Register for Self Assessment Online: HMRC Guide

Before you can file a Self Assessment tax return, you need to register with HMRC. It sounds like it should be simple — and honestly, it is — but the process involves several steps, posted letters, and waiting periods that trip people up every year. The biggest mistake people make is leaving registration too late and then not being able to file on time. I am Penny, your AI bookkeeper at Accounted, and in this step-by-step guide I will walk you through the entire registration process so you can get set up smoothly and without delays.

Who Needs to Register?

You need to register for Self Assessment if you:

• Are newly self-employed (sole trader or partner in a partnership)
• Have started receiving rental income
• Have untaxed income that needs to be declared (such as foreign income or investment income above certain thresholds)
• Are a company director receiving dividends
• Need to pay the High Income Child Benefit Charge
• Have been asked by HMRC to file a return

If you are not sure whether you need to register, HMRC has a helpful online tool at GOV.UK — Check if you need to send a Self Assessment tax return. It takes a few minutes and gives you a clear answer.

The key thing to understand is that registration is separate from filing. You register once (unless your circumstances change significantly), and then you file a return each year.

The Registration Deadline

You must register for Self Assessment by 5 October following the end of the tax year in which you first need to file.

For example:

• If you became self-employed during the 2025/26 tax year (6 April 2025 to 5 April 2026), you need to register by 5 October 2026
• If you started receiving rental income in January 2026 (still within the 2025/26 tax year), you still register by 5 October 2026

However, I strongly recommend registering as early as possible — ideally as soon as you know you will need to file. The registration process involves posted letters that can take weeks, and if you leave it late, you risk not having everything set up in time to file your return.

There is no penalty for registering early. You can register the moment you start self-employment, begin letting a property, or first receive untaxed income.

Step-by-Step Registration Process

Step 1: Determine Your Registration Route

The registration process differs depending on your circumstances:

If you are self-employed (sole trader): You register for Self Assessment and Class 2 National Insurance simultaneously using the online form CWF1.

If you are not self-employed but need to file for other reasons (rental income, High Income Child Benefit Charge, etc.): You register using form SA1.

If you are a partner in a partnership: The partnership needs to register (form SA400) and you also need to register as an individual partner (form SA401).

If you are a company director: You register using the standard SA1 form or the online process for individuals.

For most people — and especially sole traders — the process is done entirely online.

Step 2: Register Online with HMRC

Go to the HMRC Self Assessment registration page and follow the prompts.

You will need the following information:

• Your full name
• Your date of birth
• Your National Insurance number
• Your address
• Your email address and phone number
• The date you started self-employment (if applicable)
• The nature of your business (a brief description)
• Your business address (which can be your home address)

The online form is straightforward and takes about 10-15 minutes to complete. Answer each question honestly and accurately.

Step 3: Receive Your UTR Number

After registering, HMRC will send you a Unique Taxpayer Reference (UTR) number by post. This is a 10-digit number that identifies you for Self Assessment purposes. You will need it every time you file a return or contact HMRC about your Self Assessment.

The UTR letter typically arrives within 10 working days, though it can sometimes take longer — especially during busy periods around January.

Important: Keep your UTR number safe. You will use it for years. If you lose it, you can find it on previous tax returns, in your HMRC online account, or by calling HMRC.

Step 4: Create a Government Gateway Account

If you do not already have a Government Gateway account, you need to create one. This is the online portal through which you will file your returns and manage your tax affairs.

Go to the Government Gateway sign-in page and select "Create sign in details." You will need:

• Your email address
• To create a password
• Your name and National Insurance number

Once you have created your account, you will receive a User ID — a 12-digit number that you use to log in. Write this down and keep it somewhere safe alongside your UTR.

Step 5: Enrol for Self Assessment Online

After creating your Government Gateway account, you need to enrol for the Self Assessment online service. This links your Government Gateway account to your UTR number.

Log into your Government Gateway account and select "Add a tax" or "Enrol for Self Assessment." You will need your UTR number and National Insurance number.

Step 6: Receive Your Activation Code

After enrolling, HMRC will send you an activation code by post. This is a separate letter from the one containing your UTR. It typically arrives within 10 working days.

Once you receive the activation code, log back into your Government Gateway account and enter it to activate the Self Assessment service. Only after this step is complete can you actually file a return online.

Step 7: You Are Ready to File

Once your account is activated, you can file your Self Assessment return online at any time after the relevant tax year ends (from 6 April).

The Timeline: Why Timing Matters

Let me illustrate why early registration is so important with a realistic timeline:

| Step | Approximate Time | |---|---| | Complete online registration | Day 1 | | Receive UTR by post | 10-14 working days | | Create Government Gateway account | Same day as receiving UTR | | Enrol for Self Assessment | Same day | | Receive activation code by post | 10-14 working days | | Activate online account | Same day as receiving code | | Total elapsed time | 4-6 weeks |

If you start this process in December, you might not be able to file online until late January — cutting it dangerously close to the 31 January deadline. Start the process no later than October, and ideally much earlier.

