CT600 Amendments
Need to correct your CT600 after filing? Learn when you can amend, the 72-hour wait, and how to do it in TinyTax.
Need to correct your CT600 after filing? This guide explains when and how you can amend your corporation tax return.
When Can You Amend a CT600?
HMRC allows you to amend a CT600 return, but there are two timing considerations:
1. Give HMRC Time to Register Your Original
HMRC's systems take time to post your original return to the records used for amendment matching. If you amend too soon, you're more likely to see Error 3001: "The accounting period for this amended return does not match the records currently held for the company." We recommend waiting at least 72 hours after HMRC accepts your original before amending.
This is a guideline, not a hard rule — even after 72 hours, amendments are sometimes rejected with Error 3001. If that happens, retrying over the next few days often works. See Error 3001 — what to do below.
2. Statutory 12-Month Deadline
You must submit your amendment within 12 months of your original filing deadline.
| Your Period End | Filing Deadline | Amendment Deadline |
|---|---|---|
| 31 March 2024 | 31 March 2025 | 31 March 2026 |
| 30 November 2023 | 30 November 2024 | 30 November 2025 |
| 31 December 2024 | 31 December 2025 | 31 December 2026 |
How to Amend Your CT600 in TinyTax
- Go to your Dashboard
- Find the filed CT600 you want to amend
- Click Amend
- TinyTax will check timing and show you any warnings
- Your form will be pre-filled with the original values
- Make your corrections
- Review and submit the amended return
Amending a Return Filed via Another Service or HMRC's Portal
If your original CT600 was submitted via HMRC's own online services, a different piece of software, or through an accountant — not through TinyTax — there will be no "Amend CT600" button on your dashboard. That button only appears for returns originally filed through TinyTax.
TinyTax can still file the amendment. HMRC matches amendments to your original return by UTR and period dates — it does not check which software submitted the original. You can use TinyTax to file a corrected return regardless of where the original was submitted.
To start, contact support with your company number and the period end date you need to amend. We will provide a direct link to an amendment filing session for that period. The form will be blank — enter the correct figures, review everything at Review & Submit, and submit when ready.
If both your CT600 and Companies House accounts contained errors, we can correct both in a single submission.
What Can You Amend?
You can amend most items on a CT600, including:
- Trading profits - correcting turnover or expenses
- Other income - interest, property income, etc.
- Reliefs and deductions - claiming missed reliefs
- Capital allowances - correcting asset values
- Tax calculation errors - fixing computational mistakes
What You Cannot Amend
Some changes cannot be made through an amendment:
- Accounting period dates - these are fixed once filed
- Company information - registered name, company number
- Changes requiring HMRC approval - certain reliefs need formal claims
The 72-Hour Guideline
When you click "Amend" in TinyTax, we check when your original CT600 was accepted by HMRC. If it's been less than 72 hours, you'll see a warning recommending you wait. You can proceed anyway, but an early amendment is more likely to bounce.
72 hours isn't a hard cut-off on HMRC's side — it's a sensible buffer that reduces the chance of Error 3001. Amendments can still occasionally be rejected with Error 3001 beyond that window, and amendments submitted well within it sometimes succeed.
Error 3001 — What to Do
If your amendment is rejected with "The accounting period for this amended return does not match the records currently held for the company", the usual cause is that HMRC's amendment matcher can't yet line your amendment up against the original return — even when the dates, UTR, and company details you submitted are identical to the accepted original.
Step 1: Retry
Go back to Review & Submit on the amendment session and click Submit again. We've seen the same amendment bounce once and then be accepted on a later retry, sometimes within minutes, sometimes after several days. There's no harm in retrying; rejected amendments don't reach HMRC's records.
If it's still bouncing after a handful of attempts spread across several days, move on to Step 2.
Step 2: Paper Amendment (reliable fallback)
Post a paper amendment pack directly to HMRC:
- A completed CT600 for each affected period, marked "Amended return" at the top
- The amended tax computation
- A copy of your accounts as filed with Companies House
We can prepare the pack for you — reply to any open support ticket and we'll put it together.
