How to Register for Self Assessment

If you need to send a Self Assessment tax return, for example because you are self-employed, a CIS subcontractor or have untaxed income, you must register with HMRC first. This guide explains who needs to register, the deadline, and how to do it.

Who needs to register

You usually need to register for Self Assessment if any of the following apply: you are self-employed and earned more than £1,000, you are a partner in a business, you work under the Construction Industry Scheme, or you have other untaxed income such as rental or significant savings or dividend income.

The registration deadline

You must register by 5 October following the end of the tax year you need to file for. For example, if you need to file for the 2025/26 tax year, you must register by 5 October 2026. Missing this date can lead to penalties, so register as soon as you know you need to.

How to register

You register online through GOV.UK. The route depends on your situation:

• If you are self-employed, register as a sole trader, which also registers you for Self Assessment and Class 4 National Insurance.
• If you need to file but are not self-employed, register using form SA1.
• If you are a partner, register using form SA401.

Once registered, HMRC sends you a Unique Taxpayer Reference (UTR) and an activation code to set up your online account. Keep these safe, as you need them to file.

Key dates after you register

Once in Self Assessment, the deadline to file online and pay any tax owed is 31 January following the end of the tax year. Paper returns are due by 31 October. See the UK tax year explained for all the key dates.

Frequently asked questions

When do I need to register for Self Assessment?

By 5 October following the end of the tax year you need to file for. For 2025/26, that means by 5 October 2026.

How do I register for Self Assessment?

Online through GOV.UK. Self-employed people register as a sole trader, others use form SA1, and partners use SA401. You then receive a UTR and an activation code.

Do CIS subcontractors need to register?

Yes. CIS subcontractors register for Self Assessment so they can file a return and reclaim any overpaid CIS tax.

What happens after I register?

HMRC sends your Unique Taxpayer Reference and an activation code so you can set up your online account and file your return.