The deadline to register for Self Assessment is 5 October. Who needs to register for the 2025/26 tax year, how to do it, and what happens if you miss it.
If you started earning money outside your normal payslip during the last tax year, a side business, freelance work, rental income, or a chunk of untaxed income, there is a date you really do not want to drift past. The deadline to register for Self Assessment is 5 October, and for income you earned in the 2025/26 tax year that means 5 October 2026. Register in time and the rest of the process is calm. Miss it and the penalties are tied to the tax you owe, which is where it gets expensive.
This is a day-of guide. If today is the deadline, you can still get this done, because registering is quicker than most people fear. Here is who needs to do it, how, and what happens if you are late.
Registering for Self Assessment tells HMRC that you have income they are not already taxing through PAYE, and that you will file a tax return to declare it. In return, HMRC sets you up in the system and issues your Unique Taxpayer Reference, the ten-digit UTR you will need to file. Without a UTR you cannot submit a return, which is why registration has to come first and has its own earlier deadline.
The registration deadline and the filing deadline are two different things. Registration is 5 October. The online filing deadline for the same tax year is the following 31 January, so for 2025/26 income that is 31 January 2027. Registering now does not mean filing now. It just opens the door so you can file later.
You generally need to register for the 2025/26 tax year if any of these apply:
If your only income is a normal PAYE job and tax is already handled through your payslip, you usually do not need to register at all.
There is a simple floor for side income. If your gross self-employed or casual income for the year is £1,000 or less, the trading allowance usually means you do not have to register or file. This is the rule that keeps the occasional eBay seller or one-off bit of freelance work out of the system. The figure is gross income before expenses, not profit, so measure it on what came in.
Registration happens on GOV.UK, and which route you take depends on why you need to file.
The part that trips people is the ten-working-day wait for the UTR. That is exactly why the registration deadline sits months before the filing deadline. It gives the post and the paperwork time to catch up before you have to actually submit anything.
Missing the registration date is not, by itself, an instant fixed fine in the way that filing late is. The real risk is the failure to notify penalty. If you register late and then end up paying your tax late as a result, HMRC can charge a penalty based on a percentage of the tax you owe. For an unprompted disclosure the percentage can be reduced significantly, but for a deliberate or very late one it can reach up to 100 per cent of the tax due.
The practical takeaway is simple. If you have missed the deadline, do not let that become a reason to delay further. Register straight away. The sooner you register and pay what you owe, the lower any penalty, because HMRC looks at how prompt and how voluntary your disclosure was. A late registration that is sorted quickly and paid on time often ends up with no penalty at all.
Say you drove for a delivery platform in your spare time during 2025/26 and earned £6,500 gross on top of your main job. That is well above the £1,000 trading allowance, so you need to register. You register in early October 2026, your UTR arrives mid-month, and you file your return in January 2027 declaring the £6,500 less your allowable costs such as mileage. You pay the tax due by 31 January. Because you registered and paid on time, there is no failure to notify penalty and no late-filing penalty. The whole thing cost you nothing beyond the tax you genuinely owed.
Now imagine you ignored it until the following summer. You would face a late-filing penalty, interest on the unpaid tax, and a possible failure to notify penalty on top. Same income, same tax, far more pain, purely from missing the dates.
Registration goes faster if you gather a few things first, so you are not stopping halfway to dig out a number. Have your National Insurance number, your personal details, the date you started your self-employment or first received the income, and a rough idea of what you earned. If you are registering as self-employed you will describe the nature of the work, so have a short plain description ready, for example delivery driver, freelance designer, or private tutor.
You will also create or sign in to a Government Gateway account during the process. If you already have one from checking your tax code or State Pension, use the same one rather than making a second, because duplicate accounts are a common cause of confusion later. If you are creating a new account, keep the user ID and password somewhere safe straight away, because you will need exactly the same login to file your return in January.
One more thing worth knowing: registering also sorts out your National Insurance position if you are self-employed. The self-employment route signs you up for Class 2 and Class 4 National Insurance at the same time, so you do not have to do anything separately for that. Class 2 builds your entitlement to the State Pension and certain benefits, and Class 4 is the profit-based charge collected through your return. Getting registered through the right route means both are handled automatically rather than leaving a gap in your National Insurance record that you have to chase up later.
If you genuinely cannot tell whether your situation requires a return, GOV.UK has a short questionnaire that checks for you, and it is worth two minutes of your time. It is better to confirm than to assume you are exempt and discover otherwise after the deadline. If the tool says you do need to register, do it the same day while you are thinking about it.
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You must register for Self Assessment by 5 October following the end of the tax year in which you had the new income or liability. For income in the 2025/26 tax year, which ended on 5 April 2026, the deadline to register is 5 October 2026.
Register online through GOV.UK. If you are newly self-employed you use the CWF1 route, and for other income you use form SA1 or register through your personal tax account. HMRC then sends your Unique Taxpayer Reference, usually within about ten working days.
Registering late is not an automatic fine on its own, but if you then pay your tax late, HMRC can charge a failure to notify penalty based on the tax you owe, up to 100 per cent of it. Registering as soon as you can keeps any penalty as low as possible.
If your gross self-employed or casual income is £1,000 or less in the tax year, the trading allowance usually means you do not need to register or file. Above £1,000 gross, or if you have other untaxed income, you generally need to register.
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A note on financial advice: NetPay UK calculates take-home pay based on official HMRC tax rules. This article reflects rules in force at the time of publication (5 October 2026). Tax rules change. For complex situations, consult a qualified UK accountant or visit gov.uk/income-tax.
