How to register for self assessment and get your UTR.

Two dates matter in a first year of self assessment, and they are nearly four months apart. The return and the tax are due by 31 January. Telling HMRC that a return is needed is due by 5 October, six months after the tax year ends.

For the tax year that ran from 6 April 2025 to 5 April 2026, you have to tell HMRC by 5 October 2026 if you need to send a return and either:

• you have never sent a tax return before, or
• you registered in the past, but did not need to send a return for the 2024 to 2025 tax year

You tell HMRC by registering for self assessment. If you are not sure whether you need a return at all, read Do I need to file a tax return? first.

If you have started working for yourself

You can begin trading without registering anything. The duty to register arrives when your income from self-employment passes £1,000 in a tax year, counted before expenses. You may register sooner if you wish, and you have to keep records of what you earn and spend from the first day, because the return is built from them.

1. Check that you need a return. HMRC asks you to do this before you register.
2. Go to the Self Assessment registration service on GOV.UK and sign in. If you have no sign in details, you create them as you go.
3. Prove who you are if you are asked to. This usually means photo identification such as a passport or a driving licence.
4. Say why you need to send a return, and give the details the service asks for.
5. Wait for your Unique Taxpayer Reference to arrive by post.
6. File the return online, which can be done at any time from 6 April after the tax year ends.

Registered before? If you were in self assessment in the past but did not send a return last year, you may need to reactivate your account rather than register again. The service tells you how. A return filed without reactivating the account can be delayed.

If an accountant does it for you

An accountant can register you and file for you, once you have authorised them to deal with HMRC on your behalf. HMRC then sends its letters about your return to the accountant, with two exceptions: tax bills and refunds still come to you.

A Unique Taxpayer Reference, or UTR, is a ten digit number. HMRC letters sometimes call it simply your tax reference. You are given one when you register for self assessment, and a limited company is given its own when it is set up.

It usually comes by post around 15 days after you register, and takes longer if you live overseas. Once it has been issued you can also find it:

• in your Personal Tax Account
• in the HMRC app
• on earlier tax returns, and on other documents from HMRC such as a notice to file or a payment reminder

If it is in none of those places, contact HMRC.

What you use it for

You need the UTR to file a return online. When you pay, your payment reference is the UTR followed by the letter K, which makes 11 characters: get it wrong and the payment can be delayed or set against a different bill. A subcontractor in construction also gives the UTR to each contractor, so that the contractor can verify them and take 20% rather than 30%. Our note on the CIS tax return and refund covers that.

Marcus’s income passed £1,000 within weeks of starting, so there was never any doubt that a return was due. Because his bill is under £1,000, HMRC does not ask him for payments on account towards the following year. If it had been £1,000 or more, half as much again would have been due on the same day. Payments on account explained shows how that works.

Register as soon as you realise. Two things follow.

The return gets its own deadline

HMRC sends a letter or an email giving you a date to file by, which is three months from the date on it. That can fall after 31 January.

The tax does not

Whatever date the letter gives for the return, the tax is still due by 11:59pm on 31 January 2027. In practice you need the figure before then, which means preparing the return before then.

The penalty

If you register after 5 October and have not paid all of the tax by 31 January, HMRC can charge a failure to notify penalty. It is worked out on the amount still unpaid, and HMRC issues it within 12 months of receiving your return. Someone who registers late but pays in full by 31 January has nothing unpaid for it to be worked out on.

Income from earlier years. If the income goes back further than one year, tell HMRC about all of it. A penalty on income you bring forward yourself is lower than one on income HMRC finds for itself.

• Keep your records. Bank statements, invoices and receipts. You need them to fill in the return correctly.
• Estimate the bill. The return can be sent at any time after 5 April, and sending it early tells you what you owe months before you have to pay.
• Put money aside. HMRC’s Budget Payment Plan takes a weekly or monthly Direct Debit towards your next bill.
• Check Making Tax Digital. If your income from self-employment and property together, before expenses, was over £30,000 in the 2025 to 2026 tax year, you have to keep digital records and send HMRC an update every quarter from 6 April 2027. For income over £20,000 in 2026 to 2027, the start is April 2028. The self assessment page explains it.

You stay registered from year to year. If you stop needing a return, because you have stopped trading for example, tell HMRC, or it will go on expecting one.