The date, and how to work out yours
The tax year runs 6 April to 5 April. You must tell HMRC by the 5 October that falls after the end of the tax year in which you started trading. Started in June 2025, so inside the year that ended 5 April 2026, and your date is 5 October 2026.
It catches people because it is more than a year after the first job for anybody who started early in a tax year, and about six months after it for anybody who started late. It is not a rolling three months from your first invoice, which is what most people assume.
When you do not have to
If your gross trading income for the year is £1,000 or less, the trading allowance covers it and there is nothing to register or declare. Gross means everything that came in, before you take a single cost off it.
Go a penny over £1,000 and the whole thing is back in scope, so the safest reading of a borderline year is to add the income up properly rather than estimate it.
What registering actually is
One online form on GOV.UK. You get a Unique Taxpayer Reference in the post, usually inside a fortnight, and it is that reference which lets you file. There is no fee and no inspection, and being registered does not commit you to earning anything.
Register once. If you have filed a Self Assessment before, for any reason, you are re registering for self employment rather than starting from nothing, and the form is shorter.
What missing it costs
The penalty is for failure to notify, and it is worked out as a share of the tax you owed rather than a flat fine, so a quiet year costs little and a good year costs a lot. Telling HMRC before they come asking reduces it, sometimes to nothing.
The second cost is the one nobody counts: register after 5 October and HMRC sets you a later date for the return, 3 months from the date on its letter, but the tax is still due by 31 January. So a late registration in October still means paying in January, with interest on any payment you send after it.
The other dates in the same year
31 January is when you send the return and pay for the year that ended the previous 5 April, and a late return costs £100 even if there is no tax to pay. 31 July is the second payment on account, if you have one, and paying it late costs interest. 5 October is registration, and missing it can cost a share of the tax you owed, and nothing if there was none. Only the late return costs money with no tax to pay, which is the part that surprises people who had a bad year.
Lekhio works it out. You send it. Nothing goes near HMRC until you press yes.
Where HMRC says it: HMRC, Set up as a sole trader on GOV.UK; HMRC Compliance Handbook CH70000 onwards on failure to notify penalties; HMRC guidance on the trading allowance. Checked 2026-09-02. This is general information about the rules, not advice on your own position.