How to amend your Company Tax Return if you got something wrong

Updated 27 June 2026
The short answer

Yes, you can fix a Company Tax Return you've already sent. For about a year after the filing deadline you just send a corrected version, the same way you filed the first one, and HMRC uses the new figures. Fixing it sorts the tax too: if you paid too much you can get the difference back, and if you paid too little you pay it. After that window, you write to HMRC instead.

I already filed it. Can I still change it?

Yes. Sending your return is not the last word. If you spot a wrong number, a cost you forgot, or a figure you typed twice, you can correct it.

For most companies there's a comfortable window to do this. You can usually make changes for up to a year after your filing deadline. Your filing deadline is one year after your accounting period ends, so in plain terms you've often got close to two years from your year-end to fix a return. Plenty of time, but not forever.

How do I actually amend it?

You send the return again with the right figures on it. You don't ask permission first, and you don't fill in a special "sorry" form. You file the corrected version the same way you filed the original, online through filing software, and the new one replaces the old one.

If you're outside the online window (more on that below), you put it in writing to HMRC instead.

With SimpleReturns, you don't redo any of this by hand. You tell us what was wrong, we rebuild the figures and resend the corrected return for you.

What happens to the tax I already paid?

Fixing the return fixes the tax. The corrected figures work out a new, correct tax bill, and HMRC squares it up with what you've already paid:

  • If your real bill was lower than you first paid, you paid too much, and you can get the difference back.
  • If your real bill was higher, you paid too little, and you pay the rest. Because that tax was really due back at the original deadline, HMRC also adds interest on the unpaid bit for the time it was outstanding, so the sooner you settle it the less the interest is.

So a mistake that made you overpay isn't lost money, and a mistake the other way is something you can put right rather than worry about.

A quick example

Say your company's profit for the year was £40,000, but when you filed you forgot a £5,000 business cost. That made your profit look like £45,000, so you paid tax on £5,000 you shouldn't have.

You amend the return and add the £5,000 cost back in. Your profit drops to the real £40,000, your tax bill comes down, and HMRC pays back the extra tax you'd handed over on that £5,000. Same return, correct figures, money back.

It works the same way in reverse: if you'd left out £5,000 of income by mistake, amending it raises your profit to the true figure and you pay the bit of tax you still owed, plus a bit of interest for the time it went unpaid.

What if it's been too long to send a corrected return?

Once you're past that year-after-the-deadline window, you can't just resubmit online. You write to HMRC instead and explain what needs changing.

  • Paid too much? You can still ask HMRC to pay back tax you shouldn't have paid, but there's a deadline for this: you've got up to four years from the end of that accounting year to claim it, so don't leave an old overpayment sitting too long.
  • Paid too little? Tell HMRC as soon as you can, so it gets sorted and any extra tax is settled. HMRC will add interest on the unpaid tax for the time it was outstanding, so flagging it sooner keeps that smaller.

The sooner you flag it, the simpler it is, so don't sit on a known mistake just because the easy online route has closed.

Will I get in trouble for a mistake?

An honest slip you put right is normal, and fixing it is exactly what HMRC expects you to do. HMRC can charge a penalty for some errors, and owning up early and correcting them is what keeps things calm. A genuine mistake you've spotted and fixed is a very different thing from one you've hidden.

If the figures feel beyond you, that's a fair point to get help rather than guess at the fix.

How SimpleReturns handles it

Tell us which return needs changing and what was wrong. We pull your figures back up, correct them, recheck the whole calculation, and resend the corrected return to HMRC, with every number shown to you first. If your bill changes, we show you the new figure so you know exactly where you stand.


Common questions

Can I change a Company Tax Return after I've sent it?

Yes. For about a year after your filing deadline you send a corrected version and HMRC uses the new figures. After that, you write to HMRC instead.

How do I amend the return?

You file it again with the right numbers, the same way you filed the first one. The corrected return replaces the original.

Do I get my money back if I overpaid?

Yes. The corrected figures work out the right bill, and HMRC pays back tax you shouldn't have paid.

What if I underpaid?

You pay the difference. Amending the return shows the true bill, and you settle the rest, along with a bit of interest for the time the tax went unpaid.

It's been over two years. Have I missed my chance?

Not necessarily. You can no longer resubmit online, but you can write to HMRC to put it right, and you may still be able to claim back tax you overpaid for up to four years from the end of that accounting year.

Will I be fined for fixing a mistake?

Correcting an honest error is normal. HMRC can charge a penalty for some errors, so the safe move is to fix it promptly rather than leave it.

Spotted a mistake? Let us put it right

You don't need to understand any of the figures to fix a return. Tell us what went wrong, we rebuild the numbers, recheck the tax, and resend the corrected return, with every figure shown to you before anything goes to HMRC, for £99, once, no subscription.

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If your situation is complicated, like a tangled mix of years or a large correction, an accountant may be the better fit, and that's an honest call to make.

General guidance, not advice. This guide explains how the rules generally work for small UK limited companies. It isn't tax advice for your specific situation, if you're unsure, check with us or an accountant.