Self Assessment: what UK small businesses should do before 5 October 2026

The first big Self Assessment date for the 2025–26 tax year is close. HMRC says people who need to complete a return and are new to Self Assessment, or who were registered before but did not need to send a return for 2024–25, must tell HMRC by 5 October 2026. For sole traders and small-business owners, the practical job now is not to rush a tax return. It is to confirm whether a return is required, make sure the registration status is correct and start organising the records and cash plan for the deadlines that follow.

The timing is especially useful this year because HMRC launched an improved registration service on 9 September 2026. HMRC says customers using the new service can register through their Personal Tax Account, with pre-populated information, save-and-return functionality and confirmation by email or text. It also says their Unique Taxpayer Reference (UTR) should appear in the online account within 72 hours, instead of waiting up to 15 days for it to arrive by post.

Who should check the 5 October deadline?

Start with HMRC’s own eligibility checker rather than assuming every side income or new business automatically means a tax return. GOV.UK says you must send a return for 2025–26 if, for example, you were self-employed as a sole trader and earned more than £1,000 before expenses, were a partner in a business partnership, had certain Capital Gains Tax to report, or need to pay the High Income Child Benefit Charge and are not paying it through PAYE. You may also need a return for untaxed income such as property income, tips or commission, savings interest, dividends or foreign income.

If HMRC has written to you asking for a 2025–26 tax return, follow the deadline in that notice. If you believe you no longer need to send a return, contact HMRC rather than simply ignoring it; HMRC’s current guidance says you should continue to meet Self Assessment obligations until it confirms that a return is no longer required.

What exactly must happen by 5 October?

The 5 October date is about telling HMRC that you need to complete a return. GOV.UK says this applies if you have not sent a Self Assessment return before, or if you were registered in the past but did not need to send a return for 2024–25. New customers can use HMRC’s registration service. If you have registered before but did not file last year, HMRC says you may need to reactivate your Self Assessment account rather than register from scratch.

That distinction matters because the 5 October deadline is often confused with filing and payment. It is not the deadline for the normal online return, and it is not the date the 2025–26 tax bill is generally due. Treat it as the point by which your Self Assessment status should be sorted so you can prepare and file properly.

The 2025–26 Self Assessment dates to put in your calendar

If you register after 5 October, HMRC says it will give you a different filing deadline in a letter or email, normally three months from the date of that notice. The 31 January 2027 payment deadline still applies. This is important because registering late does not automatically move the date when the tax itself must be paid.

What happens if you miss 5 October?

Do not confuse a late registration with the automatic £100 penalty that applies when a Self Assessment return itself is filed late. HMRC’s penalties guidance says that if you register after 5 October and do not pay all of your tax bill by 31 January, you may receive a failure-to-notify penalty. HMRC says that penalty is based on the amount of tax left unpaid. The sensible response to a missed 5 October date is therefore to register as soon as possible, then make sure the return and payment timetable is understood.

A five-step bookkeeping checklist for the next two weeks

1. Run HMRC’s online check to confirm whether you need a 2025–26 Self Assessment return rather than relying on last year’s position.
2. Confirm whether you are registering for the first time or reactivating an existing Self Assessment account.
3. Gather bank statements, sales records, invoices, expense receipts and records of other taxable income for 6 April 2025 to 5 April 2026.
4. Separate business and personal transactions so that allowable business costs are easier to review and duplicate or private spending is not accidentally treated as an expense.
5. Estimate the January cash requirement early. Filing earlier can give you more time to understand the bill even though the normal payment deadline remains 31 January 2027.

What if you are already using Making Tax Digital for Income Tax?

HMRC’s 9 September update says customers signed up to Making Tax Digital for Income Tax still need to submit their Self Assessment tax return and pay any tax due by 31 January 2027. MTD changes the in-year record-keeping and update process for those within scope, but it does not make the January Self Assessment obligation disappear for 2025–26.

The useful takeaway is simple: use 5 October as a readiness checkpoint. Confirm that HMRC knows you need a return, make sure your account is active, get the records into a workable shape and build the expected January tax payment into cash flow. That is a much easier position than discovering in late January that registration, records and payment funding all need attention at the same time.