If you are self-employed, or a director of a company, or if you receive any other non-taxed income then you’ll need to file a self assessment tax return. It’s one of HMRC’s ways of collecting Income Tax.
You’re notified in April if you do need to file a self assessment tax return.
Anyone self-employed, is expected to provide details of all sales and takings, purchases and business expenses. This is the starting point.