How do I know which registration form to use?

It depends on why you're registering. Use the CWF1 form if you're self-employed as a sole trader — this also registers you for Class 2 and Class 4 National Insurance at the same time. Use the SA1 form if you need to file for other reasons, such as rental income, Capital Gains Tax, the High Income Child Benefit Charge, or dividend income. If you're registering online via the HMRC website, the system asks a few questions and routes you to the right form automatically. If you're unsure, drop us a message on live chat.