Registering for Self Assessment is how you tell HMRC that you have income it does not collect through PAYE. It is a one-off step: once HMRC has set you up, you file a return each year until you tell it you no longer need to. The catch is the date. Registration has its own deadline, 5 October after the tax year ends, and it falls almost four months before the 31 January filing deadline.
The registration deadline
For the 2025/26 tax year (6 April 2025 to 5 April 2026), gov.uk's register for Self Assessment page says you must tell HMRC by 5 October 2026 if you need to send a return and either:
- you have never sent a return before, or
- you registered before but did not need to send a return for 2024/25.
Tell HMRC after the deadline and you could get a penalty, according to gov.uk. The rule is in section 7 of the Taxes Management Act 1970, which requires you to give notice of chargeability within six months of the end of the tax year. That is where 5 October comes from. If you have already passed the date, register today: a short delay costs less than a long one.
Who needs to register
You do not register just because you have a job. Employees whose tax is fully handled by PAYE usually have nothing to declare. You are more likely to need Self Assessment if, in the tax year, you:
- were self-employed and your trading income was more than £1,000 (gov.uk's set up as a sole trader page uses this figure);
- had gross property income over £2,500, or between £1,000 and £2,500 where gov.uk says to contact HMRC first (see the property allowance guide);
- had to pay the High Income Child Benefit Charge through a return;
- had other untaxed income or gains HMRC cannot collect through your tax code.
If you are unsure, use gov.uk's check if you need to send a tax return tool. It does not send your details to HMRC.
You may also want to register below the £1,000 line. gov.uk lists several reasons: proving you are self-employed (for example to claim Tax-Free Childcare), paying voluntary Class 2 National Insurance, or registering as a Construction Industry Scheme subcontractor.
Sole trader or just Self Assessment?
These are not two separate registrations. gov.uk states that you register as a sole trader by registering for Self Assessment, using your National Insurance number. There is one wrinkle: if you already have a Self Assessment record (for example because you rent out a property) and then start trading, you still need to register as a sole trader so that you are registered for Class 2 National Insurance.
What happens after you register
HMRC sends you a Unique Taxpayer Reference (UTR) by post, and you use it to activate your online account. Without a UTR you cannot file, so do not leave registration to the last week. Our how to find your UTR guide covers what to do while you wait and how to recover a number you have lost.
Once your account is active you will file a return each year for the tax year just ended. The next dates to diarise are in our Self Assessment deadline calendar: 31 October for paper, 30 December if you want a small balance collected through your tax code, and 31 January for online filing and the balancing payment.
A short registration checklist
- Work out the tax year the income relates to, and check whether 5 October for that year has passed.
- Have your National Insurance number to hand.
- Register for Self Assessment on gov.uk (as a sole trader if you are self-employed).
- Note the date you started trading or receiving the income, because HMRC will ask.
- Start keeping records straight away; see what records to keep for Self Assessment.
- Diarise the first payment date. In your first year you normally pay the full tax bill plus a first payment on account on 31 January, as explained in our guide to payments on account.
How ac-co helps
ac-co does not register you with HMRC; that step stays with you and your HMRC account. Once you have a UTR, ac-co drafts your SA100 from your connected bank, books and HMRC's own data, shows you the full tax computation and files only after you approve it. Start by checking the first-year self-employed guide if you are about to begin trading.