How to amend a CT600 after filing

If you find a mistake in a Company Tax Return you’ve already filed, you can usually amend it within 12 months of the filing deadline. You send the amended return through your software, or write to HMRC. After that window, the routes are overpayment relief if you paid too much, and a disclosure if you paid too little.

Last checked against GOV.UK on 1 October 2026.

A4O Accounts prepares the CT600 from your books and files it with HMRC. See the CT600 software, or get started free.

How long you have

GOV.UK says you must usually make any changes within 12 months of the filing deadline. The filing deadline is itself 12 months after the end of the accounting period, so for most companies the window closes 24 months after the period ends.

The window runs from the deadline, not from the day you filed. Filing early doesn’t shorten it.

How to amend

GOV.UK gives two ways:

1. Use commercial software. You prepare the return again with the corrected figures and send it as an amended return. The accounts and computation go with it, as they did the first time.
2. Write to your Corporation Tax office, or send a paper return.

An amended return replaces the figures in the original, so send the whole return with the corrections made, not just the boxes that changed.

If the change means more tax is due, pay it as soon as you can, because HMRC charges interest on Corporation Tax paid late. If it means you’ve overpaid, HMRC repays the difference.

Common reasons for amending

• A cost or income item was missed or put in the wrong period.
• Capital allowances were claimed on the wrong amount, or not claimed.
• A loss is being carried back to an earlier period.
• A supplementary page was left off, or its figures have changed. See CT600 supplementary pages.
• The accounts themselves were revised after the return went in.

A change to the accounts usually means a change to the return, because the tax computation starts from the accounts profit.

When HMRC corrects the return

HMRC can correct obvious errors or omissions in a return itself, such as arithmetical mistakes. Its manual says it can’t do this more than 9 months after the day the return was delivered. If you disagree with a correction, the company can reject it in writing within 3 months of the correction notice.

If HMRC is checking the return

A company can still amend a return while HMRC has an enquiry open into it, if it’s within the time limit. HMRC’s manual says a change that affects the tax payable only takes effect once the enquiry is finished.

GOV.UK also notes that HMRC may charge a penalty for errors it finds in a return. It’s generally better to correct a mistake yourself than to wait for HMRC to find it.

After the 12 months

Once the window has closed you can’t amend the return. What you do depends on which way the mistake went:

• You paid too much. You may be able to claim overpayment relief. GOV.UK’s “Making changes” page links to the rules and how to claim.
• You paid too little. GOV.UK says to tell HMRC using its online disclosure service as soon as possible.

Amending is not the same as fixing a rejection

A return that HMRC’s system rejected was never filed, so there’s nothing to amend. You fix the problem and submit it again as a new return. Amending applies only to a return HMRC has accepted.

Amending a return in software

In A4O Accounts, once HMRC accepts a return its computation is locked, and corrections go in as an amended return, sent with the accounts and computation. A return that was rejected can be reopened as a draft, fixed and submitted again. See CT600 software and preparing and filing a CT600.

More in this series: all the CT600 guides.