There are a few key reasons you might have overpaid tax.

First, HMRC may have made an error in your tax code. Even a small mistake can mean you’re taxed more than you should be.

Second, you might not have known about certain tax allowances you were (or at the time, were) eligible for. Some allowances have been phased out, but if you didn’t know to claim them when they applied, HMRC wouldn’t have included them in your tax code. As a result, you could have paid more tax than necessary without even realising it.

If you don’t know there’s an error or missing allowance, you can’t tell HMRC about it—and HMRC will simply confirm that your tax code is correct based on the information they have from you.

The good news? It’s not too late to claim a refund. HMRC allows you to go back up to four years to correct errors or claim any allowances you missed.

However, there’s a catch. Unless you know exactly what to look for, it can be hard to spot mistakes or identify allowances you were entitled to. If you don’t claim what you’re owed in time, HMRC will simply keep your money. There’s a strict deadline for these claims—so acting quickly is essential to make sure what’s rightfully yours ends up back in your pocket, not HMRC’s.