SA401 is the HMRC form an individual uses to register for Self Assessment and Class 2 National Insurance when they become a partner in a business partnership. Every individual partner fills in their own, even if they're already registered for Self Assessment. The nominated partner can register online; any other partner joining has to send the SA401 by post.
This page uses the SA401 form (HMRC 04/26) and HMRC's partnership registration guidance, as of October 2026.
Which form for which partner
| Who | Form |
|---|---|
| The partnership itself | SA400, Register a partnership for Self Assessment |
| An individual partner | SA401 |
| A partner that isn't an individual, such as a company, LLP or trust | SA402 |
SA400, SA401 and SA402 can be sent together, and form 64-8 can go with them if you're appointing an agent. HMRC asks you not to attach anything else.
The online or post rule
Only the nominated partner can register online. The nominated partner is the one responsible for the partnership's tax returns and business records. Every other individual partner joining registers by post, either by filling in the online version and printing it, or by completing the PDF on screen or by hand. The PDF should be downloaded and opened in Adobe Reader, not in a browser.
What SA401 asks
- About you: title, name, address, phone, date of birth and National Insurance number. If you've never had one and don't need one, explain why.
- Whether you're a UK resident, and whether you came to the UK from a non-EU country in the last 12 months.
- Your existing Unique Taxpayer Reference if you've registered before, and whether you're the nominated partner.
- The date your self-employment began and the date you joined the partnership.
- The partnership's name, address, type of business and UTR if it has one.
- If there's no partnership UTR yet: the Company Registration Number for an LLP or limited partnership registered on or after 25 October 2010, or an SA400 attached for one registered earlier.
- Whether you're entitled to a share of profits, losses and other income, and whether the partnership is in sharefishing.
- The signed declaration.
The agent trap
This is the detail that catches people. An agent authorisation for the partnership doesn't cover the individual partners. If you want your accountant to deal with your own tax, you have to authorise them separately, on form 64-8 or through their online authorisation service.
Deadlines
A partnership has to register by 5 October in the business's second tax year, or it could be charged a penalty, and GOV.UK gives the same rule for someone who became a partner during a tax year. If the nominated partner sends the partnership return late, every partner pays a penalty, so registration is only the start.
After you send it
HMRC usually makes contact within 15 days, though it can take longer in busy periods. If you registered online, you may get your UTR sooner through the HMRC app or your personal tax account. If you've heard nothing after 3 weeks, you can check online when to expect a reply. Postal forms go to HMRC's National Insurance Contributions and Employer Office.
Setting up the partnership around it
If you're starting the partnership rather than joining one, SA401 is one of three jobs. GOV.UK lists them as choosing a name, choosing a nominated partner and registering with HMRC. A partnership can trade under the partners' own names or a business name, and doesn't need to register that name. It can't include limited, Ltd, LLP or plc, be offensive or copy an existing trade mark, and all the partners' names plus the business name have to appear on official paperwork such as invoices and letters. Limited partnerships and LLPs follow different rules.
Common mistakes on SA401
- Using the partnership's UTR in question 10, which asks for your own UTR if you've registered before.
- Leaving the CRN out for an LLP or limited partnership registered with Companies House on or after 25 October 2010.
- Forgetting the profit-share box in question 19, which tells HMRC you're entitled to a share of profits, losses and other income.
- Answering question 11 wrongly. Only one partner is the nominated partner, the one responsible for the partnership's returns and records, and only they can use the online service.
What comes next for a partner
- The nominated partner files the partnership return, the SA800, and should do it promptly so the other partners can complete their own returns.
- Each partner reports their share of profit on their personal SA100 return and pays tax on it.
- Partners share responsibility for the business's losses and bills, which is worth understanding before you sign.
The verdict
SA401 is a 2-page form, but it's easy to get the route wrong, so check which one applies to you before you start. If you're the nominated partner, register online; if you're joining someone else's partnership, post your own SA401 even if you already file Self Assessment, and authorise your accountant for you personally, not just for the firm. Do it well before 5 October of the second tax year.
Frequently asked questions
What is SA401?
It's the form an individual completes when becoming a partner in a partnership, to register for Self Assessment and Class 2 National Insurance contributions.
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Can I register as a partner online?
Only if you're the nominated partner. Other partners joining a partnership must register by post using SA401.
Do I need SA401 if I'm already registered for Self Assessment?
Yes. Each partner must complete a separate SA401 or SA402 even if they already send tax returns in their own right.
What is the difference between SA400 and SA401?
SA400 registers the partnership itself; SA401 registers an individual partner.
How long does HMRC take to reply to SA401?
HMRC usually contacts you within 15 days of receiving the form, though it can take longer during busy periods.
Does an agent authorised for the partnership cover me as a partner?
No. The form says a partnership authorisation doesn't cover the partners, so you must authorise your agent separately, on form 64-8 or online.
Can a company be a partner using SA401?
No. Partners that aren't individuals, such as companies, LLPs and trusts, use form SA402 instead.
Where do I send SA401?
To HM Revenue and Customs, National Insurance Contributions and Employer Office, BX9 1AN, with form 64-8 if you're appointing an agent.
