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The wrong tax code cost me £1,800 — here's how I found out
6 min read
A wave of letters from HMRC is landing on doormats across the UK right now — and many of them are wrong. The Telegraph, Yahoo Finance, and thousands of threads on Reddit's r/UKPersonalFinance all tell the same story: HMRC has been automatically adjusting PAYE tax codes to recoup what it believes is unpaid tax on savings interest, and the figures are frequently inaccurate.
According to tax accountants, over 6 million UK workers were on the wrong tax code in 2024/25, resulting in at least £1.4 billion in overpayments. With interest rates having been higher than usual since 2022, HMRC is now chasing savings interest — but its data is patchy, its estimates are often too high, and the consequence lands directly on your monthly payslip.
Why is HMRC changing tax codes for savings interest?
Your bank or building society automatically reports your savings interest to HMRC each year if the account is not an ISA. HMRC then estimates what you owe for the next year and adjusts your PAYE tax code to collect it in advance — month by month, silently, through your employer.
The problem: HMRC's estimate is based on your previous year's interest. If interest rates have fallen, if you've moved savings into an ISA, or if you've spent the money, the estimate is wrong. You pay too much, and HMRC makes you prove it after the fact.
How to spot it on your payslip
Look at the number in your tax code. For most people it should be 1257 (representing the £12,570 personal allowance). If HMRC has decided you owe savings tax, it reduces this number. A code of 857L, for example, means HMRC has deducted £4,000 from your allowance — collecting 20% of that (£800) through your PAYE deductions over the year.
Signs your code has been adjusted for savings:
- Your code number is lower than 1257 with no company car, benefit-in-kind, or known underpayment
- You received a "Notice of Coding" letter from HMRC — these are often easy to miss
- Your net pay dropped without a pay cut, and your tax line on the payslip increased
What to do if your code is wrong
Act quickly. The sooner you correct it, the sooner your payslip returns to normal.
- Log in to your HMRC Personal Tax Account (gov.uk/personal-tax-account). Under "Pay As You Earn," you will see your current tax code and the reason for any adjustments.
- Check the savings interest estimate. HMRC will show what figure it's used. If it's wrong — for example, if you've moved savings to an ISA this year — note the correct figure.
- Challenge the estimate online. In your Personal Tax Account, you can tell HMRC your expected savings interest for the current year. They will issue an updated code to your employer, usually within a few days.
- If you've already overpaid, any overcollection this tax year will be refunded automatically in your payslip once the code is corrected. For last year's overpayments, HMRC will issue a P800 refund notice — or you can claim online.
The Personal Savings Allowance: what HMRC often ignores
Basic rate taxpayers can earn £1,000 in savings interest per year completely tax-free (the Personal Savings Allowance). Higher rate taxpayers get £500. Additional rate taxpayers get nothing.
Many HMRC coding notices incorrectly include savings interest below these thresholds. If your total interest is under £1,000 and you're a basic rate taxpayer, you should owe zero tax on it — and your code should not be adjusted at all. Challenge any notice that appears to tax interest under this threshold.
Frequently asked questions
Can HMRC take back tax through my payslip without telling me?
Yes — they send a "Notice of Coding" letter, but many people never open it. The result is an unexplained drop in take-home pay with no other visible change. Check your post and your HMRC online account regularly.
How far back can HMRC claim tax?
HMRC can generally go back 4 years for tax owed due to PAYE errors. However, they cannot use your PAYE code to collect underpayments older than 2 years without your agreement.
What if I can't afford the deduction?
You can ask HMRC to spread the collection over a longer period. Call 0300 200 3300 and explain your situation. They can adjust the amount being collected through your code.
Will fixing my tax code affect my next payslip?
Yes — once HMRC updates your code, your employer applies it immediately. PAYE is cumulative, so any overpayment already collected this tax year will be refunded over the remaining months of the year through reduced deductions.
Is savings interest in an ISA affected?
No. ISA interest is completely tax-free and is never reported to HMRC. Putting savings into a Cash ISA fully removes those earnings from the savings interest equation.
Use our tax code checker to understand what your current code means, or our overpaying tax calculator to check whether your deductions look right.