The SA100 is the standard Self Assessment tax return form used by HM Revenue & Customs (HMRC) to collect income tax and National Insurance contributions from individuals, sole traders, and partners in the UK.

For agency founders, the SA100 is the default tax return you must file if HMRC asks you to complete a Self Assessment. This typically applies if you operate as a sole trader, have untaxed income from dividends or savings, earn over £2,500 from renting property, or have capital gains to report. The form covers the tax year ending 5 April, and the filing deadline for paper returns is 31 October, while online submissions must reach HMRC by 31 January following the end of the tax year.

The SA100 is a multi-page document divided into sections. Key pages include:

  • Page 1 (Personal details) – your name, address, National Insurance number, and contact information.
  • Page 2 (Employment) – income from PAYE employment, including your P60 or P45 figures.
  • Page 3 (Self-employment) – your sole trader profit or loss, calculated from your business accounts. This is where you report agency income if you are not limited by shares.
  • Page 4 (Dividends and savings) – dividend income over the £500 allowance (2025/26) and interest from savings accounts.
  • Page 5 (Capital gains) – gains from selling assets like shares or property, using the annual exempt amount (£3,000 for 2025/26).
  • Page 6 (Tax reliefs) – pension contributions, charitable donations, and other allowable deductions.

The SA100 is not used for limited companies. If you trade through a limited company, you file a CT600 corporation tax return instead. However, if you take dividends from your agency as a director, you still need an SA100 to report that dividend income personally.

HMRC calculates your tax liability based on the figures you enter. For 2025/26, the personal allowance is £12,570. Income above that is taxed at 20% (basic rate), 40% (higher rate), or 45% (additional rate). Dividends are taxed at 8.75%, 33.75%, or 39.35% depending on your tax band. Capital gains are taxed at 18% (residential property) or 24% (other assets) for higher rate taxpayers.

When this matters for agency founders: Filing the SA100 correctly ensures you pay the right amount of tax and avoid penalties. Late filing incurs a £100 automatic fine, with escalating charges after three months. If you are a sole trader agency owner, the SA100 is your primary reporting tool. If you run a limited company, you still need it for personal dividend and capital gains reporting. Getting it right saves you money and stress.