Deadline for self-assessment registration
If you or your client became self-employed or started receiving income from property, during the 2016–17 tax year, the deadline to register with HMRC was last month on 5 October 2017. Penalties may arise should registration be late.
You should submit a form CWF1 for self-employment or form SA1 for non self-employed income to HMRC.
Take a look at the toolkits HMRC has, containing comprehensive guidance that address the most common errors seen in filed self-assessment tax returns, as well as a checklist of the key issues to look out for.