If you started earning money in a new way during the 2025/26 tax year, a side business, your first rental property, dividends, crypto gains or income from abroad, there is a deadline that is easy to miss. You must tell HMRC by 5 October 2026 that you need to complete a Self Assessment tax return. This is a separate step from filing the return, and many people do not realise it exists until it is too late.
The reassuring news is that missing 5 October does not automatically mean a fine. The real reason to act now is practical: registering triggers a reference number that arrives by post and takes time, and you need it before you can file and pay. Leave it to autumn and you risk a scramble and interest charges. Here is who is caught and what to do.
In a nutshell
- Several kinds of new income trigger this. Self-employment, rent, dividends, crypto, foreign income and more.
- 5 October 2026 is a notify-by deadline, not a filing one. You are telling HMRC you have a return to complete.
- Act in July, not October. Your tax reference arrives by post and takes time.
- Being late is not an automatic fine. But it carries real risks worth avoiding.
- Registering is straightforward. Then you file and pay by 31 January 2027.
- Foreign income counts too. Including rent from a property back home.
1. Who has to register
You need to register for 2025/26 if, during that tax year (6 April 2025 to 5 April 2026), any of these applied:
- You were self-employed as a sole trader and earned more than £1,000 (the trading allowance).
- You received rental income over £1,000 gross, from a UK or an overseas property.
- You were a partner in a business partnership.
- You had dividends above the £500 dividend allowance, or other untaxed income from savings and investments.
- You made a capital gain above the annual exempt amount of £3,000, including from selling or exchanging cryptoassets.
- You had foreign income, such as rent, a pension or earnings from abroad.
- You have to pay the High Income Child Benefit Charge, where one partner has income over £60,000 and you receive Child Benefit.
If any of these is new for 2025/26, HMRC does not know about it unless you tell them. That is what registering does.
2. What the 5 October deadline actually is
This is the point people miss. 5 October 2026 is the deadline to notify HMRC that you have income to declare for 2025/26. It is not the deadline to file your return or pay your tax, those come later.
Once registered, the filing and payment deadlines are the familiar ones: a paper return by 31 October 2026, an online return by 31 January 2027, and any tax owed also by 31 January 2027. Registering by 5 October gets you into the system in good time to meet those dates.
It is also worth being clear that this is not the same as signing up for Making Tax Digital for Income Tax, a separate obligation for larger sole traders and landlords. Registering for Self Assessment is about telling HMRC you have a 2025/26 liability to report.
3. Why you should act in July, not October
When you register, HMRC issues you a Unique Taxpayer Reference, or UTR. This is the 10 digit number you need before you can file a return, and for most people it arrives by post.
HMRC says the UTR usually turns up around 15 days after you register, and can take up to 21 days, or longer during busy periods. The busiest period is December and January, exactly when late registrants are trying to file. If you register online you may see your UTR sooner in the HMRC app or personal tax account, but you should not count on it.
So the message is simple. Registering in July gives you months of breathing room. Leaving it until October or later risks a slow UTR and too little time to file and pay by 31 January. There is no downside to registering early.
4. What being late really costs
Here is where a lot of scaremongering happens, so let us be accurate. Missing the 5 October deadline does not automatically trigger a penalty. HMRC's position is that a failure to notify penalty only applies if you register after 5 October and do not pay all of your tax by 31 January. Register a little late but file and pay everything on time, and there is usually nothing to pay, because the penalty is based on tax left unpaid, and if none is unpaid the figure is nil.
That said, leaving it late carries three genuine risks:
- Not enough time. If your UTR arrives slowly, you may miss the 31 January filing and payment deadline, which brings its own late filing and late payment penalties.
- Interest. Interest runs on any tax paid after 31 January, regardless of when you registered.
- A bigger penalty if HMRC finds you first. If HMRC contacts you before you come forward, your disclosure becomes "prompted", and the minimum penalty is higher. The failure to notify penalty is a percentage of the tax due, and coming forward voluntarily keeps it as low as possible, potentially to nothing.
The takeaway is not to panic, but not to drift either. Register early and the whole issue disappears.
5. How to register, step by step
Registering is more straightforward than most people expect:
- Choose the right route. If you are newly self-employed, you register for Self Assessment and Class 2 National Insurance together. For another reason, such as rental or foreign income, you use the form for people who are not self-employed.
- Register online through your Government Gateway account, or set one up.
- Wait for your UTR, usually by post, then activate your online account.
- Keep your records ready: income, expenses, dates and any tax already paid.
- File and pay by 31 January 2027, yourself or through an accountant.
If this feels daunting, it is exactly the sort of thing an accountant handles as routine, from registering you to filing the return.
6. Foreign income and overseas property
This one catches a lot of people, especially if you have moved to the UK but kept assets at home. If you are UK resident, you generally have to report your worldwide income, including rent from a property abroad, such as a flat in Poland, through Self Assessment.
Reporting is not the same as paying UK tax on the full amount. A double taxation agreement between the UK and the other country can reduce or offset what is due, so you may not be taxed twice on the same rent. But the obligation to declare it stands, and that is what the 5 October deadline is about.
A worked example: Marek, a first-time landlord
Marek moved to the UK a few years ago and rents out a flat he still owns in Poland. In 2025/26 it produced about £7,000 of rent, his first year receiving it while UK resident. That is well over the £1,000 property allowance and it is foreign income, so he must register for Self Assessment and tell HMRC by 5 October 2026.
Marek registers in July. His UTR arrives by post about two weeks later, he activates his online account, and he has until 31 January 2027 to file and pay. Because a double taxation agreement applies, any Polish tax already paid on the rent is taken into account, so he is not taxed twice. Had he waited until October, a slow UTR could have left him filing in a panic in January. Acting early cost him nothing and removed the risk.
Things to watch out for
- Registering is not filing. They are two separate steps with different deadlines.
- The £1,000 allowances matter. Trading and property income under £1,000 gross may not need reporting at all.
- Crypto counts. Gains above the £3,000 annual exempt amount must be reported, even from swapping one token for another.
- Foreign income is reportable even if little UK tax is due. The obligation to declare does not depend on whether you end up paying.
How Ollen Services Can Help
Working out whether you need to register, and doing it correctly, is exactly the kind of thing we take off your plate. We will confirm whether your 2025/26 income means you must file, register you with HMRC, and prepare and submit your return so it is right and on time.
If you have foreign income, rental property or crypto in the mix, we are used to it, including the double taxation rules that stop you being taxed twice. We also support you in English and Polish.
Call us on 07513 491 259 or email hello@ollenservices.co.uk. The 5 October deadline is closer than it looks, and registering early makes the whole thing simple.