Every year, hundreds of thousands of people in the UK discover — usually in September, with the deadline bearing down — that they need to register for Self Assessment for the first time. If you started freelancing, took on a rental property, began earning dividend or crypto income, or need to pay the High Income Child Benefit Charge through Self Assessment during the 2025-26 tax year, registering is the first step, and it has its own deadline that’s separate from (and much earlier than) the 31 January filing deadline.
This guide covers exactly how to register, which route applies to your situation, how long it takes to get your Unique Taxpayer Reference (UTR), and what happens if you’re already close to the deadline.
Do you actually need to register?
You only need to register if you meet one of HMRC’s filing triggers for 2025-26 — broadly: self-employment income over £1,000, rental income over £1,000, untaxed income, capital gains above the reporting thresholds, dividend income above the Dividend Allowance, needing to pay the High Income Child Benefit Charge through Self Assessment rather than PAYE, or a handful of other circumstances. See our full breakdown of who needs to file if you’re not sure, or run through the Self Assessment checker — HMRC also runs its own online checking tool for this.
If you’ve filed a return before and are still required to file, you generally don’t need to register again — you already have a UTR. This guide is for first-time registrations, and for anyone who registered before but didn’t need to file for 2024-25 and needs to reactivate.
The 5 October 2026 deadline
If you need to complete a tax return for the 2025-26 tax year (6 April 2025 to 5 April 2026) and have never submitted one before, you must tell HMRC by 5 October 2026. This is the registration deadline — it is not the same as the 31 January filing deadline, and it comes nearly four months earlier. Registering late can result in a penalty even if you go on to file and pay on time.
Three registration routes — pick the right one
HMRC splits registration into three separate online journeys depending on your circumstances. Using the wrong one is the single most common cause of a delayed UTR, so check which applies before you start.
1. Self-employed
If you’re a sole trader, register through the self-employed route at gov.uk. This is an online-only service — there’s a “Start now” button that walks you through registration and, if you’ve been self-employed before but let your registration lapse, will prompt you to reactivate instead. HMRC doesn’t publish a fixed number of days for a reply on this specific page, so if you’re registering close to a deadline, use HMRC’s “check when you can expect a reply” tool to see current processing times before assuming your UTR will arrive in time.
2. Not self-employed
If your reason for registering isn’t self-employment — for example you have property income, foreign income, you need to pay the High Income Child Benefit Charge through Self Assessment rather than PAYE, trust income, other untaxed income, or Capital Gains Tax to report — use form SA1. You can complete SA1 online (sign in, or verify by email if you don’t have sign-in details yet) or print and post it. HMRC says it will “usually” contact you within 21 days of receiving your form, though this can take longer during busy periods — and September/early October, right before the registration deadline, is one of HMRC’s busiest windows.
3. Partner or partnership
If you’ve joined or are setting up a business partnership, two different forms apply:
- Registering the partnership itself uses form SA400. The nominated partner can register online with a Government Gateway ID and either an existing UTR or VAT reference; everyone else registers by post.
- Registering each individual partner (for Self Assessment and Class 2 National Insurance) uses form SA401. Only the nominated partner can register online — every other partner must register by post.
For both SA400 and SA401, HMRC says it will “usually” contact you within 15 days of receiving your form, again with the caveat that busy periods can extend this.
Government Gateway, GOV.UK One Login, and your UTR
Registering creates your Self Assessment record and triggers HMRC to post you a 10-digit Unique Taxpayer Reference (UTR) — you’ll need this for every future interaction with Self Assessment, so keep it somewhere safe. Separately, if you don’t already have sign-in details for HMRC’s online services, the registration journey will help you create a Government Gateway user ID (up to 12 characters) and password, or GOV.UK One Login credentials. These sign-in details are what let you access your personal or business tax account, view your UTR status, and eventually file your return online.
Note that getting a UTR and being able to file online for Self Assessment are two separate steps — after your UTR arrives, you (or your registration journey) still need to enrol for the Self Assessment online service specifically before you can submit a return through it.
