You've filed your CT600, received the HMRC acknowledgement, and then it hits you — you got something wrong. Maybe you forgot an expense, entered the wrong turnover figure, or missed a capital allowance claim.
Don't panic. HMRC allows you to amend your CT600 within certain time limits. Here's how it works.
Can you amend a CT600 after filing?
Yes. You can amend your CT600 within 12 months of the original filing deadline (not 12 months from when you actually filed).
Example timeline
- Accounting period ends: 31 March 2026
- Filing deadline: 31 March 2027 (12 months after period end)
- Amendment deadline: 31 March 2028 (12 months after filing deadline)
So you effectively have up to 24 months from the end of the accounting period to get your CT600 right.
What if you're outside the amendment window?
If more than 12 months have passed since the filing deadline, you can't amend the return yourself. Instead, you'd need to write to HMRC and ask them to make a "discovery assessment" or accept an overpayment relief claim. This is more complex and HMRC may investigate.
How to amend your CT600
With commercial software (including Taxpipe)
Most CT600 filing software lets you submit an amended return electronically. The process is:
- Open the original return in your software
- Make the corrections
- Submit the amended return to HMRC
HMRC's systems will recognise it as an amendment (not a duplicate) based on the company UTR and accounting period dates.
With Taxpipe, you can edit your return data and resubmit at no extra cost — we include free resubmission for amendments and corrections.
By post
You can also send a paper amendment to HMRC. Write to:
Corporation Tax Services
HM Revenue and Customs
BX9 1AX
Include your company name, UTR, the accounting period, and clearly mark which boxes are being changed (with old and new values).
Paper amendments take longer to process — typically 4-8 weeks vs. near-instant for electronic submissions.
Common reasons to amend
Forgot to claim an expense
The most common amendment. If you missed an allowable expense (office costs, travel, professional subscriptions), amend the return to reduce your taxable profit and get a Corporation Tax refund.
Wrong turnover figure
If your turnover was higher or lower than what you reported, amend boxes 145 (total turnover) and 155 (trading profits) accordingly.
Capital allowances not claimed
Capital allowances can significantly reduce your tax bill. If you bought equipment, vehicles, or other qualifying assets and didn't claim the allowances, it's worth amending.
Directors' loan balance incorrect
If the directors' loan account balance was wrong, your S455 tax calculation may be incorrect. Amend boxes 360-375 as needed.
Period dates wrong
If the accounting period start or end dates were incorrect, this is a more significant amendment that may require HMRC to reissue notices.
What happens after you amend?
If you owe more tax
HMRC will issue a revised tax calculation. You'll need to pay the additional tax plus interest from the original due date. No penalties apply if you voluntarily amend — penalties only apply if HMRC discovers the error through an enquiry.
If you're owed a refund
HMRC processes CT refunds relatively quickly — typically within 2-4 weeks of processing the amendment. The refund will include repayment interest from the later of: (a) the date you paid the original tax, or (b) the normal due date.
What if HMRC finds the mistake?
If HMRC opens an enquiry and finds errors you didn't disclose:
- Careless errors: penalty of 0-30% of the additional tax owed
- Deliberate errors: penalty of 20-70%
- Deliberate and concealed: penalty of 30-100%
Voluntary disclosure (amending yourself) always results in lower or no penalties compared to HMRC discovering the error. If you spot a mistake, amend promptly.
Can HMRC amend your return?
Yes. HMRC can open an enquiry into your CT600 within 12 months of the date you filed (not the filing deadline). During an enquiry, they can request documents, ask questions, and ultimately amend your return if they believe it's incorrect.
After the enquiry window closes, HMRC can still make "discovery assessments" if they find evidence of an error, but the bar is higher — they generally need to show the error was careless or deliberate.
Tips for avoiding amendments
- Double-check your figures before submitting — especially turnover, expenses, and directors' loan balances
- Use software with validation — tools like Taxpipe check for common errors before submission
- Keep records — bank statements, invoices, and receipts make it easy to verify your numbers
- File early — gives you more time to spot and fix errors before the deadline
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Summary
| Question | Answer |
|---|---|
| Can I amend my CT600? | Yes, within 12 months of the filing deadline |
| Is there a cost? | No cost with Taxpipe (free resubmission) |
| Will I be penalised? | No, if you amend voluntarily |
| How long does a refund take? | 2-4 weeks typically |
| Can HMRC amend my return? | Yes, via enquiry (within 12 months of filing) |
Spotted an error in your CT600? Don't wait — amend it now with Taxpipe. Free resubmission included with every filing.
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