Self-employment income that is not all collected through PAYE.
In short
An SA100 is the main UK Self Assessment tax return. After each tax year (6 April to 5 April) you report income, reliefs and tax already paid so HMRC can work out what you owe or are owed. Supplementary pages (SA102, SA103, SA105 and others) attach when they apply. Self Assessment Software prepares those pages, runs a combined calculation and files online — prepare free, pay from £39 when you file.
Who usually files an SA100?
HMRC tells you if you need to register for Self Assessment. Typical reasons include:
Rental income from UK property, alone or with other income.
Significant savings interest, dividends or other untaxed income above HMRC’s thresholds.
Many company directors and people with a partnership share (SA104) file an SA100.
The tax year the SA100 covers
UK tax years run from 6 April to 5 April. You normally file the return for that year by the following 31 January if you file online.
The 2025-26 tax year runs 6 April 2025 to 5 April 2026. Online filing is due by 31 January 2027.
Paper returns are due earlier (31 October). Most people file online. This product is an online SA desk.
See our Self Assessment deadline guide for penalties and planning.
HMRC forms
What sits with the SA100?
The SA100 is the main return. Supplementary pages attach only when they apply. The partnership’s own return is SA800 — separate from an individual’s SA100.
In our product
How Self Assessment Software handles the SA100
Same HMRC forms and box numbers — guided in a year hub, then calculation, reports and filing.
- Open a tax year Create your organisation, open the year, and tick what applies so only the right pages appear.
- Complete the pages SA100 and supplementary forms with HMRC box numbers on the fields. Partnerships use the SA800 pack alongside partners’ SA104.
- Preview tax Run the combined calculation and an SA302-style estimate before you submit.
- File when ready Prepare free. Pay for that tax year (or use a practice credit), then submit from the filing desk.
Handbook
Related guides
The partnership tax return — separate from each partner’s SA100.
31 January online and what happens if you are late.
Tax calculation summaries lenders often ask for.
How quarterly updates differ from the year-end SA100.
FAQs
SA100 — questions answered
What is an SA100 tax return?
The SA100 is HMRC’s main Self Assessment tax return for individuals in the UK. You use it after the tax year to report income, reliefs and tax already paid so HMRC can calculate what you owe or are owed.
Who needs to fill in an SA100?
People HMRC requires to be in Self Assessment — commonly sole traders, landlords, those with higher untaxed investment income, and many company directors. Simple PAYE-only employees often do not need one. HMRC’s letter or online account confirms if you must register.
What are SA100 supplementary pages?
Extra forms that attach to the SA100 when needed — for example SA102 for employment, SA103 for self-employment, SA105 for UK property, SA108 for capital gains and SA110 for the tax calculation. You only complete the pages that apply.
Is the SA100 the same as Self Assessment?
Self Assessment is the system. The SA100 is the main individual return form inside that system. Partnerships also use SA800 for the firm’s return and SA104 for each partner’s share on their SA100.
When is the SA100 due?
If you file online, HMRC must have the return by 31 January after the end of the tax year. Paper filing is due by 31 October. See our deadline guide for penalties.
Can I prepare an SA100 with Self Assessment Software?
Yes. Register free, complete the SA100 and any supplementary pages, preview the calculation, then pay when you file that tax year — Employment £39, Self-employed & property £49, or Partnership £69 (ex VAT). Accountants use filing credits for client returns.
Does an SA100 include bookkeeping?
No. The SA100 is the year-end tax return. Invoices and bank feeds belong in bookkeeping software such as FreeAgent, Xero or QuickBooks. Self Assessment Software is the return desk only.
Prepare free
Ready to prepare your SA100?
Start free, walk the same hub as the live desk, and pay only when you file that tax year.