YF Accounting8 min read
You register for Self Assessment through the free HMRC service on GOV.UK, and if you started earning untaxed income during the 2025/26 tax year, you must tell HMRC by 5 October 2026. The process is free, takes around ten minutes, and ends with HMRC issuing you a Unique Taxpayer Reference (UTR), which is the number you need before you can file a tax return or pay anything.
Most people who get caught out do not do so because the process is hard. They get caught out because nobody told them the deadline exists. Below is the whole thing in plain English: who has to register, what you need to hand, the steps, and what it costs if you leave it too late.
Who has to register for Self Assessment
You need to register if, at any point in the 2025/26 tax year (6 April 2025 to 5 April 2026), any of the following applied to you:
- You were self-employed as a sole trader and earned more than £1,000, before taking off any expenses.
- You were a partner in a business partnership.
- You had Capital Gains Tax to pay on something you sold or gave away.
- You had to pay the High Income Child Benefit Charge and do not pay it through PAYE.
- You are an off-payroll worker repaying a student or postgraduate loan.
You may also need to register if you had untaxed income of other kinds, such as rent from a property, tips and commission, income from savings, investments and dividends, or income from abroad. If you are not sure whether any of this applies to you, our guide on whether you need to file a tax return walks through the tests one by one.
There are also good reasons to register voluntarily. You can file a return to claim certain Income Tax reliefs, to prove you are self-employed when claiming Tax-Free Childcare or Maternity Allowance, or to pay voluntary Class 2 National Insurance so you protect your State Pension record.
The 5 October deadline
HMRC must be told by 5 October following the end of the tax year in question. So for income you earned between 6 April 2025 and 5 April 2026, the deadline to register is 5 October 2026.
That deadline applies if you have never sent a tax return before, or if you registered previously but did not need to send a return for the 2024/25 tax year. In that second case, your Self Assessment record may have been closed down, and you will need to reactivate it rather than start from scratch. Filing without reactivating the account first is one of the quiet ways people end up with a delayed return.
If you register late, HMRC will write to you with a different filing deadline, usually three months from the date on their letter or email. That does not buy you extra time to pay. Any tax you owe for 2025/26 is still due by 31 January 2027, whatever date your return is due.
What you need before you start
- Your National Insurance number. You cannot register without one, so apply for a number first if you do not have one.
- Your date of birth and home address, plus the date you started trading or first received the income.
- A description of what your business actually does, and your business name if you trade under one.
- A Government Gateway account. If you do not already have one, you will create it as part of the process.
How to register, step by step
1. Check how you need to register
Start on the GOV.UK page Check how to register for Self Assessment. It asks a few questions and points you to the right route, because how you register depends on why you need to. A sole trader, a landlord and a company director with a dividend problem all take slightly different paths.
2. Sign in or create a Government Gateway account
You will be asked to sign in with your Government Gateway user ID, or to set one up. Keep the user ID and password somewhere safe. You will need them every year, and recovering a lost login from HMRC is far more painful than storing it properly in the first place.
3. Give HMRC your details
Fill in your personal details, the date your self-employment or untaxed income started, and what you do. If you started trading part way through the year, use the real start date rather than rounding it to the start of the tax year.
4. Wait for your UTR to arrive
HMRC then sends you a ten digit Unique Taxpayer Reference. It comes by post, so build in time for it. This is the single biggest reason not to leave registration until the end of September: the deadline is not just about clicking submit, it is about having a working Self Assessment record in time to actually file.
5. Activate your account and file
Once the UTR and any activation code arrive, sign in, activate the Self Assessment service, and you are ready to file. You can submit your 2025/26 return any time from 6 April 2026 onwards. You do not have to wait until January, and filing early tells you what you owe long before you have to pay it.
If you are already registered but have now gone self-employed
This one catches people out. If you already file a tax return for another reason, say rental income or a high income child benefit charge, and you have now started trading as a sole trader, you still need to register again as a sole trader. That second registration is what gets you into the Class 2 National Insurance system, which is what builds your entitlement to the State Pension and certain benefits. Skip it and you may quietly build gaps in your NI record.
