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Guide

Self Assessment

Self Assessment is how you report income that isn't taxed automatically through PAYE. You file a return once a year online and pay any tax due by 31 January.

Step by step

  1. 1Check whether you need to file
  2. 2Register by 5 October after the tax year you first need to file for
  3. 3Get your UTR (Unique Taxpayer Reference) by post, in your HMRC online account, or in the HMRC app
  4. 4Gather income and expense records
  5. 5File online by 31 January
  6. 6Pay what you owe, including any payments on account

Common mistakes

  • Leaving registration too late
  • Forgetting payments on account
  • Mixing personal and business expenses
  • Missing the 31 January deadline (a £100 fine straight away on a non-MTD return; MTD users get penalty points from 2026/27)

Related guides

Self Assessment tax calculator

Last checked against GOV.UK: 28 October 2026. Tax year 2026/27.

Not affiliated with HMRC. General information, not professional tax advice. Check GOV.UK or speak to a qualified adviser for your situation.