Two separate questions hide inside this one. The first is whether you have to register at all, which turns on how much you earned from trading rather than on whether you think of yourself as a business. The second is when, which has a fixed answer: 5 October following the end of the tax year the income arose in. This page separates them, because people who assume they must register often need not, and people who assume they need not are often already late.
Whether you must register at all
Trading income of £1,000 or less in a tax year is covered by the trading allowance, and if that is all you had, there is usually nothing to register and nothing to report. Above that, registration follows. The threshold is on gross income, not profit, so £1,200 of takings against £400 of costs is over it even though the profit is not. It is also per tax year, so a good year followed by a quiet one can put you in and out of the requirement, and it does not apply if you are already filing for another reason.
When: 5 October after the end of the tax year
The deadline is 5 October following the end of the tax year in which the income arose. Start trading in June and the tax year ends the following 5 April, so registration is due by the 5 October after that, some sixteen months after the first invoice. It is generous, and it is missed constantly, because nothing prompts you. The return for that year is then due by the following 31 January, which is where the impression that January is the deadline comes from.
What registering actually does
It opens a Self Assessment record, issues a UTR, and sets you up for Class 2 and Class 4 National Insurance on self-employed profits. That last part is why registering as self-employed is a different form from registering for Self Assessment as, say, a landlord: the National Insurance treatment differs. Registering also starts the notices to file arriving, which is a good thing, because a return filed without a notice can be harder to place against the right year.
If you are late
Register anyway, immediately. Late registration on its own is not the expensive part; the penalties that bite are for filing late and paying late, and both of those deadlines run regardless. Registering promptly gets the UTR moving so the return can be filed on time even if the registration was not, which is the outcome that keeps the £100 filing penalty away.
Questions people ask about when do i need to register as self employed
Do I have to register if I only made a few hundred pounds?
Usually not. Trading income of £1,000 or less in a tax year is covered by the trading allowance, so there is nothing to report unless you are filing for another reason or want to preserve a National Insurance record.
Does the £1,000 apply to profit or to takings?
Takings. It is gross trading income before expenses, so a year with £1,200 in and £400 out is over the threshold even though the profit is £800.
I have a job as well. Does that change it?
Not the registration test, which is about the trading income. It changes the arithmetic on the return, because employment income uses up the personal allowance first and the trading profit is taxed on top of it.
What if I registered but earned nothing?
File the return anyway if HMRC has issued a notice to file. A nil return submitted on time costs nothing; the same return submitted late attracts the fixed penalty even though no tax is due.