When should I notify HMRC that I need to complete a self-assessment return?

You must notify HMRC within 6 months following the end of the tax year (5 October) that you need to complete a tax return. This is to ensure that HMRC has enough time to issue a tax return to you and for you to complete and submit it to HMRC. Failure to notify the HMRC by this date will result in a £100 penalty.