HM Revenue and Customs (HMRC) deadline for 2016/2017 Paper Self-Assessment Tax Return is 31 October 2017. If you fail to meet this deadline then your Self-Assessment Tax Return will have to be submitted electronically as the deadline for Online Tax Returns is the 31 January 2018.
Any deadline missed will be amount to a Penalty. The minimum penalty is £100 if your return is up to 3 months late. You may be able to appeal this penalty if you have a reasonable excuse for submitting your Self-Assessment tax return late. Note that a penalty will apply even if you do not owe tax.