How to Register for Self Assessment and Get a UTR (2026/27)

This guide is general information, not advice.

This is for anyone who needs to send a Self Assessment tax return for the first time, or who registered years ago and needs to start again: a new sole trader, a new partner, a landlord, someone with untaxed income. It walks the exact route HMRC’s own service takes you down, in the order the screens appear.

By the end you will have told HMRC you need a return, know which form applies to you if you cannot do it online, and know what arrives in the post and when. What goes on the return itself, and the wider deadlines and penalties, are in the Self Assessment 2026/27 guide.

Before you start

Have these to hand:

• your full name, postal address (it can be outside the UK), date of birth and a daytime telephone number
• your National Insurance number, if you have one
• your reason for registering (self-employment, rental income, a pension and so on)
• for partners: the date your self-employment started, the date you joined the partnership, and the partnership’s name, address and UTR
• your old Unique Taxpayer Reference (UTR) if you have ever been in Self Assessment before - you will be reactivating, not registering
• Government Gateway sign-in details if you already have them; if not, you create them when you first sign in

Step-by-step

1. Check you actually need a return. GOV.UK’s who must send a tax return page says you must if, in the last tax year, you were self-employed as a sole trader and earned more than £1,000 before reliefs, were a partner in a business partnership, had Capital Gains Tax to pay, had to pay the High Income Child Benefit Charge outside PAYE, or are an off-payroll worker repaying a student loan; untaxed rent, tips, interest, dividends or foreign income may also mean one. If in doubt, run HMRC’s check if you need to send a tax return tool first.

2. Open HMRC’s Check how to register for Self Assessment service and press Start now. The first screen asks why you need to send a return and gives three options: self-employed (for example a sole trader), a partner in a business partnership, or another reason (for example pension or property income). The second screen asks whether you sent a tax return last year: yes, no but I have registered before, or no, registering for the first time. The answers decide the route.

3. Self-employed, first time. The outcome page says you register for both Self Assessment and Class 2 National Insurance in one go. Press Start now and sign in, creating sign-in details if you have none. If you cannot register online, use form CWF1, which the page links, and post it to Self Assessment, HM Revenue and Customs, BX9 1AN, United Kingdom. Have everything ready: GOV.UK says you cannot save a partly completed CWF1.

4. Not self-employed, first time. The outcome page lists what the screen asks for: full name, postal address, date of birth, daytime telephone number, UK National Insurance number if you have one, and an explanation of why you are registering. Sign in to register online. If you cannot use the online service, fill in form SA1, print it and post it to the address shown on the form. This route does not register you for Class 2 National Insurance.

5. Partner in a partnership. The outcome page sends you to form SA401, which registers you for Self Assessment and Class 2 National Insurance. HMRC’s SA401 guidance says only the nominated partner can use the online version; any other partner registers by post. The nominated partner must also register the partnership itself using form SA400.

6. Registered before, but sent no return last year. Do not register again. The service’s reactivation page tells a self-employed person to sign in with the details used before and reactivate the account, which also covers Class 2 National Insurance; those who cannot use the online form send a CWF1 to the BX9 1AN address quoting their UTR. On the other-reason route you sign in to your tax account and reactivate, and you will need your UTR. If you sent a return last year you are already registered; HMRC adds that someone who became self-employed after registering still needs to register as a sole trader so that Class 2 is covered.

7. Wait for two letters. The first is your UTR, a 10-digit number that may just be called a tax reference. The registration service says it usually arrives by post in 15 working days, 21 days if you are abroad, longer in busy periods, and sooner in the HMRC app or personal tax account. The second, if you created a new online account, is an activation code, usually within 7 working days (21 abroad). When it arrives, sign in and enter it to activate Self Assessment. You can ask for a replacement code if it never turns up or you lose it.

8. Wait to be told to file. GOV.UK says HMRC will contact you by email or post telling you to complete a return before the deadline. You then file through the online service with your UTR and sign-in details; it may ask you to prove your identity with photo ID.

Deadlines and what happens next

Every GOV.UK page in this chain gives the same rule: tell HMRC by 5 October after the end of the tax year in which you first needed a return. The pages currently say 5 October 2026 for the year from 6 April 2025 to 5 April 2026; a first trading year of 2026/27 falls under the same rule a year on, so 5 October 2027.

Register after 5 October and the deadlines page says HMRC will send a letter or email with a different filing deadline of 3 months from the date on it; the registration service words it as 31 January 2027 or 3 months from the letter, whichever is later. Either way the money does not wait: tax is still due by 11:59pm on 31 January 2027 for the 2025/26 year.

The penalties page describes the late-registration consequence as a failure to notify penalty, charged if you register after 5 October and do not pay all of your tax bill by 31 January. It is based on the amount still left to pay, and you receive it within 12 months after HMRC gets your return. HMRC’s factsheet CC/FS11 sets the ranges as a percentage of that unpaid tax: non-deliberate 0% to 30%, deliberate 20% to 70%, deliberate and concealed 30% to 100%, with the lower minimums for a disclosure you make before HMRC prompts you. No penalty is charged where you had a reasonable excuse, the failure was not deliberate and you told HMRC without unreasonable delay once the excuse ended.

If your UTR has not arrived in 15 (or 21) days, use HMRC’s check when you can expect a reply tool. If you had one once and have lost it, the find your UTR page lists where it appears: personal tax account, HMRC app, old returns and HMRC letters.

Common mistakes

• Registering twice. An old UTR never expires. If you have ever been in Self Assessment, answer no, but I have registered before and reactivate; GOV.UK says filing without reactivating can delay your return.
• Using SA1 when you are a sole trader. SA1 is the not-self-employed form and does not cover Class 2 National Insurance. The self-employed route (or CWF1) does both.
• Waiting for the UTR before doing anything else. The letter takes up to 15 working days; the activation code is a separate letter. Register early enough that both have arrived well before 31 January.
• Assuming late registration moves the payment date. It can move the filing date. The tax is still due on 31 January.

Worked example

You start trading as a sole trader on 1 June 2026 and expect to earn more than the £1,000 sole-trader threshold on the who must send a return page. Your first tax year is 2026/27, ending 5 April 2027. Applying the rule on the deadlines page, which currently shows the 2025/26 cycle (register by 5 October 2026, paper return by 31 October 2026, online return and payment by 31 January 2027), the 2026/27 cycle runs a year later: register by 5 October 2027, paper return by 31 October 2027, online return and payment by 31 January 2028. Register in June 2026 and both letters have arrived more than a year before that deadline.

Related reading

• Self Assessment 2026/27 - who must file, the deadlines in order and the penalty ladder.
• Self-employed first-year setup - what to do after registering: bookkeeping, expenses, Class 2 and Class 4.
• How to pay your Self Assessment bill - every payment method, the UTR-plus-K reference and the time each takes.