Can You Amend a Submitted CT600 Form with HMRC?

Team Nomi

Jun 09, 2025
5 minutes read

If you have submitted a CT600 Corporation Tax return and later find an error or need to make changes, you can amend your submitted CT600 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, typically up to two years from the end of the relevant accounting period. Making timely amendments ensures your company’s tax affairs remain accurate and compliant, helping you avoid potential penalties or issues with HMRC.

What is a CT600 Form?

A CT600 form is an important part of a Company Tax Return in the UK. It must be submitted to HMRC. The CT600 form, along with other supporting documents like accounts and tax calculations, helps HMRC understand your company’s financial position. It includes important details such as income, expenses, profits, and tax reliefs.

Limited companies use the information in the CT600 to work out how much Corporation Tax they owe for the accounting period.

Submitting a correct and complete CT600 form is essential to stay compliant and avoid penalties.

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 filing deadline to make any 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.

It’s important to make sure your CT600 form is correct. If it’s wrong, you could pay too much or too little Corporation Tax. This can cause problems with HMRC.

When Can You Amend a CT600 Form?

You can amend a CT600 form within 12 months of the original filing deadline. This means if your company’s accounting period ends on 31 December 2023, the filing deadline is 31 December 2024. You then have until 31 December 2025 to make any changes. 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 can’t amend the return in the usual or simple way. You would need to contact HMRC directly and may need to apply for a correction or overpayment relief.

How to Amend a CT600 Form

You need to fix a mistake on your CT600 form, you must amend it electronically. You cannot send paper changes to HMRC. There are two main ways to amend a CT600 form:

Use Commercial Tax Software

Using commercial tax software is the simplest and reliable way to amend a CT600 form. This software is built to meet HMRC’s standards and help reduce filing errors.

Nomi is software that makes it simple to update taxes, calculate Corporation Tax, and submit the return online. 

• • Open the original return
  • Make changes to accounts or tax figures
  • Tick the box to show it’s an amended return
  • Submit it again to HMRC

Use HMRC’s Online Service

If your return is simple, you may be able to amend it using HMRC’s website. This is usually for small businesses with basic tax returns. Important to check:

• • Always mark the return as “amended”
  • Check that your figures are correct before resubmitting.
  • Send all supporting documents if needed.

Amending a CT600 in Tax Software

Amending a CT600 form is easy when using reliable tax software. Nomi provides strong Corporation Tax software designed for accountants and bookkeepers, making the amendment process quick and stress-free. With Nomi, you can:

• • Select the company and the accounting period that needs updating.
  • Correct the accounts, tax computations, or CT600 form as needed.
  • Mark the return clearly as “amended”.
  • Resubmit the updated return directly to HMRC.

If your amendment also involves changes to your company accounts, Nomi lets you file amended accounts directly to Companies House (COHO), with no need for paper forms.

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. You must amend it within 12 months of the filing deadline.

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. Book a free demo and see how Nomi can help. 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.