likely Wrong
A 23T code that reflects your real position is unusual but legitimate. A 23T built on stale data is a different problem, and it costs you money on every payslip until it’s fixed. Common causes of an incorrect code include:
- HMRC is still deducting for a company car you returned or medical cover that ended, because your employer’s annual P11D hasn’t caught up with the change.
- HMRC’s estimate of your income is too high, so the tapering calculation is eating more allowance than it should.
- HMRC still has you on the books at a former employer and has split your allowance across jobs you no longer hold.
- A prior-year underpayment has already been collected but wasn’t removed from this year’s coding calculation.
What to Check Before You Contact HMRC
Pull the paperwork before you make the call, because without figures the conversation goes nowhere.
Start with your P60, which shows your total pay and tax for the year ending 5 April.10GOV.UK. Your P45, P60 and P11D Form – P60 If you changed jobs, find your P45 from the previous employer.11GOV.UK. Your P45, P60 and P11D Form If you get workplace benefits, your P11D lists their value; compare that list against what you actually received during the year.12GOV.UK. Your P45, P60 and P11D Form – P11D A car shown on the P11D that you handed back in September is a strong candidate for the source of the problem. Also check the tax code, tax to date, and gross-pay-for-tax fields on your most recent payslip so you know exactly what your employer has been told to apply.
How to Get the Code Changed
The quickest route is HMRC’s online Check your Income Tax service, where you can see your current coding notice, update income estimates, and remove benefits you no longer receive.13GOV.UK. Tax Codes – If You Think Your Tax Code Is Wrong Changes made online feed directly into HMRC’s systems and tend to be processed faster.
If you’d rather call, the Income Tax helpline is 0300 200 3300. Have your National Insurance number to hand.14GOV.UK. Income Tax – Enquiries An advisor can update your record during the call using the figures from your P60 or P11D.
Once HMRC agrees to change the code, they send your employer a notification (a P6) with the new figure.15GOV.UK. Understanding Your Employees Tax Codes HMRC aims to notify you and your employer within 15 working days, so if you’re paid monthly, expect the new code on your next payslip or the one after; if you’re paid weekly, by around the third payslip after the change.13GOV.UK. Tax Codes – If You Think Your Tax Code Is Wrong
What Happens If You Leave It
If the code is wrong and you do nothing, you’ll overpay tax all year. HMRC does run an end-of-year reconciliation and will send a P800 or Simple Assessment letter if their records show you’ve overpaid, and you can then claim the refund online or wait for a cheque.16GOV.UK. Tax Overpayments and Underpayments Relying on that is a poor plan for two reasons. The P800 doesn’t arrive until after 5 April, so you’d spend a full year with reduced take-home pay. And the reconciliation only catches what’s visible in employer-submitted data; if the underlying issue is a wrong benefit valuation your employer also reported wrong, the P800 won’t flag it.
If the wrong code has caused an underpayment rather than an overpayment, HMRC charges interest on the shortfall at 7.75% as of January 2026, which is the Bank of England base rate plus 4%.17GOV.UK. HMRC Interest Rates for Late and Early Payments
If HMRC Insists the Code Is Right
If HMRC won’t change the code and you still disagree, you can appeal in writing within 30 days of the decision letter.18GOV.UK. Disagree With a Tax Decision or Penalty HMRC will either amend their decision or confirm it. If they confirm, you can ask for an internal review by a different officer, and if that doesn’t resolve it, you can appeal to the First-tier Tax Tribunal. Missing the 30-day window doesn’t permanently close the door, but you’ll need a reasonable excuse or the tribunal’s permission to file late.