Amending a CT600

Key facts

You can amend a CT600 within 12 months of the original filing deadline (i.e. up to 24 months after the accounting period end).
Amendments are filed online as a revised CT600 return.
HMRC can open an enquiry into your return within 12 months of the date you filed (or amended).
After the amendment window closes, you may need to make a formal claim or disclosure instead.

When Can You Amend?

A company can amend its CT600 at any time within 12 months of the filing deadline. Since the filing deadline is itself 12 months after the accounting period end, this means you have up to 24 months from the end of the accounting period to submit an amended return.^[1]

Accounting Period End	Filing Deadline	Amendment Deadline
31 March 2025	31 March 2026	31 March 2027
31 December 2025	31 December 2026	31 December 2027
30 June 2025	30 June 2026	30 June 2027

Important: The amendment window runs from the filing deadline, not from the date you actually filed. So even if you file your CT600 on the last possible day, you still get a full 12 months to amend it.^[1]

How to File an Amendment

To amend a CT600, you file a revised return electronically. The process depends on how you originally filed:^[1]

Commercial software — most packages allow you to re-open the return, make changes, and re-submit it as an amended CT600
Agent software — your accountant can file the amended return on your behalf
In writing — where a software amendment is not possible (for example, the original was filed on HMRC’s old online service, which closed on 31 March 2026), you can send the correction to HMRC by post or, in limited cases, make an eligible paper submission

The amended return replaces the original in its entirety. HMRC will recalculate the tax position and issue a revised statement showing any additional tax due or overpayment to be refunded. HMRC-recognised packages such as GoFile’s CT600 software let you re-open a filed return, make the changes, and re-submit it as an amendment.

Common Reasons for Amending

Typical reasons companies amend their CT600 include:

Correcting errors — wrong figures, miscalculations, or transposition mistakes
Claiming a relief that was missed — for example, capital allowances, R&D relief, or marginal relief
Adjusting for a loss carry-back — a loss in a later period can be carried back to the previous period, requiring an amendment
Revised accounts — if the statutory accounts are restated (e.g. due to an audit adjustment), the CT600 must also be updated
Reclassifying income or expenses — correcting the treatment of an item after taking professional advice

Tip: If the amendment results in additional tax being due, pay it as soon as possible. Interest will be charged from the original payment deadline (9 months and 1 day after the period end), not from the date of the amendment.^[3]

What If the Amendment Window Has Closed?

If more than 12 months have passed since the filing deadline, you can no longer amend the CT600 in the normal way. Instead, you may need to:^[1]

Make a stand-alone claim to HMRC (e.g. for overpayment relief under Schedule 1AB TMA 1970)
Write to HMRC requesting a determination be set aside if HMRC issued an estimated assessment
Make a voluntary disclosure if you discover an error that has resulted in too little tax being paid

Claims for overpayment relief must generally be made within 4 years of the end of the accounting period.

HMRC Enquiries

HMRC has the right to open an enquiry into a Company Tax Return. The enquiry window is:^[2]

12 months from the date HMRC received the return (if filed on time)
12 months from the quarter-day following the actual filing date (if filed late) — the quarter-days are 31 January, 30 April, 31 July, and 31 October
If you file an amended return, HMRC gets a fresh 12-month window from the date the amendment is received

During an enquiry, HMRC may request documents, ask questions, and ultimately adjust the company’s tax position. The company has the right to appeal any amendments HMRC proposes.

Frequently Asked Questions

How long do I have to amend a CT600?

You can amend a CT600 within 12 months of the original filing deadline. Since the filing deadline is 12 months after the accounting period end, this gives you up to 24 months from the end of the accounting period.

How do I file an amended CT600?

You file a revised CT600 return electronically, either through your commercial software or via your accountant’s agent software. HMRC’s old online service closed on 31 March 2026 and can no longer be used for amendments — if a software amendment is not possible, you can write to HMRC or, in limited cases, make an eligible paper submission. The amended return replaces the original in full.

What happens if I miss the CT600 amendment deadline?

After the 12-month amendment window closes, you may need to make a stand-alone claim to HMRC (e.g. overpayment relief) or a voluntary disclosure. Claims for overpayment relief must generally be made within 4 years.

Will HMRC charge interest on additional tax from an amended CT600?

Yes. If your amendment results in additional tax due, interest is charged from the original payment deadline (9 months and 1 day after the period end), not from the date of the amendment.

Sources

Company Tax Returns: corrections and amendments — GOV.UK
HMRC compliance checks — GOV.UK
Company Tax Returns — GOV.UK