What if your UTR hasn’t arrived and the deadline is close?
If you registered in good time but your UTR is taking longer than the 15–21 day guideline, check HMRC’s “check when you can expect a reply” tool, which is updated weekly with current processing times for Self Assessment registrations. If you registered online, you may see your UTR appear in the HMRC app or your personal/business tax account before the paper confirmation arrives by post, so it’s worth checking there rather than waiting on the letter.
If the 31 January filing deadline is approaching and you still don’t have a UTR because of a genuine HMRC processing delay (rather than a late registration on your part), this is generally accepted as a reasonable excuse for late filing if you can show you registered in good time — but you still need to keep chasing HMRC and file as soon as you’re able. Don’t simply wait silently for the deadline to pass.
What happens if you register late
If you register after 5 October 2026, that alone doesn’t stop you filing a return — but HMRC can charge a “failure to notify” penalty if you also fail to pay all of your tax bill by 31 January. This penalty is based on the amount of tax still outstanding, and HMRC can issue it within 12 months of receiving your return. Registering late and then paying everything owed by 31 January reduces your exposure to this penalty; registering late and paying late compounds the two problems (see the late-filing and late-payment penalties below, which apply on top of a failure-to-notify charge).
What happens if you miss the later filing/payment deadlines too
Registration is only the first deadline in the Self Assessment calendar — the return itself is due later. If you go on to miss the filing or payment deadlines for 2025-26, HMRC’s standard penalty structure applies:
| Situation | Penalty |
|---|---|
| Return filed 1 day late | £100 fixed penalty |
| Return filed 3+ months late | £10 per day, up to a maximum of £900 |
| Return filed 6+ months late | 5% of the tax due, or £300 if greater |
| Return filed 12+ months late | A further 5% of the tax due, or £300 if greater |
| Tax paid late (30 days) | 5% of the tax unpaid |
| Tax paid late (6 months) | A further 5% of the tax unpaid |
| Tax paid late (12 months) | A further 5% of the tax unpaid |
Use our Self Assessment penalty calculator to estimate your exposure if you’re already behind. For the full deadline calendar, see our Self Assessment deadlines guide.
First-year timeline: from registration to your first payment
For someone registering for the first time in September or early October 2026 for the 2025-26 tax year, the realistic sequence looks like this:
- Now – 5 October 2026: Register using the correct route (self-employed, SA1, or SA400/SA401).
- Within 15–21 days of registering (longer if HMRC is busy): Your UTR arrives by post; online registrants may see it sooner in the HMRC app or personal tax account.
- After your UTR arrives: Enrol for the Self Assessment online service using your Government Gateway or GOV.UK One Login details, if you haven’t already.
- From 6 April 2026: HMRC opens the online return for the 2025-26 tax year — you can file as soon as you have all your income and expense records together.
- By 31 January 2027: Submit your online return and pay any tax owed for 2025-26. If you’re liable for payments on account, your first payment towards 2026-27 is also due this day — use the payment on account calculator to work out how much to set aside.
- By 31 July 2027: Pay your second payment on account for 2026-27, if applicable.
Registering as early as possible relative to the 5 October deadline gives you the most buffer against HMRC processing delays, and means your UTR and online access are sorted well before the January rush.
Key takeaways
- The registration deadline (5 October 2026 for the 2025-26 tax year) is separate from, and much earlier than, the 31 January 2027 filing and payment deadline.
- There are three registration routes — self-employed, not self-employed (SA1), and partner/partnership (SA400/SA401) — and using the correct one avoids delays.
- HMRC’s guideline response times are around 21 days for SA1 and 15 days for SA400/SA401, both longer during busy periods; the self-employed online route doesn’t publish a fixed figure.
- Getting a UTR and being able to file online are two separate steps — you need both before 31 January.
- Registering late can trigger a “failure to notify” penalty if tax is also unpaid by 31 January, on top of any late-filing or late-payment penalties that follow.