Depending on your income and circumstances, you may need to register for self-assessment. This may be the case even if most of your income is taxed through PAYE.

You will usually need to file a self-assessment tax return if you are self-employed as a sole trader and your gross income exceeds £1,000 before expenses. Partners in business partnerships must also submit a self-assessment tax return.

A self-assessment tax return may also be required if your total taxable income exceeds £150,000 in the 2026-27 tax year, although people with lower income levels can still fall within self-assessment depending on their circumstances. This often applies where there is other untaxed income, such as rental income, foreign income, savings or investment income, including dividends.

Other common reasons for filing include paying Capital Gains Tax after selling assets or being liable to the High Income Child Benefit Charge. Although some smaller amounts of income relating to online selling or property income may be covered by allowances, it is important to check the position carefully.

If you need to complete a tax return for the first time, HMRC must generally be notified by 5 October following the end of the relevant tax year. For the 2026-27 tax year ending on 5 April 2027, the registration deadline will usually be 5 October 2027. HMRC also provides an online checker to help determine whether you need to file a return that can be found at www.gov.uk/check-additional-income-tax.