The SA1 form is the form that you need to use to register for Self Assessment with HMRC.
You have to register for Self Assessment if:
- you have any untaxed income (for example, from self-employment, renting out a property, etc.), usually over £1,000
- you want to claim a tax relief, for instance, SEIS/EIS
- you earned over £100,000 in the last tax year (including from full-time employment), etc.
You can download the SA1 on HMRC’s website, if you want to register by post.
However, you can also register for Self Assessment online, in which case you don’t need to worry about this form. The link is on the same HMRC page.
Once you’ve registered for Self Assessment, you’ll receive a UTR number – also by post. You’ll need your UTR to file your tax return.