You need to register for Self Assessment if you have to send a tax return and didn’t send one last year.
How you do this depends on whether:
You register online even if you want to send a paper return.
When to register
Register by 5 October following the end of the tax year you need to send a tax return for. If you’re late, you won’t get a penalty as long as you pay your Self Assessment bill in full by the deadline.
The tax year is from 6 April to 5 April the following year.
It can take 20 working days (this is usually 4 weeks) to complete the registration process. Allow time so you can send your tax return by the deadline. This is 31 January, or 31 October (3 months earlier) for paper returns.