You need to pay your own tax and NI with a self-assessment tax return, even if you're employed on a PAYE basis for some of your work.
Income tax on self-employed earnings is paid at the same rate as for PAYE employees, as set out in the following table. These rates apply from April 6, 2017:
||£11,501 - £45,000
||£45,001 - £150,000
But the NI rates for the self-employed are different. Most self-employed individuals pay Class 2 and Class 4 NI contributions (NICs).
For 2017-18, class 4 NICSs are paid at 9% on profits between £8,164 and £45,000, and 2% on profits above £45,000. Class 2 NICs are paid at £2.85 a week on profits above £6,025. Class 2 NICs are planned to be scrapped from April 2018. It's well worth checking the gov.uk website for all the information you need.