Hi,
Where your turnover from self employment exceeds £1000 or you are in receipt of untaxed income of £2500 or more and there is tax liability and you need to register for Self Assessment.
If you are self employed, you would register for Self Assessment as self employed, as you may have National Insurance to pay or you may want the opportunity to pay class 2 National Insurance contributions to go towards your State Pension.
Check how to register for Self Assessment
Thank you.