Registering a client for Self Assessment

Key points

Deadline

5 October after the tax year ends; 5 October 2026 for 2025 to 2026 (GOV.UK)

Who registers

Anyone needing a return who has not sent one before, or who did not need to send one for 2024 to 2025

Unique Taxpayer Reference

A 10-digit number, usually posted around 15 days after registering (GOV.UK)

Late registration

GOV.UK says registering after the deadline could bring a penalty

Who has to register

GOV.UK's registration guidance says a person must tell HMRC by 5 October 2026 if they need a return for the 2025 to 2026 tax year and either have not sent a tax return before or registered before but did not need to send a return for 2024 to 2025. Someone who registered in the past but missed a year may need to reactivate their existing Self Assessment account, which the online service handles.

The reasons for needing a return are in who must file a Self Assessment return.

Registering by type of client

1. Sole traders

   Register online through GOV.UK. The paper equivalent is form CWF1. HMRC's Self Assessment manual says someone starting a new business or self-employment completes one form only, the CWF1, and should register as self-employed no later than six months from the end of the tax year in which they started.

2. People who are not self-employed

   Landlords, people with untaxed investment income, company directors with other income and those paying the High Income Child Benefit Charge register through the online service. The paper form is the SA1, which the Self Assessment manual describes as the form for individuals who are not self-employed but wish to register.

3. A new partnership

   The nominated partner registers the partnership on form SA400, or online. GOV.UK says the nominated partner must complete the SA400.

4. Each partner

   Each partner registers for Self Assessment and Class 2 National Insurance as a partner. HMRC's guidance says this can be done online, and a partner who cannot use the online service uses the postal form SA401. They need the partnership's name, address and UTR.

The Unique Taxpayer Reference

Once registration is processed HMRC issues a Unique Taxpayer Reference (UTR). GOV.UK says it is a 10-digit number that usually arrives by post around 15 days after registering, and takes longer for someone living overseas. An individual can also find it in their personal tax account, the HMRC app and on earlier returns and HMRC letters.

The UTR is needed before the return can be filed, so registering close to the deadline leaves less time for the reference to arrive before 31 January.

Registering late

GOV.UK's deadlines page says that where someone registers after 5 October 2026, HMRC will send a letter or email with a different filing deadline, which will be 3 months from the date on the letter or email. Registering late can also bring a penalty, according to the registration page.

Questions