Do I need to send a SA104 form if I do my Self Assessment online?
You only need to complete an SA104 form if you submit your Self Assessment tax return by post.
Why do HMRC need an SA104 form?
If you submit Self Assessment online, HMRC tailor your tax return to collect the relevant information from you. When you file your tax return using the paper form, partners in partnerships must include an SA104.
This is the section which deals with the income you earn as a partner in a partnership. If you’re a partner in multiple businesses, you’ll complete a separate section for each partnership.