How to register for self assessment

Not everyone with extra income has to register. Trading income under the £1,000 trading allowance, for example, usually does not need reporting at all. GOV.UK asks you to check whether you need to send a tax return before you register, and our guide on whether you need to do a tax return walks through the common situations.

If you have filed before, you may not need to register again. GOV.UK says people who were registered previously may need to reactivate their self assessment account instead, and the service explains how.

GOV.UK's registration service starts by asking why you need to file, and the answer decides the form. Most people can do the whole thing online after signing in, and you can create sign in details the first time if you do not already have them. GOV.UK says you will be told when you sign in if you need to prove your identity, which usually involves photo ID such as a passport or driving licence.

If you have started working for yourself, you register as a sole trader rather than simply for a tax return. GOV.UK notes that this registration also registers you for Class 2 National Insurance contributions. You will be asked for your personal details and information about your self-employment, including when you started working for yourself. If you were already registered for self assessment for another reason, you still need to register as a sole trader once you start trading.

Partnerships work slightly differently. GOV.UK says the nominated partner must register the partnership itself, using form SA400, while each partner registers for self assessment and Class 2 National Insurance using form SA401.

Your Unique Taxpayer Reference is a 10-digit number that identifies you for self assessment. GOV.UK gives these timings:

• if you register online, the UTR will be in your personal tax account within 72 hours
• if you register by post, you will usually get it by post in 15 working days, or 21 days if you are abroad, and it may take longer during busy periods
• if you were registered before and are reactivating online, the activation code usually arrives within 7 working days, or 21 if you are abroad

Keep the UTR somewhere safe. You need it when you deal with HMRC about your return, and HMRC uses it, followed by the letter K, as your payment reference.

GOV.UK says you must tell HMRC by 5 October if you need to complete a tax return for the previous tax year and have not sent one before, or registered before but did not need to send one for 2024 to 2025. For income received in 2026 to 2027, the usual pattern means registering by 5 October 2027, ready for an online return due by 31 January 2028. GOV.UK has not yet published those dates for 2026 to 2027, so check them nearer the time.

Registering late is better than not registering at all. If you register after 5 October, HMRC sends you a letter or email with a filing deadline of 3 months from its date, though the tax is still due by the usual 31 January. For partners registering late, GOV.UK says the return is due on 31 January or 3 months after the letter, whichever is later. A penalty for failing to notify can apply, and GOV.UK says it is based on the amount still left to pay. The self assessment penalties guide explains how this works.

GOV.UK's registration page points agents to a separate route for registering a client, so an accountant acting for you may be able to do it. GOV.UK is clear that you remain legally responsible for your own tax even when an agent deals with HMRC for you. Please treat this as general information, not tax advice. If you would like help setting up, our sole trader accountants page explains what is usually involved, and you can tell us what you need through get in touch. We read every enquiry and, where we can, we will put you in touch with a local accountant who could help.