How to Register for Self Assessment: Deadlines, Forms and What Happens Next

You must register for Self Assessment by 5 October after the end of the tax year in which you first had income HMRC needs to know about. Sole traders register through the online CWF1 route, which also sets up Class 2 National Insurance; everyone else uses form SA1; partners use SA401. HMRC then posts a UTR and an activation code, and the first return is due the following 31 January. Here is who needs to register, how, and what to expect afterwards.

Who has to register

You need to register if, in the last tax year, you:

• Were self-employed with gross income over the £1,000 trading allowance.
• Were a partner in a business partnership.
• Received rental income over £1,000 after the property allowance, or any rental profit if you also have other reasons to file.
• Had untaxed income such as dividends, savings interest or foreign income above the relevant allowances.
• Had to pay the High Income Child Benefit Charge, unless you pay it through your tax code.
• Sold assets and owe Capital Gains Tax, unless you have reported it through the real-time service.
• Were a company director with income not fully taxed at source. Directors with only PAYE salary no longer have to file just because they are directors.

If you are not sure, GOV.UK has a short "check if you need to send a tax return" tool. Our Self Assessment guide for business owners goes into more depth.

The 5 October deadline

Tax years run from 6 April to 5 April. The deadline to tell HMRC you need to file is 5 October after the tax year ends. So for income first received between 6 April 2025 and 5 April 2026, the registration deadline was 5 October 2026 and the online filing deadline is 31 January 2027. Registering late does not move the filing deadline, which is why it pays to register as soon as you know you will need to file.

Self-employed: the CWF1 route

If you have started working for yourself, register for Self Assessment and Class 2 National Insurance together. On GOV.UK search "register for Self Assessment", choose "self-employed" and sign in or create a Government Gateway account. You will need:

• Your National Insurance number.
• Your business name (your own name is fine), address and the date you started trading.
• A description of the work.
• Your contact details.

Class 2 contributions are no longer compulsory for most people, but registering this way makes sure you are treated as having paid them, which protects your state pension record.

Not self-employed: form SA1

Landlords, people with investment income, high earners and anyone else who needs to file without a trade use form SA1. It asks why you need to file and from which date. It can be completed online with a Government Gateway account, or printed and posted if you cannot verify your identity online. The result is the same: a UTR by post.

Partners and partnerships

Each partner registers with form SA401 (SA402 for a corporate partner). If the partnership itself is new, the nominated partner also registers the partnership with form SA400, which gives the partnership its own UTR for the partnership return (SA800). The partnership return allocates profit between partners; each partner then reports their share on their own return.

What happens after you register

1. HMRC posts your 10-digit UTR, normally around 15 days after you register, in a letter headed "Welcome to Self Assessment".
2. Add Self Assessment to your online account using the UTR. HMRC posts an activation code, valid for 28 days.
3. Enter the code to complete enrolment. You can now file online. Our login guide covers the common snags.
4. Each spring, HMRC sends a "notice to file" for the year just ended. Returns are due by 31 October on paper or 31 January online.
5. Tax is paid by 31 January, with payments on account if your bill is over £1,000.

Penalties for registering late

Failing to notify HMRC by 5 October can attract a penalty, calculated as a percentage of the tax that was unpaid at 31 January as a result. If you register late but still file and pay on time, the percentage is applied to nothing and no penalty arises. Separate penalties apply for filing late (£100 immediately, rising after three, six and twelve months) and for paying late, which are explained in our guide to paying your tax bill.

What Cloudkeepers can do

We register new clients, keep an eye on the post for the UTR and activation code, and set up the agent authorisation so that from then on the return is prepared by us from your records. If you have just started trading, a discovery call is the right first step; the Self Assessment service page explains what is included, and the self-employed tax calculator shows roughly what to set aside.