Think you only need to worry about Self Assessment if you are a sole trader? Many limited company directors discover otherwise when HMRC writes to them about dividend income. 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. It is the personal tax return, distinct from the CT600 corporation tax return your company files.

The form covers the tax year ending 5 April. For online submissions, the filing deadline is 31 January following the end of the tax year. Paper returns must reach HMRC by 31 October. HMRC calculates your tax liability from the figures you provide. For 2025/26, the personal allowance stands at £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 attract 18% (residential property) or 24% (other assets) for higher rate taxpayers.

The SA100 is a multi-page document with distinct sections. Key pages include:

  • Page 1 (Personal details), your name, address, National Insurance number, and contact information.
  • Page 2 (Employment), income from PAYE employment, using 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 trading 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.

A common pitfall: if you are a director of a software company or a construction subcontractor operating through a limited company, you may assume you have no personal filing obligation. Wrong. Any dividends you draw personally must be reported on an SA100, even if your company pays corporation tax on its profits. Similarly, if you sell shares in your business, the capital gain is yours to declare, not the company's.

Filing the SA100 correctly matters because late submission triggers a £100 automatic penalty, with escalating charges after three months. For a manufacturing SME owner with rental property income alongside director dividends, getting the figures right on the SA100 can mean the difference between a modest tax bill and an unexpected HMRC surcharge. The form is your personal reporting tool, not your company's. Use it properly and you avoid fines and interest. Get it wrong and HMRC will come looking.