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 submit one, you fill it in after the end of the tax year (5 April) it applies to. The last tax year started on 6 April 2018 and ended on 5 April 2019.
Paper Returns must be in by 31 October following the end of the tax year. On-line returns may be submitted three months later – by 31 January. E.g. the Tax Return to 5 April 2019 is due by 31 October 2019 on paper and by 31 January 2020 if submitted on-line.
You’ll get a penalty if you need to send a tax return and you miss the deadline for submitting it or paying your bill. You’ll get a penalty of £100 if your tax return is up to 3 months late. You’ll have to pay more if it’s later, or if you pay your tax bill late. You’ll also be charged interest on late payments.