You need to register for Self Assessment before you can file a tax return. It tells HMRC you have income to declare. There are three routes, and the one you choose depends on why you need to file. The deadline is 5 October after the tax year ends.
HMRC then sends you a Unique Taxpayer Reference. You cannot file without it. Register early so the paperwork arrives in time.
Disclaimer: This article is for general information only. Your position depends on your income and circumstances. Speak to a qualified accountant before you act on it.
What do you need before you register?
You need your National Insurance number and full name. HMRC also asks for your date of birth and postal address. Add a daytime phone number and the date your income started. Have everything ready before you open the form.
HMRC asks why you are registering for Self Assessment. Sole trading, rental income and foreign income all count as reasons. HMRC runs a short online tool for this. Use it to check if you need to send a tax return if you are unsure.
The paper forms do not save your progress. You have to complete them in one sitting. Gather everything first so you do not lose your work.
Your postal address can be outside the UK. HMRC accepts an overseas address on the form. The process usually takes longer if you live abroad.
Registration also starts your record keeping duty. Keep bank statements and receipts from day one. You will use those records when you fill in the return later.
How do you register for Self Assessment with HMRC?
Choose the right route first. Sole traders sign in to a business tax account and add Self Assessment. Partners use form SA401 and everyone else uses form SA1. HMRC then opens your record and posts your Unique Taxpayer Reference.
| Why you're registering | Route |
|---|---|
| Self-employed sole trader | Business tax account, or form CWF1 by post |
| Partner in a business partnership | Form SA401, plus form SA400 for the partnership |
| Any other reason, such as rental or pension income | Form SA1 |
The route matters more than people expect. If you register as a sole trader with form SA1, HMRC opens a tax record but not a self-employment record.
If you choose the wrong route, fixing it takes longer than the registration itself. An accountant will pick the right one for you and file the return in the same job. That is what our self assessment tax return service does.
How do you register if you’re self-employed?
Sign in to your HMRC business tax account. Choose the option to add a tax to your account. This registers you for Self Assessment and Class 2 National Insurance together. You can create sign in details on your first visit.
Class 2 National Insurance matters more than the tax record. HMRC says this registration helps you qualify for the State Pension. Register through form SA1 alone and that link does not get made.
Some people cannot use the online service. Form CWF1 does the same job on paper. The postal address is Self Assessment, HM Revenue and Customs, BX9 1AN, United Kingdom.
One trap catches existing filers. You may already send a return for rental income, then start trading as a sole trader. You still need to register again as a sole trader. That second step opens your Class 2 National Insurance record.
How do you register if you’re not self-employed?
Use form SA1 for this route. It covers rental income, pension income, foreign income and capital gains. You can send it online through your HMRC account, or print and post it. HMRC needs the same details either way.
Posting the form does not skip the online account. You still create an HMRC account to file later. The form only opens your tax record.
Landlords use this route. So do people with foreign income and people paying the High Income Child Benefit Charge. Company directors with untaxed income use it too.
HMRC asks you to explain why you are registering. Keep the answer short and factual. Name the income source and the date it started.
How do you register as a partner in a partnership?
Each partner in a partnership registers separately, using form SA401. The nominated partner also registers the partnership itself with form SA400. Only the nominated partner can use the online service. Every other partner has to register by post.
Form SA401 asks for more than your own details. You need the partnership name, address and UTR. You also need the date you joined the partnership.
Limited liability partnerships need the Company Registration Number too. That applies to LLPs registered at Companies House on or after 25 October 2010.
HMRC usually replies within 15 days of getting the form. Allow three weeks before you chase it. Registering online gets you the UTR sooner through the HMRC app.
Each partner needs their own UTR to file. The partnership gets its own UTR as well. One business can mean two returns.
What if you registered before but stopped filing?
Your old Self Assessment record goes dormant. You still tell HMRC by 5 October that you need to file again. HMRC then reactivates the record and reissues the notice to file. Filing without reactivating first can delay your return.
Your Unique Taxpayer Reference does not change. HMRC keeps the same number and switches the record back on.
Most guides skip this step. People assume an old UTR is enough to file. It is not. The return can sit unprocessed while HMRC works out why it arrived.
The registration service asks whether you sent a return last year. Answer honestly and it routes you to reactivation. Answer wrongly and you start a duplicate record.
What is the deadline to register for Self Assessment?
The deadline is 5 October after the tax year ends. The tax year runs 6 April to 5 April. Income earned in the year ending 5 April 2026 needs registration by 5 October 2026. HMRC can charge a penalty after that date.
The date is not random. HMRC's rules give you six months from the end of the tax year. The tax year ends 5 April. Six months later is 5 October.
Two things happen if you register after 5 October. HMRC sends you a letter with your filing date. That date is 31 January or three months from the letter, whichever is later.
Almost nobody writes that second rule down. It means late registration does not always mean a late return. The payment deadline does not move though. You still pay by 31 January.
