Deadline for 2016/2017 Paper Self-Assessment Tax Return to HM Revenue and Customs (HMRC) is 31 October 2017.

Electronically Submitted Returns Deadline

Failure to to meet this deadline will result in your only other option for your Self-Assessment Tax Return will have to be submitted electronically.
The Deadline for Online Tax Returns is the 31 January 2018.

Penalties Charged by HMRC

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.