How to Register for Self Assessment with HMRC?

So you've worked out that you need to file a Self Assessment tax return. The first step isn't actually filling anything in. It's telling HMRC you exist as someone who needs to file. That's what registering does.

Registering is a one-off task. Once you're in the system, you stay in it until HMRC tells you otherwise, so you only have to do this part once. This guide walks you through how to register, how long it takes, and the deadline you need to hit.

If you're not yet sure whether you even need to file, it's worth checking that first before you register.

The deadline that matters most

You need to register by 5 October following the end of the tax year you need to report on.

The UK tax year runs from 6 April to 5 April. So if you started earning untaxed income during the 2025/26 tax year (6 April 2025 to 5 April 2026), you need to register by 5 October 2026.

Don't leave it to the last minute. Registering isn't instant, and the part that takes time, getting your reference number from HMRC, can hold up everything else. The earlier you register, the less stressful the whole thing is.

How you register depends on your situation

There are three routes, and which one you use depends on why you need to file. The good news is that HMRC walks you through it once you start, so you don't need to memorise form numbers.

If you're self-employed (a sole trader)

This is the most common route. You register as self-employed, which also sets you up for National Insurance at the same time. The form behind the scenes is called the CWF1, but if you register online HMRC handles that for you.

You'll need to give your business start date and a short description of the work you do. There's nothing complicated here, just be honest about when you started.

If you're not self-employed but still need to file

This covers landlords, people with Capital Gains Tax to pay, those caught by the High Income Child Benefit Charge, and anyone with other untaxed income. You register using a form called the SA1, which again you can complete online.

If you're in a business partnership

Partnerships have an extra step. The partnership itself needs to be registered (one partner, the nominated partner, does this), and then each individual partner registers separately too. If this is you, it's worth getting advice so nothing is missed.

What you'll need to hand

Before you start, have these ready:

• Your National Insurance number
• Your date of birth and home address
• Your contact details
• The date you started your self-employment or began receiving untaxed income
• A brief description of your work or income source

The online registration itself takes around 15 to 20 minutes. It isn't difficult, it just asks for the basics.

Getting your UTR

Once you've registered, HMRC sends you a UTR. This stands for Unique Taxpayer Reference, and it's a 10-digit number that identifies you for tax. You'll use it every year on your return and whenever you contact HMRC, so keep it somewhere safe.

Your UTR arrives by post, usually within about 10 working days (or up to 21 days if you live overseas). You can't file your return without it, which is exactly why registering early matters so much.

A separate letter follows with an activation code for your online account. You enter this code once to switch your account on, and then you're ready to file.

A quick example

Daniel started doing freelance photography alongside his main job in August 2025. By the spring he realised his freelance income had passed £1,000, so he needed to register.

Because he's a sole trader, Daniel registered as self-employed online. He had his National Insurance number ready and described his work as freelance photography, with a start date of August 2025. About a week and a half later, his UTR arrived in the post. His activation code came a few days after that.

Daniel registered in June, months before the 5 October deadline. That meant when January came around, the only thing left to do was the return itself, with no last-minute panic about missing paperwork.

What happens after you've registered

Once you're registered and your account is active, you're set up to file. You don't need to register again next year, your UTR stays the same, and you simply file a return each year for as long as HMRC expects one from you.

If your situation changes and you no longer need to file, for example you stop being self-employed, tell HMRC so they can take you out of the system. Otherwise they'll keep expecting a return, and a missed one means an automatic penalty even if you owe nothing.

Let us handle it

Registering is the first step, and filing is the part most people would rather not think about. That's what we're here for. A chartered accountant takes care of your return from start to finish, so once you're registered, you can hand the rest over. You fill out one form, and we take it from there.