The wrong tax code cost me £1,800 — here's how I found out

It started with a conversation at work. A colleague mentioned she'd received a £400 cheque from HMRC. She hadn't done anything special — just logged into her Personal Tax Account and found she'd been overpaying for two years.

I checked mine that evening. I'd been on a 943L code for 18 months instead of 1257L. The difference? £314 less in personal allowance every month — taxed at 20% when it shouldn't have been. Over 18 months: £1,131 overpaid. With interest: just over £1,200 back in my account six weeks later.

Why this happens so often

Tax codes go wrong for surprisingly mundane reasons. The most common:

• A benefit-in-kind (company car, private medical) was added to your code but then removed from your employment — and nobody told HMRC.
• You have outstanding tax from a previous year being collected through your code.
• HMRC made an error when updating your records after a job change.
• You started a new job without handing over your P45, triggering an emergency code.

The exact number to check

Look at your payslip right now. Find your tax code — it's usually in the top section. The number part (e.g. "1257" in 1257L) multiplied by 10 is your personal allowance for the year.

Most people should have 12,570 (code 1257L). If your number is lower with no obvious reason (like a company car benefit), you've likely got a wrong code.

What I did to get it fixed and claimed back

1. Called HMRC on 0300 200 3300 with my National Insurance number ready.
2. Explained the discrepancy. The agent confirmed the error — a medical benefit from a previous employer had never been removed from my code.
3. They issued an updated code to my current employer immediately.
4. For the current year's overpayment, my next payslip automatically adjusted (larger net pay as the cumulative calculation corrected).
5. For the previous year, I completed a simple online form and received a bank transfer within 5 weeks.

The whole process took about 35 minutes. The cheque — well, direct transfer — was genuinely life-changing for a relatively small amount of admin.

How to check yours right now

Use our tax code checker — it explains every UK tax code in plain English and tells you what your code means for your personal allowance. If something doesn't add up, it'll flag it.

And if you've potentially been on the wrong code for more than a year, consider using a tax refund service — they'll check up to 4 years for free and only charge if they find something.