Yes, is the short answer. You certainly must sign up for Self Assessment with HMRC if you earned more than £1,000 through self-employment.  Once you register for Self Assessment, you will be given a Unique Taxpayer Reference number as proof that you are a self-employed taxpayer.  You may also have to pay National Insurance contribution payments which will go towards state benefits and the state pension that you will receive in future years.

It is possible to be employed in a PAYE job and to also work as a sole trader and, even though you might pay tax through your job (PAYE), you will still need to register the ‘trade’ for Self Assessment and declare both sets of income to HMRC in a Self Assessment Income Tax Return.