Do I need an accountant if I'm self-employed?

No, it's not a legal requirement — you can complete your own Self Assessment return and keep your own records.

In practice, most self-employed people find an accountant pays for themselves: through expenses and allowances you'd otherwise miss, through avoiding the penalties that come with a late or incorrect return, and through the time saved not wrestling with HMRC's online forms every January. It's particularly worth having one once your income becomes regular, you're close to a new tax band, you're deciding whether to move from sole trader to limited company, or Making Tax Digital is about to bring you into quarterly reporting. If you've been managing on your own so far and want a second opinion on whether you're paying more tax than you need to, that's a conversation worth having before your next return rather than after it's filed.