Can You Amend a Submitted CT600 Form with HMRC?

Team Nomi

Jul 16, 2026
6 minutes read

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Yes, you can amend a submitted CT600 Corporation Tax form. Amending a CT600 is an easy
process that allows companies to correct mistakes, update information, or provide additional documentation, such as
for research and development claims or loss adjustments.

HMRC allows amendments to a submitted CT600 within specific time limits. While the CT600
filing deadline is generally 12 months after the end of the accounting period, companies can usually amend their
return within 12 months of that filing deadline. 

Can you amend a submitted CT600 form?

You can amend a submitted CT600 form, but only within a time limit. If you realise
you’ve made a mistake after submitting your Corporation Tax return, HMRC allows you to correct your tax
return. 

You have 12 months from the original CT600 submission
deadline to make changes to your CT600 form. This includes fixing
errors in the return itself, the company accounts, or the tax calculations.

Amendments must be sent online. You cannot send changes by post. Most businesses use
commercial tax software to amend and resubmit their return.

When can you amend a CT600 form?

You can amend a CT600 form within 12 months of the original filing deadline. For
example, if your company’s accounting period ends on 31 March 2026, the CT600 deadline for filing the return is 31
March 2027. You would then have until 31 March 2028 to amend the return if needed.

During this time, you can correct:

• • Mistakes in your company
    accounts.
  • Wrong figures in your tax
    calculations.
  • Missing income or
    expenses.
  • Any other errors in your
    CT600 form.

After the deadline of 12 months, you cannot amend the return through the normal
amendment process. If you need to amend Corporation Tax return after 12 months, you may need to contact HMRC
directly and explore options such as overpayment relief or voluntary disclosure, depending on the
circumstances.

How to amend a CT600 form

If you find an error after filing your Company Tax Return, you can amend your CT600 form
online. HMRC does not accept paper amendments, so all changes must be submitted online before the amendment window
closes.

It’s important to make corrections before the relevant CT600 filing deadline for
amendments to ensure your Corporation Tax position remains accurate.

Use commercial tax software

Most businesses use commercial tax software to amend a CT600 form. This is usually the
quickest and easiest method, as the software is designed to meet HMRC requirements and reduce filing errors.

The process typically involves:

• • Opening the original CT600
    return
  • Updating the accounts or tax
    figures
  • Marking the return as an
    amended submission
  • Resubmitting it to HMRC
    online

Use HMRC’s online service

For simple Corporation
Tax returns, you may be able to make changes through HMRC’s online
service. Before resubmitting:

• • Mark the return as
    “amended”
  • Review all figures
    carefully
  • Include any supporting
    information if required

Remember that amendments can only be made within HMRC’s permitted time limits. Once the
CT600 submission deadline for amendments has passed, the process becomes more complex.

Amending a CT600 in tax software

Tax software can simplify the amendment process by allowing you to update accounts, tax
computations, and Corporation Tax figures in one place before resubmitting the return to HMRC.

If you need to amend Corporation Tax return after 12 months, you generally cannot make a
standard online amendment. In these cases, you may need to contact HMRC directly and explore alternative options
such as overpayment relief, depending on the circumstances.

To avoid issues, always keep track of your accounting period end date, the original
CT600 deadline, and any amendment deadlines that apply to your return.

What happens after submitting an amended CT600 form

Once you submit an amended CT600 form, HMRC will send an acknowledgement to confirm they
have received it. They will then review the updated return and process the changes.

If everything is correct, you’ll receive an updated Corporation Tax calculation or a revised
statement. HMRC may also contact you if they need more details or have questions about the amendment.

Processing times can vary, but most changes are reviewed within a few days to a few
weeks. It’s important to keep a copy of the amended return and monitor any communication from HMRC. Amending a CT600
form is possible, but only within the right time limit. 

If you need to submit an amended CT600 form, using Nomi software makes the process
simple and easy. Designed specifically for accountants and bookkeepers, Nomi lets you update your company accounts
and tax calculations. With Nomi, you can quickly return as amended and resubmit it directly to HMRC. 

Experience the benefits with a 30-day free trial to explore its features
and functionalities. Nomi helps you avoid common mistakes with built-in checks and smooth filing of amended accounts
to Companies House.

Note: We can’t advise on how to fill in your tax return or work out your tax. The
information shared is general and for guidance only. We’re not responsible for any problems caused by using it. If
you’re unsure, please speak to HMRC or a qualified tax advisor.

Frequently asked questions

1. Can I amend a CT600?
   Ans: Yes, you can amend a CT600 if you find a mistake after submitting your Corporation Tax return. HMRC allows companies to make changes to their return, accounts, or tax calculations within the permitted amendment period.

2. How long do I have to amend my Corporation Tax return?
   Ans: You normally have 12 months from the original CT600 filing deadline to amend your Corporation Tax return. After this period, you may need to contact HMRC directly to discuss alternative options.

3. What happens if I submit the wrong figures on my CT600?
   Ans: If you submit incorrect figures, you should amend the return as soon as possible. Incorrect information could result in paying too much or too little Corporation Tax and may lead to questions from HMRC.

4. Can I amend my CT600 more than once?
   Ans: : Yes, you can amend your CT600 multiple times, provided you are still within HMRC’s amendment deadline. Each amended return should include the latest and most accurate information.

5. Do I need to pay a penalty for amending a CT600?
   Ans: No, HMRC does not usually charge a penalty simply for amending a CT600. However, penalties or interest may apply if errors result in underpaid tax or if the return was filed late.

6. Will HMRC notify me after I amend my CT600?Ans: Yes, HMRC will normally send an acknowledgement confirming receipt of the amended return. They may also contact you if they need further information or clarification about the changes.