Amend the return as a controlled replacement—not an unexplained overwrite.
A company can usually amend its return within twelve months of the filing deadline. Rebuild the affected package, document the reason and retain both versions.
Normal amendment window
HMRC says a company must usually make amendments within twelve months of the filing deadline. The relevant deadline is the statutory filing deadline, so a late original return does not necessarily create a fresh twelve-month period.
Identify what changed
Before rebuilding the return, document:
- The error or new information discovered
- The affected accounting entries or tax adjustments
- Whether the statutory accounts also need correction
- Changes to tax payable, losses, claims or supplementary pages
- Interest, payment or repayment consequences
Reconcile the complete amended package
Do not change an isolated CT600 box without checking the accounts, tax computation, supplementary pages and attachments that support it. The amended return should remain internally consistent and use the correct filing format.
Preserve the audit trail
Retain both the original and amended records.
- Original submitted files and acceptance response
- Reason for amendment and supporting evidence
- Reviewer and authoriser
- Amended accounts, computation and CT600 package
- New submission response and any payment action
Act promptly on errors
HMRC guidance advises telling HMRC promptly when an error is found. If the normal amendment window has passed, the next action depends on the issue and should be checked against current HMRC guidance or professional advice.
Where professional judgement is needed
This guide explains a filing workflow, not the accounting treatment, tax position or legal duties of a particular company. Confirm the applicable accounting framework, reliefs, disclosures and filing obligations against current official guidance and, where needed, a qualified accountant or tax adviser.
FAQ
How long do I usually have to amend a Company Tax Return?
Usually twelve months from the statutory filing deadline.
Does filing the original return late extend the amendment window?
Not generally. HMRC guidance ties the normal window to the filing deadline rather than the late submission date.
Should I keep the original return?
Yes. Retain the original submission, reason for change, amended package and both authority responses.
Can an amendment affect payment?
Yes. It can change tax payable, interest, losses, claims or a repayment, so payment consequences need a separate check.
Official sources
Checked on 2026-08-11. Official guidance takes priority if rules change.