# SA401: registering as a new partner for Self Assessment

- **Canonical URL:** https://formspedia.com/uk/sa401/
- **Country & Language:** United Kingdom (`en-GB`)
- **Form:** SA401
- **Revision:** HMRC 04/26
- **Issuing Agency:** HM Revenue and Customs
- **Tax Area:** Registration, representation and benefits
- **Who Must File:** Individuals becoming a partner in a partnership, including those already registered for Self Assessment
- **Filing Method:** Online service for the nominated partner; other partners post the form to HMRC, BX9 1AN
- **Filing Deadline:** By 5 October in the business's second tax year
- **Official Form PDF:** https://assets.publishing.service.gov.uk/media/6a20104e8666b6ae4dc56bb9/SA401-registering-partner-self-assessment-2026.pdf

Joining a partnership creates a separate registration task for the individual partner, even if they already submit Self Assessment returns. SA401 tells HMRC about that person and the partnership they have joined. The form used here is HMRC 04/26.

### Who uses SA401, and can you register online?

SA401 is for an individual becoming a partner. Companies, LLPs, trusts and other non-individual partners use SA402. Each partner completes their own applicable form, while the partnership registers separately through SA400.

HMRC's SA401 guidance allows the nominated partner to register through the sign-in service. A person joining who is not the nominated partner must register by post. The online fill-and-print version is another postal preparation method: completing it on screen does not send the registration to HMRC, and that version cannot save progress.

### Filling in SA401 questions 1 to 12

Have your National Insurance number, any existing personal UTR and the date your self-employment began available. The address entered in question 4 is where HMRC will send the reference. Dates on the form use day, month and four-digit year.

| Questions | Information to enter | Conditional detail |
| --- | --- | --- |
| 1–3 | Your title, surname and first names. | Use your own details as the individual partner. |
| 4–6 | Address and postcode, telephone number and date of birth. | The reference goes to the address in question 4. |
| 7 | National Insurance number. | If you do not need a UK number, explain why in the space provided. |
| 8 | Whether you are UK resident. | Use HMRC's residence guidance if you cannot determine the answer. |
| 9 | Whether you arrived from a non-EU country within the last 12 months. | There is a separate box for a recent arrival whose length of stay is uncertain. |
| 10 | Your existing Self Assessment UTR, if previously registered. | This is your personal reference, distinct from the partnership UTR requested later. |
| 11–12 | Whether you are the nominated partner and when self-employment began. | The partnership joining date is a separate question, number 16. |

For a missing National Insurance number, the form points to a payslip, P60, or a tax, pension or benefit letter, and to GOV.UK's find-your-number service. Someone who has never had a number and needs one should use the application service referenced on the form. The explanation box is for a person who does not need a UK number.

### Recording the partnership in questions 13 to 20

Enter the partnership's name in question 13, address and postcode in 14, and a description of its business in 15. Question 16 asks when you joined. Put the partnership's UTR in question 17 if one has already been allocated.

If no UTR has been issued, question 18 separates LLPs and limited partnerships by their Companies House registration date. For one registered before 25 October 2010, attach SA400 to register the partnership. For one registered on or after that date, enter the company registration number.

Tick question 19 if you are entitled to a share of the partnership's profits, losses and other income. Otherwise leave it blank. Question 20 is ticked only if the partnership is engaged in sharefishing. Sign and date the declaration, which confirms the information is complete and correct to the best of your knowledge and commits you to tell HMRC straightaway if your circumstances change in a way that affects your answers.

### When and where to send the registration

GOV.UK's partnership registration guidance sets a 5 October registration deadline in the business's second tax year and warns that late registration can lead to a penalty. For example, joining during the 2026 to 2027 tax year makes the corresponding date 5 October 2027. The same guide keeps the returns separate from registration: the nominated partner sends the partnership return, and every partner sends an individual Self Assessment return.

Download the [SA401 PDF](https://assets.publishing.service.gov.uk/media/6a20104e8666b6ae4dc56bb9/SA401-registering-partner-self-assessment-2026.pdf), complete and print it, then send it to HM Revenue and Customs, National Insurance Contributions and Employer Office, BX9 1AN. HMRC recommends downloading the form and opening the saved file in Adobe Reader if completing it on screen.

Include form 64-8 if you are using it to authorise an agent. The package may also contain SA400 and other partners' SA401 or SA402 forms; the form says not to attach anything else. An agent's authorisation for the partnership does not cover your personal tax affairs, so arrange separate authorisation for yourself through 64-8 or the appropriate online service.

### What happens after HMRC receives SA401?

HMRC's registration page says it usually contacts applicants within 15 days of receiving the form, with longer waits possible during busy periods. If you registered through the online service, the UTR may appear sooner in the HMRC app or personal tax account. After three weeks without contact, use the reply-time checker linked from the registration page.

The questions help HMRC decide your tax return and Class 2 National Insurance requirements. For Class 2 rates, the form points to the GOV.UK page on [self-employed National Insurance rates](https://www.gov.uk/self-employed-national-insurance-rates). For help completing SA401, its listed telephone numbers are 0300 200 3500 within the UK and +44 191 203 7010 from abroad.

### Sources

- [HMRC SA401, April 2026](https://assets.publishing.service.gov.uk/media/6a20104e8666b6ae4dc56bb9/SA401-registering-partner-self-assessment-2026.pdf)
- [HMRC: register an individual partner](https://www.gov.uk/guidance/register-a-partner-for-self-assessment-and-class-2-national-insurance)
- [GOV.UK: register the partnership](https://www.gov.uk/set-up-business-partnership/register-partnership-with-hmrc)