Step 3: Pay Any Tax You Owe Now (separate from the amendment)
If your amendment increases the tax due, pay HMRC the correct amount now — it stops further interest accruing from the original CT payment due date (9 months and 1 day after your period end), regardless of which amendment route eventually lands. HMRC will reconcile their records once the amendment is processed.
The Statutory Deadline
If you try to amend a CT600 after the 12-month deadline has passed, TinyTax will show:
[[ui:alert:error:Amendment deadline has passed - The statutory deadline to amend this CT600 was [date]. You can no longer amend this return electronically.]]
After the deadline, you cannot amend electronically through TinyTax or any other software. Your options are:
- Contact HMRC directly to request a late amendment
- Call the Corporation Tax helpline: 0300 200 3410
- Write to HMRC explaining the error and requesting correction
Multiple Amendments
You can submit multiple amendments to the same CT600, as long as each is within the deadline. Each amendment replaces the previous version.
| Amendment | Replaces |
|---|---|
| Amendment #1 | Original filing |
| Amendment #2 | Amendment #1 |
| Amendment #3 | Amendment #2 |
Extended Period Amendments
If your original filing was an extended period (two CT600 returns), you can amend either or both:
- Period 1 only - if the error was in the first 12 months
- Period 2 only - if the error was in the remaining period
- Both periods - if the error affects the full period
Tax Impact of Amendments
If your amendment changes the tax due:
| Change | Result |
|---|---|
| More tax owed | HMRC will issue a revised bill with interest |
| Less tax owed | HMRC will process a refund |
| No change to tax | No financial adjustment needed |
Common Questions
"I made a mistake but the deadline has passed"
Contact HMRC directly. They may accept corrections under "overpayment relief" rules if you've paid too much tax, or through their error correction process for other issues.
"Can I amend an amended return?"
Yes, you can amend as many times as needed within the deadline. Each new amendment replaces the previous one.
"Will HMRC know I made an error?"
Yes, HMRC sees both your original and amended returns. However, making corrections is normal and expected - it's better to amend than leave errors uncorrected.
"What if I'm amending to reduce my tax?"
This is fine. If you forgot to claim a relief or overstated your profits, you can amend to correct this and receive a refund.
"How long does it take for HMRC to process an amendment?"
Typically 2-4 weeks, though it can be longer during busy periods. You'll receive confirmation once processed.
Changed Your Mind? Deleting an Amendment Draft
If you started a CT600 amendment but no longer want to proceed, you can delete the draft without submitting it. Your original accepted CT600 will not be affected in any way.
How to delete an amendment draft:
- Go to your Dashboard
- Find the period you started the amendment for — you'll see a Continue Amendment ▾ button
- Click the ▾ chevron to open the dropdown
- Select Delete Draft
- Confirm when prompted — the amendment draft is permanently removed
If you decide to amend later, you can always start a new amendment from the Actions dropdown on the same period.
"I cannot see the losses brought forward field (Box 220) in my amendment"
If your original CT600 had "Is this your first year of trading?" set to Yes, that setting is carried into the amendment — and with the first-year flag set, the Losses section is hidden. Box 220 (Losses brought forward used against income) will not appear until you change the flag.
To access Box 220 in your amendment:
- Open the amendment from your dashboard.
- In the Tax computation section, find "Is this your first year of trading?" near the top of that section.
- Change the answer to No.
- The Losses section will appear — click Show next to it to expand it.
- Enter your losses brought forward in the Losses brought forward from prior years field. This feeds into Box 220.
"Will my CT600 amendment appear on Companies House?"
No. CT600 amendments are filed with HMRC only — Companies House is a separate registry that does not receive your corporation tax return, original or amended.
Companies House holds your annual accounts (the balance sheet and profit and loss that appear on the public register). Your CT600 is a separate submission to HMRC. There is nothing to check on the Companies House register as a result of submitting or amending a CT600.
If an error in your figures also affects your accounts at Companies House, you would need to amend those separately. See How to Amend Companies House Accounts.
Still Have Questions?
Amendment rules can be complex. If you're unsure about your specific situation, we're here to help.
Last updated: April 2026
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