Why the 5 October deadline is easy to miss

Most people assume HMRC will tell them when to file a tax return. In practice, the responsibility sits with you. If you start freelancing in June, or let out a flat in September, nobody sends a reminder. You are expected to work out whether self assessment applies and tell HMRC yourself.

The critical date is 5 October following the end of the tax year in which your new income started. The UK tax year runs from 6 April to 5 April, so if you began self-employment in, say, August 2025, you need to notify HMRC by 5 October 2026. Miss that date and you can face a failure to notify penalty, even if you owe no tax.

The good news is that registering is straightforward, and there is plenty of support if you are unsure. The trick is knowing whether one of the common triggers applies to you.

The main triggers for registration

Self assessment is not just for the self-employed. HMRC expects a return from anyone whose tax affairs are not fully dealt with through PAYE. The most frequent reasons include:

• Sole trader income — you are trading and your gross income is more than £1,000 in a tax year.
• Rental income — you receive more than £1,000 a year in gross rent, whether from a property, a room, or a holiday let.
• Company directors — being a director does not automatically mean a return, but it often does if you receive dividends, use a company car, or have untaxed benefits.
• Untaxed income — savings interest above your personal savings allowance, dividends beyond the dividend allowance, or foreign income.
• Capital gains — selling shares, a second property or other assets where a gain exceeds the annual exempt amount.
• High Income Child Benefit Charge — if you or your partner claim Child Benefit and one of you has adjusted net income above the threshold, you need a return to pay the charge or claim relief.
• Pension and Gift Aid relief — higher or additional rate taxpayers claiming extra relief on contributions or donations often need to do it through a return.

There are also cases where HMRC simply writes to you and asks for a return. If that happens, you must file one, regardless of how small your income is.

The £1,000 allowances that decide a lot

Two allowances do a great deal of heavy lifting here. The trading allowance lets you earn up to £1,000 in a tax year from occasional trading — selling crafts, tutoring, driving work — without needing to register, provided you do not claim expenses. The property allowance works the same way for rental income.

Cross the £1,000 mark, however, and the picture changes. You generally need to register and declare the income, though you can still choose to deduct the allowance instead of actual expenses if that leaves you better off. Many people mistake these allowances for a tax-free bonus. They are not: they are a simplified way of calculating profit, and going over them brings you into the system.

Situations that catch people out

A few scenarios are notoriously overlooked:

• Income from a side hustle that grew quietly over a couple of years.
• Savings interest over £10,000 in a year, which HMRC cannot collect through PAYE.
• Letting a room or a driveway where the income exceeds the relevant relief.
• Selling cryptocurrency or shares where gains exceed the annual exempt amount.
• Working from abroad for part of the year, which can affect your residence position.
• Acting as a landlord through a joint arrangement, where the split of income matters.

If any of these sound familiar, check before the deadline rather than after. Amending a return is far simpler than dealing with a compliance letter.

How to register and what happens next

Registration is done through your Government Gateway account. You will need your National Insurance number and details of your income sources. Once registered, HMRC posts you a Unique Taxpayer Reference, usually within ten working days, though allow longer if you are overseas.

From there, the deadlines are fixed:

• 31 October — paper returns for the previous tax year.
• 31 January — online returns, plus payment of any tax owed.
• 31 July — second payment on account, where applicable.

Payments on account catch out plenty of first-timers. If your tax bill exceeds £1,000, HMRC normally asks for half of next year's expected bill in advance, which can make the first January payment feel unexpectedly large. A good accountant will flag this early so you can set money aside.

Do you need to keep filing once you're in?

Not necessarily. If your income changes and you no longer meet any of the criteria, you can ask HMRC to remove you from self assessment. Keep your records for at least five years after the 31 January filing deadline in case of a query.

If you are genuinely unsure whether you should be registered, the safest approach is to check with HMRC or a qualified accountant before the 5 October deadline. A short conversation now is far cheaper than a penalty later — and for many small business owners, it turns out to be the moment they finally get their tax affairs properly organised.