What is the SA100 tax return?
The SA100 is HMRC's main Self Assessment tax return. You use it to report your income, claim tax reliefs and any repayment due to you. Supplementary pages are completed and sent with it when your circumstances call for them. The 2026 version covers 6 April 2025 to 5 April 2026.
Figures checked against gov.uk on . General information, not tax advice.
One return, plus the pages you need
Everyone who files uses the SA100 for personal details, a summary of income and the tax calculation. Where your income does not fit the main form, you add supplementary pages. gov.uk says some are for particular types of taxpayer, such as employees and company directors, and others cover specific kinds of income, deductions or reliefs.
A sole trader typically adds the self-employment pages, and a landlord the UK property pages.
Online, software or paper
The same return can be filed on HMRC's own service, through commercial software, or on paper. The deadline depends on the route: 31 October 2026 for a paper return and 31 January 2027 for an online one, for the 2025/26 tax year.
Whichever route you take, the return is not final until HMRC receives it, and you can still change it afterwards within 12 months of the deadline.