Most employees never touch self-assessment, because PAYE handles their tax automatically through their payslip. Self-assessment exists for the income PAYE doesn't see — and working out whether that applies to you is usually simpler than the form itself looks.
Who typically needs to register
You may need to register for self-assessment if you're self-employed or a sole trader, have significant income from a side activity, earn rental income, have substantial untaxed investment income, or want to claim certain reliefs. Some thresholds (such as how much side income triggers a requirement to register) are reviewed periodically, so check the current rules on gov.uk rather than relying on a fixed number here.
The key dates
- Registering. If you need to file for the first time, there's a deadline to register after the tax year in which you started — check gov.uk for the exact date, as it's tied to the tax year structure.
- Filing online. The standard deadline for online returns is 31 January following the end of the tax year being reported.
- Paying. Any tax owed is normally due by that same 31 January deadline, and some people also make "payments on account" toward the following year at the same time.
What you'll need before you start
Your Unique Taxpayer Reference (UTR), records of your income and allowable expenses for the year, and any relevant documents such as a P60, P45, or dividend vouchers. HMRC's online personal tax account is the most reliable place to check your own specific position.
For how PAYE works for everyone else, see our guide to understanding your tax code.