Your Self Assessment Tax Return (SA100, SA800, SA900) normally has to be completed and submitted to HM Revenue and Customs by 31st October following the tax year end (5th April) if you file on paper, or by 31st 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 – usually around May after the end of the tax year.
HMRC Online Filing Availability
The following years remain open for online filing of self assessment returns.
Earlier years' tax returns will require paper filing.
Filing Deadline and Online Filing Window
Original corporation tax returns should usually be filed within 12 months of the company accounting period end to avoid late filing penalties and interest. You can file original corporation tax returns online after the filing deadline, for periods going back to 2013/14 (that we are aware of). These will still usually incur the late filing penalties and any interest chargeable.
Amended corporation tax returns can be filed online within 12 months of the original filing deadline. As the original filing deadline is 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.