Part of our complete guide to Self Assessment.
Before you start
If you're not sure you actually need to file a return, check our guide on who needs to file self assessment first, there's no need to register if none of those situations apply to you. If you do need to, registration is the first, unavoidable step before you can file anything.
The deadline: 5 October
You must register by 5 October following the end of the tax year in which you need to start filing. The UK tax year runs 6 April to 5 April. So if you started self-employment, began renting out a property, or crossed into another filing trigger at any point in the 2026/27 tax year (6 April 2026 to 5 April 2027), you must register by 5 October 2027, even though your actual filing deadline isn't until 31 January 2028.
Key point: Registration and filing are two separate deadlines. Missing the October registration deadline doesn't usually trigger an automatic penalty on its own, but it eats into the time you have to actually prepare your return before the 31 January filing deadline, and failure-to-notify penalties can apply if tax ends up unpaid as a result.
Which form do you need?
- Form CWF1 — if you're registering as self-employed or a sole trader for the first time
- Form SA1 — if you need to file for another reason (rental income, high income, dividends, etc.) but aren't self-employed
- Form SA401 / SA402 — for partners joining a business partnership
All three are completed online through HMRC's website, you don't need to post anything in most cases.
What you'll need
- Your National Insurance number
- An email address and mobile number, to set up your Government Gateway account if you don't have one
- Your business start date (for CWF1) or the date the relevant income started
- Your personal details: full name, date of birth, address, and contact details
What happens after you register
HMRC posts you a Unique Taxpayer Reference (UTR), a 10-digit number that identifies you for tax purposes permanently. This typically takes up to 10 working days, longer if you're overseas. You'll then need to activate your Government Gateway account using an activation code sent separately, so leave time for two pieces of post to arrive, not one.
Once registered, HMRC will expect a tax return from you every year going forward, even in years with little or no tax to pay, until you formally tell them you no longer need to file.
Want this handled for you? Book a free consultation and we'll register you, chase your UTR, and get your Government Gateway account working, so the only thing left for you to do is hand us your numbers in January. See our first self assessment service for the full picture.
Frequently asked questions
What is the deadline to register for self assessment?
5 October following the end of the tax year in which you need to start filing. For example, if you became self-employed in the 2026/27 tax year (6 April 2026 to 5 April 2027), you must register by 5 October 2027.
How long does registration take?
HMRC says up to 10 working days to receive your Unique Taxpayer Reference (UTR) by post, longer if you're abroad. Start well before the deadline, not at it.
Do I need a Government Gateway account?
Yes, if you don't already have one from a previous HMRC service, you'll create one as part of registering. Keep the login details safe, you'll need them every year.
What if I've registered before but stopped, and now need to again?
If you already have a UTR from previous self-employment, you re-register using form CWF1 (self-employed) or SA1 (other income) rather than starting from scratch, quoting your existing UTR speeds things up.