According to HMRC, you should register at the earliest opportunity. To remain within the legal times standard, you will need to register by 5th October. This would need to be after the end of the tax year when you became self-employed. For example, if you started your business in July 2021, then you would need to register with HMRC by 5th October 2022. If you need help registering for self-assessment, we would be happy to help.