SA400: Registering a Partnership for Self Assessment

Everything here comes from GOV.UK's SA400 guidance, the SA400 form (HMRC 08/23) and the Set up a business partnership guide, as of October 2026.

Who needs SA400, and who doesn't

Business	Register the partnership on SA400?
Ordinary partnership	Yes
Limited partnership or LLP registered at Companies House before 25 October 2010	Yes
Limited partnership or LLP registered from 25 October 2010	No, it's registered for Self Assessment through its Companies House registration
Each individual partner	Not SA400: they use SA401
A partner that's a company, another partnership or a trust	Not SA400: they use SA402

A partner doesn't have to be a person. A limited company counts as a legal person and can be a partner, which is why SA402 exists. Even partners already registered for Self Assessment still complete the right partner form.

The nominated partner's job

Every partnership chooses a nominated partner, who's responsible for the partnership tax return and keeping the business records. That person completes SA400 and later files the partnership return on SA800. All the other partners still send their own individual returns.

What SA400 asks

1. The partnership's name, address and phone number.
2. Any trading name and trading address, if different.
3. The nature of the business, such as plumbers or electrical engineers.
4. The date the business started.
5. The partnership type: partnership, LLP or limited partnership, with the Company Registration Number for an LLP or LP.
6. The partnership's accounting date.
7. The nominated partner's name, correspondence address and 10-digit UTR if they already have one.
8. A signed declaration, and a count of any SA401, SA402 or 64-8 forms enclosed.

Before you start, have the start date and the accounting date ready. Those two dates are the ones GOV.UK says you'll need.

Online or by post

• Online: you need a Government Gateway user ID and password, which you can create while registering, and your UTR or VAT reference from your business tax account.
• Online version of the paper form: fill it in, print it and post it. You can't save your progress, so gather everything first.
• PDF: complete on screen in the free Adobe Reader or print and fill in by hand, then post it.
• Postal address on the form: HM Revenue and Customs, National Insurance Contributions and Employer Office, BX9 1AN. Don't attach anything other than the listed forms.

How long until you hear back

HMRC usually makes contact within 15 days of getting the form, though it can take longer at busy times. If you registered online, you may get the UTR sooner through the HMRC app or your business tax account. If 3 weeks pass with nothing, GOV.UK has a tool to check when you can expect a reply. The helpline on the form is 0300 200 3500, or +44 191 203 7010 from outside the UK.

The penalty that lands on every partner

Here's the part partnerships underestimate. If the partnership return is sent late, each individual in the partnership has to pay a penalty, not just the nominated partner. The partnership return can go in any time after 5 April, and GOV.UK suggests the nominated partner files it as soon as possible so the others can complete their own SA100 returns. Each partner then pays their Self Assessment bill by 31 January.

Other things to set up at the same time

• A name: you can trade under the partners' names or a business name, but it can't include limited, Ltd, LLP or plc, and every partner's name must appear on official paperwork such as invoices.
• VAT: registration is required if VAT taxable turnover goes over £90,000, as of October 2026, and you can choose to register below that.
• An agent: authorise one on form 64-8 or through the agent's online service.

Choosing a name that won't be refused

You don't have to register a partnership name, but the Set up a business partnership guide sets limits. The name can't be offensive, can't be the same as an existing trade mark, and can't use a sensitive word or suggest a link with government or local authorities without permission. Its own example: to use the word Accredited, you need permission from the government department the guide names. If you want to stop others trading under your name, you'll need to register it as a trade mark.

After registration: the yearly cycle

1. Keep records so the nominated partner can complete the partnership return and every partner can complete their own return.
2. From 6 April, the nominated partner files the partnership return and shares each partner's figures.
3. Each partner reports their share on their individual return, with the partnership pages.
4. Everyone pays their own Self Assessment bill by 31 January.

The verdict

Pick the nominated partner first, register the partnership on SA400 or online, and have every partner register on SA401 or SA402 in the same week. Put 5 October of your second tax year in the diary as the latest date, and treat the partnership return as everyone's deadline, because a late one costs every partner a penalty.

Frequently asked questions

What is the SA400 form?

It's HMRC's form for registering a new partnership for Self Assessment, completed by the nominated partner.

When do I need to register a partnership?

By 5 October in the business's second tax year, or you could be charged a penalty.

Do LLPs need to complete SA400?

Only those registered with Companies House before 25 October 2010; later LLPs and limited partnerships are registered through Companies House.

Do partners also need to register?

Yes. Each individual partner completes SA401, and partners that aren't individuals complete SA402.

How long does it take to get a partnership UTR?

HMRC usually contacts you within 15 days, and online registrations may see the UTR sooner in the HMRC app or business tax account.

Where do I send a paper SA400?

To HM Revenue and Customs, National Insurance Contributions and Employer Office, BX9 1AN, with any SA401, SA402 or 64-8 forms.

Who completes the SA400?

The nominated partner, the person the partnership has chosen to receive and submit the partnership returns.