Register for Self Assessment by 5 October

If you started working for yourself, took on a side income, or began letting a property at any point between 6 April 2025 and 5 April 2026, HMRC needs to know by 5 October 2026. It is the most commonly missed deadline of the tax year, and the penalty is a percentage of the tax you end up owing.

Who has to register

Self-employed with turnover over £1,000 in the tax year (the trading allowance covers you below that).
Landlords with rental income over £1,000 (property allowance), or over £2,500 of untaxed income after allowable expenses.
Partners in a partnership, and the partnership itself.
People with untaxed income HMRC cannot collect through PAYE: dividends over your allowances, foreign income, capital gains, crypto disposals, high-income child benefit charge.
Anyone HMRC has sent a notice to file — even if you owe nothing.

You do not need to register again if you already file returns and your circumstances have not changed.

How to register

Self-employed, first time: gov.uk/register-for-self-assessment → "Register for Self Assessment and Class 2 National Insurance" (form CWF1). You need a Government Gateway account and your National Insurance number.
Not self-employed (landlord, dividends, other income): the SA1 form on the same page.
Partnership: SA400 for the partnership and SA401 for each partner.

The UTR takes time — this is why the deadline is early

After registering, HMRC posts your Unique Taxpayer Reference, usually within 10 working days (longer if you are abroad). You then need an activation code for the online service, which arrives separately. Neither is instant, and you cannot file without them. Registering on 4 October is legal but leaves you dependent on the post in January.

What happens if you miss 5 October

The penalty is for "failure to notify" and is based on the tax unpaid at 31 January, not a flat fee: up to 30% of it for a careless, unprompted disclosure, and more if HMRC contacts you first. In practice, if you register late but pay everything by 31 January, HMRC often reduces the penalty to nil — but that is discretion, not a right. Register as soon as you realise; a voluntary, unprompted disclosure is treated far better than waiting.

After registering: the dates that follow

31 October 2026 — deadline if you file on paper.
30 December 2026 — file online by this date if you owe under £3,000 and want it collected through your PAYE code.
31 January 2027 — online return for 2025/26, the balancing payment, and the first payment on account for 2026/27 if your bill is over £1,000.
7 November 2026 — quarterly update if you are in Making Tax Digital for Income Tax (combined self-employment and property income over £50,000).

Work out what you will owe

Registering does not tell you the size of the bill. Put your figures into the sole trader calculator (income tax + Class 4 NI + payments on account) or, if you let property, read what landlords can deduct first. Knowing the number in October is what makes January survivable.

Get the 2026/27 self-employed tax calendar (free PDF)

Rates and deadlines for 2026/27, checked against GOV.UK on 20 September 2026. England, Wales and Northern Ireland. Not tax advice — verify on gov.uk/register-for-self-assessment before relying on it.