The SA100 is the main Self Assessment tax return form for an individual. It collects personal details, the income and reliefs that have no page of their own, student loan and High Income Child Benefit Charge information and the declaration, and it lists which supplementary pages are attached.
Who completes it
Anyone HMRC has asked to file a Self Assessment return for a tax year: the self-employed, partners, landlords, company directors with untaxed income, people with income over £150,000 or with Capital Gains Tax to pay, and anyone who owes the High Income Child Benefit Charge. Taxpayers in Making Tax Digital for Income Tax send quarterly updates and a final declaration instead of an SA100.
What it asks for
- Your details (page TR1): name, address, date of birth, National Insurance number and Unique Taxpayer Reference.
- Which pages you are attaching (TR2): a Yes or No for each supplementary page, from employment to residence.
- Income (TR3): bank and building society interest, dividends, state and other pensions, taxable state benefits and other income that has no page of its own.
- Tax reliefs (TR4): pension contributions, Gift Aid and Blind Person’s Allowance.
- Student loans, the High Income Child Benefit Charge and Marriage Allowance (TR5).
- Finishing the return (TR6 to TR8): tax already refunded, how a repayment should be paid, your tax adviser, any other information and the declaration.
Everything else goes on a supplementary page: SA102 employment, SA103 self-employment, SA104 partnership, SA105 UK property, SA106 foreign, SA107 trusts, SA108 capital gains, SA109 residence and SA101 additional information. The tax itself is summarised on the SA110.
Deadlines
For 2025-26 a paper return is due by 31 October 2026 and an online return by 31 January 2027, which is also the date the balancing payment and the first payment on account for 2026-27 fall due. A return filed late carries an automatic £100 penalty.
In Tax Optimiser
Open Self Assessment (Beta) in an individual’s menu, add the sections the return needs from the section picker, and enter each box with its HMRC reference shown beside it. The tax calculation updates live as you type, Return Preview shows the return on HMRC’s own forms, Download full SA100 return PDF produces the completed forms for a client to check, and the Submission section files the return to HMRC.
Read more: Self Assessment tax returns in Tax Optimiser and getting started with a client’s return.
