Register as soon as you can after starting your business. At the latest, you should register by 5 October in your business's second tax year.
If you start working as a sole trader between April 2016 to March 2017, you should register before 5 October 2017.
How to register
This depends on your circumstances.
New sole trader who's not sent tax returns before
Tell HM Revenue and Customs (HMRC) you want to be a sole trader by registering as a new business. You'll register for Self Assessment tax returns and Class 2 National Insurance at the same time.
You'll then be able to report your self-employment income through a tax return after the end of each tax year.
New sole trader who's sent tax returns before
You may have sent Self Assessment tax returns before as an individual, eg for rental or investment income.
You now need to register as self-employed and for Class 2 National Insurance using form CWF1. You can keep your existing Self Assessment account.
You need your 10-digit Unique Taxpayer Reference (UTR) from when you registered for Self Assessment, so HMRC can link your accounts.
Former sole trader who wants to start up again
You need to re-register using form CWF1. You'll need your UTR for the same reasons as new sole traders already in Self Assessment.
How long it takes
Allow 10 working days to complete the registration process because HMRC needs to post you an activation code. You'll be prompted for this when you first log in to your online account (also known as a Government Gateway account).
Allow 21 days if you're abroad.