You can check how to register for Self Assessment on GOV.UK. The tool confirms your route and your date.
Filing dates and payment dates sit apart from registration. Read our guide to self assessment deadlines for the full calendar.
What penalty do you get for registering late?
HMRC calls this a failure to notify penalty. It is a percentage of the tax you should have paid. An honest mistake carries a band of 0% to 30%. Choosing not to tell HMRC pushes that to 20% to 70%.
Two things set the percentage. HMRC looks at why you failed to tell them. HMRC also looks at how the problem came to light.
Telling HMRC before they spot it counts as an unprompted disclosure. That gets the lowest band. An unprompted disclosure within 12 months of the tax falling due starts at 0%. Waiting until HMRC asks lifts the floor to 20%. Hiding the problem takes the ceiling to 100%.
HMRC works the penalty out from the potential lost revenue. That is the tax you owed and paid late. Clear the bill by 31 January and there is nothing left to charge on.
No tax to pay means no failure to notify penalty. You still need to register.
A reasonable excuse can cancel the penalty on an honest mistake. HMRC judges each excuse on its own facts. Tell the officer if illness or a family crisis got in the way.
You can also reduce the penalty by helping HMRC. Telling them what happened is worth up to 30%. Helping them work it out is worth up to 40%. Giving access to your records is worth up to 30%.
Waiting three years or more costs you. HMRC then limits the reduction to ten points above the minimum. HMRC's failure to notify factsheet sets out every band in a table.
What happens after you register?
HMRC posts your Unique Taxpayer Reference within 15 working days. Allow 21 days if you live abroad. New online accounts also get an activation code within 7 working days. The HMRC app shows your UTR sooner than the post does.
You need the UTR to file. It is a ten digit number and it never changes. Some people lose the letter. Our guide on how to get a UTR number covers lost references and replacements.
The activation code is separate from the UTR. It switches on your online account. Lose it and you can ask HMRC for another one.
HMRC then sends a notice to file each year. That notice arrives by email or by post. HMRC picks the channel from your account settings. The notice keeps coming until you leave Self Assessment.
HMRC warns that these times stretch during busy periods. Registering in the summer beats registering in January.
What goes wrong when people register?
Four mistakes come up again and again. A sole trader registers with form SA1 and misses Class 2 National Insurance. An old filer submits a return without reactivating a dormant record. A partner tries the online route without being the nominated partner.
The fourth one is timing. People leave registration until January. Post takes 15 working days for a UTR. That leaves no room if something goes wrong.
The Class 2 mistake costs the most over a working life. Gaps in the record can cut your qualifying years.
The dormant record mistake is the quietest. Your return goes in and then sits there. Nothing tells you it has stalled.
The partnership mistake is the easiest to avoid. Check who the nominated partner is before anyone starts.
Do you need to register every year?
No, you register once. HMRC then sends a notice to file every year after that. The notice keeps coming until HMRC agrees you can leave Self Assessment. Tell HMRC when your income changes and you no longer need to file.
One exception catches people out. You register again when you start a new type of work. You already file for rental income. Then you start trading. Register as a sole trader on top of what you have.
Without that step, HMRC has no self-employment record for you. The tax return still works. The National Insurance record does not.
Registration takes ten minutes. The return takes longer. Your first choice of route decides how smoothly the rest goes.
Get a quote and we will send you our current pricing. DASA's self assessment accountants handle registration and filing from start to finish.
Author: Raqeeb Marzook ACCA, Manager at DASA Accountancy. For corrections or updates to this article, contact: raqeeb@dasaaccountancy.co.uk
This article gives general information about registering for Self Assessment. It is not tax advice. Every situation is different. Speak to a qualified accountant about your own circumstances.
FAQs
How do you register for Self Assessment with HMRC?
Choose the right route first. Sole traders sign in to a business tax account and add Self Assessment. Partners use form SA401 and everyone else uses form SA1. HMRC then opens your record and posts your Unique Taxpayer Reference.
What is the deadline to register for Self Assessment?
The deadline is 5 October after the tax year ends. The tax year runs 6 April to 5 April. Income earned in the year ending 5 April 2026 needs registration by 5 October 2026. HMRC can charge a penalty after that date.
What do you need before you register for Self Assessment?
You need your National Insurance number and your full name. HMRC also asks for your date of birth and postal address. Add a daytime phone number and the date your income started.
What penalty do you get for registering late?
HMRC calls this a failure to notify penalty. It is a percentage of the tax you should have paid. An honest mistake carries a band of 0% to 30%. Choosing not to tell HMRC pushes that to 20% to 70%.
How long does it take to get a UTR after registering?
HMRC posts your Unique Taxpayer Reference within 15 working days. Allow 21 days if you live abroad. The HMRC app shows it sooner than the post does.
Do you need to register for Self Assessment every year?
No, you register once. HMRC then sends a notice to file every year after that. The notice keeps coming until HMRC agrees you can leave Self Assessment.