The 31 January 2021 tax return filing deadline is not just the final date for submission of your Self-Assessment tax return but also an important date for payment of tax due to HMRC. This includes the payment of any balance of Self-Assessment liability for the 2019-20 plus the first payment on account due for the current 2020-21 tax year. The 31 January 2021 is also the payment date for any CGT due on residential property sales made before the rules changed on 6 April 2020.
If you missed the filing deadline then you will usually be charged a £100 fixed penalty if your return is up to 3 months late, regardless of whether you owed tax or not. If you do not file and pay before 1 May 2021 then you will face further penalties unless you have made an arrangement with HMRC.