A self-assessment is a tax return that individuals and sole traders in the UK complete in order to calculate how much tax they owe. Self-assessment forms are available online, and must be submitted by the end of January each year.
Individuals and sole traders can complete their self-assessment forms either themselves or with the help of an accountant. Once the form is completed, it is submitted to HMRC (Her Majesty’s Revenue and Customs).
How much tax you owe will depend on your income and any allowable expenses. If you have paid too much tax during the year, you will be entitled to a refund. Similarly, if you have not paid enough tax, you will need to pay HMRC the balance.