Self Assessment is a system HM Revenue and Customs (HMRC) uses to collect Income Tax.
Tax is usually deducted automatically from wages, pensions and savings. People and businesses with other income must report it in a tax return. If you need to send one, you fill it in after the end of the tax year (5 April) it applies to.
Many people find it complicated and time-consuming so it’s no wonder that almost 1 million people a year are fined for failing to return it to H M Revenue & Customs.