If you picked up any new untaxed income during the 2025/26 tax year, which ran from 6 April 2025 to 5 April 2026, there is a date you cannot afford to ignore: 5 October 2026. That is the deadline to tell HM Revenue and Customs that you need to complete a Self Assessment tax return. Miss it and you open the door to a failure to notify penalty, even before you have filed anything or paid a penny of tax.
This guide walks through who has to register, how to do it, the full run of deadlines that follow, and exactly what the penalties cost if you leave things too late. It is written for people who had a first taste of self employment, a side hustle, rental income, dividends or crypto gains in the last tax year and are not sure what HMRC expects of them.
Why 5 October 2026 is the date that matters
Registration and filing are two separate steps, and people trip up because they assume the January deadline is the only one. It is not. Before you can file a return online, HMRC has to know you exist as a taxpayer inside Self Assessment. That is what registration does. The rule is simple: if you had income to declare for a tax year, you must notify HMRC by 5 October following the end of that tax year. Because the 2025/26 tax year ended on 5 April 2026, your registration deadline is 5 October 2026.
Registering triggers HMRC to issue you a Unique Taxpayer Reference, or UTR, and set up your online account. Both can take a couple of weeks to arrive by post, so registering close to the deadline is risky. Leave enough time to receive your codes, activate your account and still file comfortably before the January cut off.
Who actually has to register
You generally need to register for Self Assessment for 2025/26 if any of the following applied to you and the income was not already taxed at source:
- You were self employed as a sole trader and earned more than 1,000 pounds in gross trading income.
- You had income from renting out property.
- You received dividends or other investment income that pushed you into needing to report it.
- You made gains on crypto assets, shares or other chargeable assets.
- You earned untaxed income from tips, commission, or casual freelance work.
- Your total income was over 150,000 pounds, or you or your partner claimed Child Benefit and income was over the High Income Child Benefit Charge threshold.
According to HMRC, you must register if you need to send a return and have either never filed before, or filed previously but did not need to send one for the prior year. You can check your position and start the process on the official page to register for Self Assessment. If you are unsure how much tax a given level of profit produces, our UK Self Assessment tax calculator gives you a quick estimate before you commit numbers to a return.
The 1,000 pound trading allowance
Not everyone with a little extra income has to register. HMRC offers a trading allowance of up to 1,000 pounds a year. If your gross trading income for 2025/26 was 1,000 pounds or less, you generally do not need to tell HMRC or file a return at all. The allowance covers casual services, small side jobs and hobby style income.
There are important exceptions. You cannot use the allowance if the income came from a company you control or a partnership you are part of. You must still register and file if you want to claim losses, pay voluntary National Insurance to protect your record, claim Tax Free Childcare, or claim Maternity Allowance. And once your gross trading income passes 1,000 pounds, the clock starts and you must register by 5 October in the following tax year. Even when you fall under the allowance, keep records of the income in case HMRC ever asks.
The key dates you cannot miss
Here is the full timeline for income earned in the 2025/26 tax year. Print it, pin it, set reminders.
| What | Deadline | Detail |
|---|---|---|
| Register for Self Assessment | 5 October 2026 | Notify HMRC you need to file for 2025/26 income. |
| Paper tax return | 31 October 2026 | HMRC must receive a paper return by 11:59pm this date. |
| Pay tax through your tax code | 30 December 2026 | Only if you file online and owe under 3,000 pounds and want it collected via PAYE. |
| Online tax return | 31 January 2027 | Submit your online return by 11:59pm this date. |
| Pay the tax you owe | 31 January 2027 | Balancing payment for 2025/26 plus any first payment on account. |
| Second payment on account | 31 July 2027 | Due only if payments on account apply to you. |
You can confirm every one of these on the official HMRC page for Self Assessment deadlines. Note that if you register after 5 October, HMRC may set an alternative deadline of three months from the date you notify them, but your tax is still due by 31 January 2027 regardless.
How to register with HMRC
The process depends on your situation. If you are newly self employed as a sole trader, you register for Self Assessment and Class 2 National Insurance at the same time. If you are registering for another reason, such as rental or investment income, you use the general Self Assessment registration route. In both cases you will need details such as your National Insurance number, address, and the date your income started.
Once you register, HMRC posts you a UTR, usually within about ten working days. You then set up your online Self Assessment account and receive an activation code, again by post, which you use to complete sign in. Only after all that can you file. If you registered before but did not need to file last year, you may simply need to reactivate your existing account. The order of operations is the reason 5 October matters so much: the paperwork chain takes time.
What the penalties really cost
Two different penalty regimes can bite. The first is the failure to notify penalty, which applies when you do not register on time and HMRC loses tax as a result. It is calculated as a percentage of the tax due and depends on whether the failure was careless or deliberate, and whether you came forward voluntarily. Telling HMRC before they contact you almost always reduces the penalty, sometimes to nothing, so acting early is your best protection.
The second is the late filing regime, which is fixed and unforgiving. According to HMRC, the structure is as follows: an initial 100 pounds penalty as soon as you miss the filing deadline, even if you owe no tax; after three months, daily penalties of 10 pounds a day up to a maximum of 900 pounds; after six months, a further penalty of 5 percent of the tax due or 300 pounds, whichever is greater; and after twelve months, another 5 percent or 300 pounds, whichever is greater.
Late payment carries its own charges on top: penalties of 5 percent of the tax unpaid at 30 days, at 6 months and at 12 months, plus interest on the outstanding amount. You can read the detail on the HMRC page covering Self Assessment penalties. The takeaway is blunt: the fixed 100 pounds arrives whether or not you owe tax, so a nil return filed late still costs you.
Payments on account explained
Once you are in the system, a feature called payments on account often catches first timers by surprise. If the tax you owe for a year is more than 1,000 pounds, and less than 80 percent of your tax was collected at source, HMRC asks you to make advance payments towards next year's bill. Each payment on account is half of your previous year's tax liability, and includes Class 4 National Insurance if you are self employed.
The two instalments are due by midnight on 31 January and midnight on 31 July. This means that in your first busy January you could face your full balancing payment for 2025/26 plus a first payment on account for 2026/27, which can be a nasty shock to cash flow. If your income later turns out lower than expected you can apply to reduce your payments on account, and if it is higher you settle the difference with a balancing payment the following January.
National Insurance for the self employed in 2025/26
Self employed people pay National Insurance through Self Assessment, but the rules have shifted in recent years. For 2025/26, most self employed people with profits above the small profits threshold do not have to pay Class 2 National Insurance: it is treated as paid to protect your entitlement to the State Pension and certain benefits. Those with profits below the threshold can still choose to pay Class 2 voluntarily to keep their record complete, and they do this through the tax return.
Class 4 National Insurance still applies to profits above the lower profits limit of 12,570 pounds, charged at 6 percent on profits between 12,570 pounds and 50,270 pounds, and 2 percent on profits above 50,270 pounds. If you want to see how Class 4 and income tax stack up on your sole trader profit, our UK sole trader tax calculator models both in one place.
What to do before you file
Do not wait for January. Start now by gathering your records for the whole 2025/26 year: invoices, bank statements, platform payout reports, rental statements, dividend vouchers and any crypto transaction history. Separate business income from personal, and note allowable expenses, because these reduce the profit you are taxed on. Then register by 5 October 2026 if you have not already, so your UTR and account are ready well before the deadline.
Registering early costs nothing and buys you peace of mind. It means that when 31 January 2027 arrives you are filing a return, not scrambling to get into the system while penalties accrue. If you had new income in the last tax year, treat 5 October 2026 as the real starting line, not the finish.