Do I need to register with HMRC before filing, and what is the deadline?

If this is your first time filing Self Assessment, yes — you need to register with HMRC before you can file. The deadline to register is 5 October following the end of the tax year you're reporting. For the 2025/26 tax year (which ended 5 April 2026), that means registering by 5 October 2026. Once registered, HMRC sends you a Unique Taxpayer Reference (UTR) number by post, which you need before you can file. If you haven't registered yet, it's worth doing as early as possible to avoid delays. We can guide you through this — just drop us a message on live chat.