Filing an Amended CT600

If HMRC already has your CT600 for a period and you need to make changes, you'll need to file an amended return. TinyTax makes this straightforward.

When Do You Need an Amended Return?

You need to file an amendment when:

• You've already submitted a CT600 for this period and HMRC accepted it
• You've discovered an error in your figures (income, expenses, tax calculation)
• You need to correct information (director details, company type, etc.)
• HMRC has queried your return and you need to submit corrections

How to Recognise the "Original Already Received" Error

If you try to submit a CT600 for a period that's already been filed, HMRC will reject it with this message:

This isn't really an error - it's HMRC telling you they already have your return. To make changes, you need to file an amended return instead.

Method 1: From the Dashboard (Recommended)

If you've previously filed via TinyTax, the easiest method is:

1. Go to your Dashboard
2. Click on the company row to open the details drawer
3. Find the period you want to amend
4. Click the Amend CT600 button

This will open the submission form with your previous data pre-filled and the amendment flag set automatically.

Method 2: From the Error Panel

If you've just received the "Original already received" error, you'll see a blue information panel:

Click the button provided:

Your previous form data will be preserved.

The Amendment Banner

When filing an amended return, you'll see a blue banner at the top of the form confirming you're in amendment mode:

This banner confirms:

• Your submission will be sent as an amendment, not a new return
• HMRC will replace your previous filing with this updated version
• All your previous form data is loaded for you to edit

Making Your Changes

Once in the amendment form:

1. Review all sections - Your previous data is pre-filled
2. Make your corrections - Update the figures that need changing
3. Check the declaration - Ensure the declarant details are still correct
4. Preview and submit - Follow the normal submission process

What Happens After Submitting

1. TinyTax generates your amended CT600 with `ReturnType="amended"`
2. The return is submitted to HMRC via the Government Gateway
3. HMRC replaces your previous return with this new version
4. You'll receive a new submission receipt

Time Limits for Amendments

Common Questions

"Can I amend if I filed elsewhere (not via TinyTax)?"

Yes! Start a new filing for that period in TinyTax. When you try to submit, you'll get the "original already received" error, and can then proceed with an amendment.

Alternatively, mark the period as "Filed Elsewhere" on your dashboard first, then click "Undo" and file the amendment through TinyTax.

"Will HMRC know this is an amendment?"

Yes. TinyTax sets the `ReturnType` to "amended" in the CT600 XML. HMRC's systems recognise this and will replace your previous submission rather than treating it as a duplicate.

"Can I amend multiple times?"

Yes, you can file multiple amendments to the same period. However:

• Each amendment replaces the previous version
• HMRC may query frequent amendments
• Keep records of why each amendment was needed

"What if I'm outside the 12-month window?"

If you're outside the amendment window, you'll need to contact HMRC directly:

• Phone: 0300 200 3410 (Corporation Tax helpline)
• Post: Write to your company's Corporation Tax office

"I filed accounts too - do I need to amend those?"

Possibly. If your amendments affect the financial statements:

• Balance sheet changes: Yes, re-file accounts with Companies House
• P&L changes: Yes, if they affect the filed figures
• Tax calculation only: Usually no accounts amendment needed

Still Need Help?

If you're unsure whether you need an amendment or how to proceed, we're here to help.

Last updated: December 2025