Amended corporation tax returns can be filed online within 12 months of the original filing deadline. As the original filing deadline is normally 12 months after the accounting period end, you will usually have 2 years from the accounting period end to file an amended return. Once this submission window has closed, an amended return has to be filed by paper.
Note, if you are attempting to complete a submission for an earlier period due to the extended loss carry back, this may not actually be required. Please see the below extract from the HMRC guidance: HMRC Extended Loss Carry Back for Business
Amended company tax returns
You do not need to submit an amended company tax return for claims for the accounting periods covered by the extended relief. If you submit amended returns on the online portal for the accounting periods for the extended relief claim, they will be rejected as the time limit on amendments will have passed. A claim made in the company tax return for the accounting period of the loss will be treated as an amendment to those earlier returns.
If you make a claim in a company tax return, make sure box 45 on CT600 form is completed. More details on the claims should be explained in the computations in the same way that any claim for one year carry back is made.
Our article for CT600 losses and how to carry them back further explains how to ensure the entries are complete within TaxCalc to meet the above requirements.