Your Self Assessment Tax Return (SA100, SA800, SA900) normally has to be (completed and submitted to HM Revenue and Customs) by 31 October following the tax year end (5 April) if you file on paper, or by 31 January following the tax year end if you file online. So the 2020 tax return (for the tax year 2019-2020) is due by 31st October 2020, if filed by paper, or by 31st January 2021, if filed online. You will be sent your tax return – or a notice to file online – about May after the end of the tax year.
Anything outside of the above listed tax years, you will be required to paper file.
For Corporation tax returns, HMRC leave the gateway open for 2 years (to include time for filing any online amendments). Therefore 2 years ago today can be filed online as an amendment but 2 year + 1 day will present you with a failure message.
The Corporation Tax amendments should usually be made within 12 months of the filing deadline. The deadline depends on the company year-end date. The return can be amended online or by writing to the appropriate tax office (regardless of how it was originally processed). If the submission window has closed (i.e. the accounting period is more than 2 years), a paper submission will be required.
However if agreed with HMRC under exceptional circumstances, you will be allowed to file an amended submission online if the 2 year deadline has passed. You will need to get in contact with them first before you will be able to do this.