How to Amend a CT600 Return After Submission | TinyTax Support

How to Amend a CT600 Return After Submission

Made a mistake on your CT600? Learn how to amend your corporation tax return after submission. Step-by-step guide to correcting errors with HMRC.

How to Amend a CT600 Return After Submission

Discovered an error on your CT600 after submitting it? Don't panic. HMRC allows you to amend corporation tax returns within specific time limits. This guide explains exactly how to correct mistakes.

Can You Amend a CT600?

Yes. You can amend a CT600 within 12 months of the original filing deadline.

Amendment Window

Year EndOriginal DeadlineAmendment Deadline
31 March 202431 March 202531 March 2026
31 December 202431 December 202531 December 2026
30 June 202430 June 202530 June 2026
Important: The 12-month amendment window starts from the filing deadline, not when you actually filed.

Types of Amendments

Minor Corrections

For small errors that don't change your tax liability significantly:

  • Typos in company details
  • Minor expense recategorisation
  • Correcting box numbers

Tax-Affecting Amendments

For errors that change your tax bill:

  • Missed income
  • Forgotten expenses
  • Incorrect capital allowances
  • Wrong loss figures

How to Amend Your CT600

Using TinyTax

  1. Log into your TinyTax account
  2. Select the submission you need to amend
  3. Click "Amend Return"
  4. Make your corrections
  5. Review the changes
  6. Submit the amended return
TinyTax automatically marks it as an amendment and submits to HMRC.

Using Other Software

Most CT600 software has an amendment function:

  1. Open the original return
  2. Select "Amend" or "Correct"
  3. Enter the correct figures
  4. Submit as an amended return

Manual Amendment

If you filed via HMRC's online service (before it closes April 2026):

  1. Log into Government Gateway
  2. Access your company's Corporation Tax account
  3. Select the return to amend
  4. Make changes and resubmit

What Happens After Amendment

If You Owe More Tax

TimingConsequence
Amended before payment deadlinePay the additional amount by original deadline
Amended after payment deadlinePay immediately plus interest
Interest is charged from the original payment deadline.

If You're Due a Refund

HMRC will:

  1. Process your amendment (usually 2-4 weeks)
  2. Issue a refund if the figures are accepted
  3. Pay interest on overpaid tax (repayment interest rate)

Common Reasons for Amendment

1. Forgot to Claim Expenses

Solution: Amend to include the forgotten expenses, reducing your tax bill.

2. Missed Capital Allowances

Solution: Add the capital allowances claim. This is common when equipment purchases were overlooked.

3. Income Was Understated

Solution: Amend to include all income. Better to self-correct than have HMRC discover it.

4. Wrong Accounting Period

Solution: Submit a corrected return with the right period dates. This may require professional help.

5. Claimed Losses Incorrectly

Solution: Amend the loss claim. Ensure you're not carrying forward more than you're entitled to.

What You Cannot Amend

After the 12-month window closes, you cannot amend via the normal process. You may need to:

  • Request an error or mistake relief claim - For errors in closed periods
  • Write to HMRC - Explain the situation and request correction
  • Accept HMRC's decision - Not all late corrections are accepted

Penalties for Amendment

No Penalty If...

You won't face penalties if you:

  • Amend within the allowed window
  • Are correcting a genuine mistake
  • Pay any additional tax owed

Potential Penalty If...

SituationPossible Penalty
Careless error discovered by HMRCUp to 30% of extra tax
Deliberate errorUp to 70% of extra tax
Deliberate and concealedUp to 100% of extra tax
Key point: Self-correcting before HMRC discovers the error usually results in no penalty or a reduced penalty.

Before You Amend

Check These First

  • Is the amendment window still open?
  • Do you have correct figures?
  • Will this affect previous years?
  • Have you considered all consequences?

Get Your Documents Ready

  • Original CT600 submission receipt
  • Corrected figures with supporting documents
  • Explanation of what went wrong
  • Bank statements if relevant

Frequently Asked Questions

How many times can I amend?

There's no limit on amendments within the 12-month window. However, frequent amendments may attract HMRC attention.

Will amending trigger an enquiry?

Not usually. Amendments are normal and expected. However, significant changes to tax liability may be reviewed.

What if I discover an error after 12 months?

You'll need to contact HMRC directly. Use form CA3822 for error or mistake relief, though success isn't guaranteed.

Does amending extend the enquiry window?

The standard enquiry window is 12 months from submission. Amendments don't restart this clock, but can extend it in some cases.

What if the error benefits HMRC?

Even if the error meant you overpaid, you should still correct it. You're entitled to an accurate tax position.

Summary

AspectDetails
Amendment window12 months from filing deadline
How to amendThrough software or HMRC portal
PenaltiesNone for genuine self-correction
After window closesContact HMRC directly
Best practiceCorrect errors promptly
Mistakes happen. The important thing is to correct them promptly and accurately. Most amendments are processed without issue.

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