The deadlines that follow registration
| Date | What is due |
|---|---|
| 5 October 2026 | Tell HMRC you need to file for 2025/26 (register for Self Assessment) |
| 31 October 2026 | Deadline for a paper 2025/26 tax return |
| 30 December 2026 | File online if you want the tax collected through your PAYE tax code |
| 31 January 2027 | Online return deadline, and the date your 2025/26 tax must be paid |
| 31 July 2027 | Second payment on account, if you make them |
That last line surprises a lot of first-time filers, because the January bill can arrive with a second payment attached. Our guide to payments on account explains how they work and how to reduce them.
What it costs if you get it wrong
Registering late is not automatically fined on its own. The problem starts if you register after 5 October and have not paid all of your tax by 31 January, because HMRC can then charge a failure to notify penalty based on the amount still outstanding.
Filing late is charged separately and adds up quickly:
- An initial £100 penalty, even if you owe no tax at all.
- After 3 months, £10 a day for up to 90 days, so a maximum of £900.
- After 6 months, a further 5% of the tax due or £300, whichever is greater.
- After 12 months, another 5% or £300, whichever is greater.
Paying late is charged on top of all that, at 5% of the unpaid tax at 30 days, again at 6 months, and again at 12 months, with interest running the whole time. A return that is a year late with a modest bill behind it can easily cost more than the tax itself.
One more thing for 2026: Making Tax Digital
If your combined self-employment and property income is over £50,000, you are now inside Making Tax Digital for Income Tax, which started on 6 April 2026. That means digital records and quarterly updates through compatible software, rather than one annual return typed in from a shoebox of receipts. Registering for Self Assessment is still the first step, but it is worth knowing what is coming next. We cover it in full in our post on Making Tax Digital for Income Tax.
Frequently asked questions
How long does it take to get a UTR number?
HMRC posts your Unique Taxpayer Reference to you once your registration is processed, so allow a couple of weeks rather than assuming it will land the next day. Times vary depending on how busy HMRC is, and they get busier the closer you get to a deadline. If you are approaching 5 October or 31 January, register now rather than waiting, because you cannot file without the UTR.
Do I need to register if I earned under £1,000?
Usually no. If your gross self-employed or casual trading income for the tax year was £1,000 or less, the trading allowance covers it and there is nothing to report. Two things to watch: the £1,000 is your income before expenses, not your profit, and it is a single allowance across all your side ventures rather than one per activity. You can still register voluntarily if you want to pay Class 2 National Insurance or prove you are self-employed.
What happens if I miss the 5 October deadline?
Register as soon as you realise. HMRC will send you a revised filing deadline, normally three months from the date on their letter, but your tax is still payable by 31 January. A failure to notify penalty only bites if tax remains unpaid after that date, so the fastest way to limit the damage is to register, file, and pay the tax. Penalties can be appealed if you have a reasonable excuse.
Can I deregister if I stop being self-employed?
Yes. If you stop trading, or your circumstances change so that you no longer meet any of the criteria, you tell HMRC and they close your Self Assessment record. Do not simply stop filing, because HMRC will keep expecting a return and will keep issuing £100 penalties for the ones that never arrive. You will still need to file a final return covering the period up to when you stopped.
Can an accountant register me and file on my behalf?
Yes, and for most people it is the easier route. We register you, get you set up as our client with HMRC, keep track of the deadlines, and file the return for you, so you are not the one deciphering HMRC letters. Everything is a fixed monthly fee with one point of contact, and sole trader packages start from £35 a month. You can see how we work on our Self Assessment guide or just get in touch.
Registering for Self Assessment is one of those jobs that feels bigger than it is, right up until you miss the deadline and it genuinely becomes bigger than it is. If you have started something new this year and you would rather someone else handled the registration, the record keeping and the return, get in touch and we will get you set up properly from the start.
Founder of YF Accounting — a fixed-fee, fully digital accountancy practice in Manchester serving SMEs, sole traders and landlords across the UK. One point of contact, unlimited support, no surprise bills.
Related reading
The £60,000 Child Benefit Trap: How the High Income Charge Works in 2026/27
Companies House Identity Verification: What Every UK Director Must Do in 2026
Tax on Rental Income: What UK Landlords Pay in 2026/27This article is part of our Self Assessment guide. See the full topic for related reads.