If you earned income that was not taxed at source during the 2025/26 tax year (6 April 2025 to 5 April 2026), you likely need to register for Self Assessment with HMRC. The registration deadline is 5 October 2026, just days away, and missing it triggers automatic penalties even if you have no tax to pay.

The Key Deadlines

According to HMRC, if you need to complete a Self Assessment tax return for the first time, you must register by 5 October following the end of the tax year (HMRC, 2026). Once registered, you have two filing deadlines: 31 October 2026 for paper returns, or 31 January 2027 for online submissions. Any tax owed is also due by 31 January 2027, with a second payment on account due by 31 July 2027 if applicable.

Who Must Register

You must register for Self Assessment if any of the following applied during the 2025/26 tax year:

Self-employment and business income. You were self-employed as a sole trader and earned more than £1,000 (after deducting allowable expenses), or you were a partner in a business partnership.

Rental income. You received rental income from UK property exceeding £1,000 (after expenses), or from overseas property at any level.

Untaxed income over £2,500. This includes income from savings interest, dividends, or foreign income not already taxed in the UK. If your total untaxed income exceeded £2,500, or if you received untaxed tips and commission, you must file.

High earners. You or your partner earned over £50,000 and one of you claimed Child Benefit. HMRC recovers some or all of that benefit through the High Income Child Benefit Charge, reported via Self Assessment.

Capital gains. You sold assets (shares, a second property, valuable possessions) and your total gains exceeded the annual Capital Gains Tax allowance (£3,000 for the 2025/26 tax year).

Other situations. You were a company director (unless it was for a non-profit organisation and you earned less than £1,000), a trustee of a trust or registered pension scheme, or you had income from abroad that you need to claim UK tax relief on.

Read also: UK Self Assessment Deadline January 31: Gather Your Documents Now

If you are already registered from a previous year and your circumstances have not changed, you do not need to register again. But if you are newly self-employed, became a landlord, or crossed any of the thresholds above for the first time, registration by 5 October 2026 is mandatory.

How to Register

Registration is done online through HMRC’s Government Gateway. You will need your National Insurance number, personal details, and information about your income sources. HMRC issues a Unique Taxpayer Reference (UTR) number within 10 working days by post, and an activation code follows separately. Both are required to file your return, so late registration can leave you scrambling to meet the January filing deadline.

To register, visit the Self Assessment section on GOV.UK and follow the prompts for new users. If you miss the 5 October deadline, you can still register afterwards, but HMRC may charge an immediate penalty.

Penalties for Missing the Deadline

Failing to register by 5 October incurs an automatic £100 penalty, even if you have no tax to pay or later discover you did not need to file. If you then miss the 31 January filing deadline, further penalties apply: another £100 after three months, then £10 per day (up to £900) after six months, and percentage-based penalties after 12 months.

If you genuinely did not know you needed to register and can demonstrate a reasonable excuse, you may appeal the penalty. But HMRC’s position is clear: ignorance of the rules is not normally accepted as reasonable.

What If You Are Unsure?

Tax rules and allowances change each tax year. If you are uncertain whether your circumstances require Self Assessment, consult HMRC’s online tool (Check if you need to send a Self Assessment tax return) or speak to an accountant. General guidance is available from MoneyHelper and Citizens Advice, but specific cases benefit from professional advice, particularly if your income structure is complex.

Self Assessment is educational information only. Nexzoe is not authorised by the Financial Conduct Authority. For personalised tax advice, consult HMRC directly or engage a qualified accountant registered with a recognised professional body. Tax rules are correct as of October 2026; verify current rates and deadlines with HMRC before taking action.