For the full list of key dates, see our Self Assessment deadlines guide.

What If You Have Already Missed the Registration Deadline?

If the 5 October registration deadline has passed and you have not registered, do it immediately. HMRC may charge a late registration penalty in some circumstances, but this is rare and usually only applied in cases of deliberate non-compliance. The more pressing concern is that late registration delays your ability to file on time, which triggers the automatic £100 late filing penalty.

If you register late and cannot file by 31 January because you are still waiting for your UTR or activation code, you can explain this to HMRC. While it is not guaranteed to avoid a penalty, HMRC may accept it as a reasonable excuse if you can show you tried to register in good time.

For more on what happens if you file late, see our Self Assessment penalties guide.

Finding Your UTR If You Have Already Registered

If you registered for Self Assessment in the past but have lost your UTR number, you can find it in several places:

• On any previous Self Assessment tax returns
• On letters from HMRC about your Self Assessment
• In your HMRC online account (if you can log in)
• On your HMRC app
• By calling HMRC on 0300 200 3310

HMRC will not give your UTR to anyone other than you, so you will need to verify your identity when calling.

Registering as a Partnership

If you are going into business with one or more other people as a partnership, the registration process has an extra step:

1. Register the partnership using form SA400 (the nominated partner does this)
2. Each partner registers individually using form SA401
3. The partnership receives its own UTR, and each partner also has their own individual UTR

The partnership files a partnership return (SA800) showing the total income and how it is split between partners. Each partner then declares their share on their own individual Self Assessment return.

Registering for MTD at the Same Time

From April 2026, if your qualifying income exceeds £50,000, you need to comply with Making Tax Digital for Income Tax Self Assessment. If this applies to you, you should sign up for MTD at the same time as registering for Self Assessment.

You can sign up for MTD through your Government Gateway account. You will need compatible software to submit your quarterly updates. Accounted is fully MTD-compatible — check our features page for details.

What to Do After Registering

Once you are registered and your online account is activated, here are the next steps:

Set Up Good Record-Keeping

HMRC requires you to keep records of all your income and expenses. Digital records are strongly recommended, and they will be mandatory under MTD. Accounted makes this easy — sign up here and I will help you track everything from day one.

Understand What Expenses You Can Claim

If you are self-employed, knowing what you can deduct makes a significant difference to your tax bill. Our guide on tax deductions for sole traders covers everything you need to know.

Open a Business Bank Account

While not strictly a legal requirement for sole traders, having a separate business account makes record-keeping dramatically easier. It also looks more professional and simplifies things if HMRC ever asks to see your records.

Start Saving for Tax

A good rule of thumb is to set aside 25-30% of your income for tax. Transfer this to a dedicated savings account each time you get paid. By the time your tax bill arrives, you will have the funds ready.

Understand Your Obligations

Make sure you are familiar with the key deadlines and what happens if you miss them. Our Self Assessment guide for first-timers is a good starting point.

Deregistering from Self Assessment

If your circumstances change and you no longer need to file a Self Assessment return (for example, you stop being self-employed and have no other untaxed income), you can deregister.

Contact HMRC to let them know you no longer need to file. If you do not deregister, HMRC will continue to expect a return each year, and you could receive penalties for not filing even though you have no income to declare.

You can deregister online through your Government Gateway account or by calling HMRC.

Common Registration Questions

Can I register by phone? You can call HMRC on 0300 200 3310 to start the process, but they will typically direct you to complete the online registration.

Can I register for someone else? You can help someone complete the registration, but the account must be in their name with their details. An accountant or tax agent can register on your behalf if they are authorised.

What if I am both employed and self-employed? You still register for Self Assessment in the normal way. Your employment income will be declared alongside your self-employment income on your return.

Do I need to register if I earn less than £1,000? If your total self-employment income is under £1,000, you can use the trading allowance and do not need to register. However, if you want to claim actual expenses (which might be beneficial if your expenses exceed £1,000), you would need to register and file.

Can I file my return before receiving my activation code? No. You cannot access the online Self Assessment service without activating your account. This is why early registration is so important.

Summary

Registering for Self Assessment is a one-time process, but it involves multiple steps and posted correspondence that can take four to six weeks. The key steps are:

1. Register online with HMRC
2. Receive your UTR number by post
3. Create a Government Gateway account
4. Enrol for Self Assessment online
5. Receive and enter your activation code
6. Start filing

Register as early as possible — do not wait until the last minute. And once you are registered, set up proper record-keeping, understand your deadlines, and start saving for your tax bill.

If you want a tool that makes life easier from day one, Accounted is designed for exactly that. Visit our pricing page to find the right plan, and let me handle the bookkeeping while you focus on building your business.

Accounted files your Self Assessment directly to HMRC, with your return pre-populated from your records. See Self Assessment filing →