If you need to file a Self Assessment return and have not filed before, or did not need to file for the previous year, you must register with HMRC by 5 October after the end of the tax year. Individuals register online or on form SA1, self employed people online or on form CWF1, partners on form SA401, partnerships on form SA400, and trustees through the Trust Registration Service. Registering produces a 10 digit Unique Taxpayer Reference (UTR), which you can find again in your personal tax account, the HMRC app or on earlier HMRC letters if it is lost.

When do you have to register?

Section 7 of the Taxes Management Act 1970 requires anyone chargeable to Income Tax or Capital Gains Tax who has not been sent a notice to file to notify HMRC within 6 months of the end of the tax year. In practice that is 5 October.

For the 2025 to 2026 tax year, gov.uk says you had to tell HMRC by 5 October 2026 if you needed to file and either had not sent a return before or had registered before but did not need to send a return for 2024 to 2025. Registering is how you tell HMRC.

If you register on or before 5 October 2026, the online filing deadline is 31 January 2027. If you register later, HMRC confirms your deadline by letter or email: gov.uk says it will be 31 January 2027 or 3 months from the date of the letter, whichever is later. The tax itself is still due by 31 January 2027.

Missing 5 October is not automatically penalised. gov.uk explains that a failure to notify penalty may arise if you register late and do not pay all your tax by 31 January, and it is based on the amount still unpaid. See failure to notify penalties. If you are not sure whether you need to register at all, start with who needs to file a tax return.

Which registration route applies to you?

Who Online By post
Individual who is not self employed (for example rental income, Capital Gains Tax, trust income) Yes, through your HMRC sign in Form SA1
Sole trader Yes Form CWF1
Partner in a partnership Only the nominated partner Form SA401
The partnership itself Yes, by the nominated partner Form SA400
Trustees of a taxable trust Trust Registration Service Paper SA900 is the return, not the registration

Individuals

gov.uk says you sign in to register online, may be asked to prove your identity with photo ID, and can save your progress. You cannot register online if you do not live in the UK; use form SA1 instead, giving your personal details and the reason you need to file, such as Capital Gains Tax or trust income.

Sole traders

Registering as self employed also registers you for Class 2 National Insurance, which gov.uk says helps you qualify for benefits like the State Pension. If you were already registered for Self Assessment for another reason and then started a business, gov.uk says you still need to register as a sole trader. Form CWF1 is the paper route, and it needs your existing UTR.

Partners and partnerships

Each partner who is an individual registers for Self Assessment and Class 2 National Insurance. Only the nominated partner can do this online; a partner joining an existing partnership who is not the nominated partner registers by post on SA401. The nominated partner separately registers the partnership, online or on SA400, giving the start date and accounting date. Limited partnerships and limited liability partnerships also register with Companies House; see our guide to limited liability partnerships.

Trustees

Trustees use the Trust Registration Service. HMRC guidance says a UK trust liable to Income Tax or Capital Gains Tax must register, as must a trust that needs a UTR, and a failure to register can lead to a £5,000 penalty. The deadline depends on the trust:

  • a taxable trust created on or after 6 April 2021 registers within 90 days of becoming liable for tax;
  • a trust created before 6 April 2021 that becomes liable for Income Tax or Capital Gains Tax for the first time registers by 5 October after the tax year in which that happens;
  • a trust created before 6 April 2021 that has been liable before registers by 31 January after the tax year.

Section 7(2) applies the same notification rule to the trustees of a settlement.

What if you registered before but stopped filing?

You may need to reactivate your existing record rather than register again. gov.uk says you sign in to reactivate online, or use form CWF1 with your UTR. It warns that filing a return without reactivating an existing account can delay the return. If you did not use the online service last year, you create a tax account and sign up for Self Assessment; an activation code usually arrives by post within 7 working days (21 if you are abroad).

How long does it take to get a UTR?

gov.uk gives these timescales:

  • registered online: the UTR is in your personal tax account within 72 hours, and you get a personal tax account as part of registering if you do not already have one;
  • registered by post: usually 15 working days (21 if you are abroad), longer in busy periods;
  • partners and partnerships registering on paper: HMRC usually makes contact within 15 days, and you can check expected reply times after 3 weeks.

Do not wait for the UTR before acting. The notification duty is met when you register, so register before 5 October even if the UTR will arrive afterwards.

What should you do if you have lost your UTR?

gov.uk describes the UTR as a 10 digit number, sometimes just called the tax reference. To find an individual UTR, look:

  • in your personal tax account;
  • in the HMRC app;
  • on previous returns and other HMRC documents, such as notices to file or payment reminders.

For a partnership or trust, the UTR appears on earlier returns and HMRC documents for that organisation. If it cannot be found, contact HMRC. A company’s Corporation Tax UTR is separate from a director’s personal UTR; it can be requested online and HMRC sends it to the registered office address held by Companies House. If an accountant acts for you, they can also see the UTR once authorised through the agent services account.

What does a typical registration look like?

Illustration only. Sam started letting a flat in June 2025. Net rental profit for 2025 to 2026 was £4,000, which is over the £2,500 limit for reporting without a return. Sam has never filed, so Sam needed to register online (as an individual, not as self employed) by 5 October 2026. The UTR appears in the personal tax account within 72 hours, and the return for 2025 to 2026 is due online by 31 January 2027 along with the tax.

If Sam only realises in November 2026, Sam should register immediately. HMRC will set the filing deadline, and as long as the tax is paid by 31 January 2027 there is no unpaid tax for a failure to notify penalty to be based on.

What are the common registration mistakes?

  • Using the wrong route. A landlord who registers as self employed, or a sole trader who uses SA1, can end up with the wrong record and a delayed UTR.
  • Relying on a company UTR. A director with personal income to report needs a personal UTR as well.
  • Registering twice. If you have a UTR from earlier years, reactivate rather than starting again.
  • Leaving it until January. Paper registrations can take 15 working days or more, and filing late triggers its own penalties. See Self Assessment deadlines and penalties.

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Sources

Reviewed by Christine Anderson, Broker, Solar Protect. Last reviewed 8 October 2026. Part of Accountapedia, the Solar Protect guide to UK tax and accounting. This page is general information, not personalised advice.