Cannot file 2nd CT600 for Extended Accounting Period - Error: "Editing Locked" | TinyTax Community
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Cannot file 2nd CT600 for Extended Accounting Period - Error: "Editing Locked"

Posted by Xiaotian Liao 13 Apr 2026 at 15:47
Hi everyone,

I am filing for a long accounting period (17 Apr 2024 to 30 Apr 2025). HMRC requires two separate CT600 returns for this (a 12-month return and a 14-day stub).

I have successfully submitted the first CT600 (Period 1). However, when I try to file the second CT600 for the final 14 days, the system blocks me with the error: "Your original CT600 has been filed and editing locked."

I need to report turnover and balance figures manually for both periods (Actual Basis) to match specific tax payments already made. How can I unlock the period to submit the second original return for the remaining 14 days?

Thanks!

1 Comment

T
Tiny (AI Assistant) Staff 13 Apr 2026 at 17:08
The "Editing Locked" message is working as designed — after Period 1 is accepted by HMRC, the session is locked to protect the submitted return. This is expected behaviour and Period 1 does not need to be unlocked or amended.

For extended accounting periods, Period 2 must be filed as a completely separate, standalone CT600. Here are the steps to do that:

1. Go to your TinyTax dashboard and open the company
2. Click Start Filing and select CT600 only from the dropdown
3. On the filing form, find the accounting period row and click "Need to file for different dates?"
4. Select Other reason and click Continue
5. Enter the Period 2 dates: Start 17 April 2025, End 30 April 2025
6. Enter the actual figures for just those 14 days (your turnover and balance sheet for that period)
7. Submit to HMRC as normal

Since this period is only 14 days, the system will treat it as a single CT600 — no further splitting. You can enter the actual figures directly rather than using automatic apportionment, which is exactly what you need for the Actual Basis approach.

Our support guide on extended accounting periods has the full step-by-step walkthrough: {{siteUrl}}/support/extended-accounting-period