Self-assessment is a method of collecting tax with the idea being that you are responsible for completing a tax return each year if you need to and for paying any tax due for that tax year. It is your responsibility to tell HM Revenue & Customs (HMRC) if you think you need to complete a tax return.
Under self-assessment, certain taxpayers must complete a self-assessment return (also known as a form SA100) where they need to show all their income and capital gains as well as claim any applicable allowances and reliefs.
As a legal document, it’s important that you don’t omit any sources of income from the return and also to include any tax has been deducted at source.
Most employees pay their taxes through the PAYE system, however, if you are in receipt of any untaxed income (such as rental income) or your tax affairs are more complex, then a self-assessment return should be completed.