CT600 Amendments

Need to correct your CT600 after filing? This guide explains when and how you can amend your corporation tax return.

When Can You Amend a CT600?

HMRC allows you to amend a CT600 return, but there are two important timing restrictions:

1. Wait 72 Hours After Filing

HMRC's systems need up to 72 hours to process your original submission. If you try to amend too soon, HMRC will reject your amendment with an error like "accounting period does not match records".

2. Statutory 12-Month Deadline

You must submit your amendment within 12 months of your original filing deadline.

How to Amend Your CT600 in TinyTax

1. Go to your Dashboard
2. Find the filed CT600 you want to amend
3. Click Amend
4. TinyTax will check timing and show you any warnings
5. Your form will be pre-filled with the original values
6. Make your corrections
7. Review and submit the amended return

What Can You Amend?

You can amend most items on a CT600, including:

• Trading profits - correcting turnover or expenses
• Other income - interest, property income, etc.
• Reliefs and deductions - claiming missed reliefs
• Capital allowances - correcting asset values
• Tax calculation errors - fixing computational mistakes

What You Cannot Amend

Some changes cannot be made through an amendment:

• Accounting period dates - these are fixed once filed
• Company information - registered name, company number
• Changes requiring HMRC approval - certain reliefs need formal claims

The 72-Hour Waiting Period

When you click "Amend" in TinyTax, we check when your original CT600 was accepted by HMRC. If it's been less than 72 hours, you'll see:

You can proceed anyway, but there's a high chance HMRC will reject the amendment. We recommend waiting until the date shown.

The Statutory Deadline

If you try to amend a CT600 after the 12-month deadline has passed, TinyTax will show:

[[ui:alert:error:Amendment deadline has passed - The statutory deadline to amend this CT600 was [date]. You can no longer amend this return electronically.]]

After the deadline, you cannot amend electronically through TinyTax or any other software. Your options are:

1. Contact HMRC directly to request a late amendment
2. Call the Corporation Tax helpline: 0300 200 3410
3. Write to HMRC explaining the error and requesting correction

Multiple Amendments

You can submit multiple amendments to the same CT600, as long as each is within the deadline. Each amendment replaces the previous version.

Extended Period Amendments

If your original filing was an extended period (two CT600 returns), you can amend either or both:

• Period 1 only - if the error was in the first 12 months
• Period 2 only - if the error was in the remaining period
• Both periods - if the error affects the full period

Tax Impact of Amendments

If your amendment changes the tax due:

Common Questions

"I made a mistake but the deadline has passed"

Contact HMRC directly. They may accept corrections under "overpayment relief" rules if you've paid too much tax, or through their error correction process for other issues.

"Can I amend an amended return?"

Yes, you can amend as many times as needed within the deadline. Each new amendment replaces the previous one.

"Will HMRC know I made an error?"

Yes, HMRC sees both your original and amended returns. However, making corrections is normal and expected - it's better to amend than leave errors uncorrected.

"What if I'm amending to reduce my tax?"

This is fine. If you forgot to claim a relief or overstated your profits, you can amend to correct this and receive a refund.

"How long does it take for HMRC to process an amendment?"

Typically 2-4 weeks, though it can be longer during busy periods. You'll receive confirmation once processed.

Still Have Questions?

Amendment rules can be complex. If you're unsure about your specific situation, we're here to help.

Last updated: January 2026