How to Register a Partnership for Self Assessment Using Form SA400

To register a partnership for Self Assessment using Form SA400, the nominated partner submits the form to HMRC either through the online service on GOV.UK or by post to BX9 1AN, giving the partnership’s name, address, start date, nature of business, accounting date, and the nominated partner’s details. Registration must be completed by 5 October in the partnership’s second tax year, and HMRC usually issues the partnership’s Unique Taxpayer Reference (UTR) within 15 days.¹ Every partner also has to register individually, which is a separate step.

Which Partnerships Actually Use SA400

SA400 is for general (ordinary) partnerships that have started trading or receiving income. The form lists three partnership types on its face — general partnership, limited liability partnership, and limited partnership — but a note on the form itself explains that LLPs and LPs have not needed to file SA400 since 25 October 2010. Those entities are registered for Self Assessment automatically when they incorporate at Companies House.² So if you are forming an LLP or LP, you can stop here: Companies House handles the HMRC side for you.

For a general partnership, the obligation to register begins the moment the business starts trading, even if it has not yet made a profit. Without a UTR the partnership cannot file its annual return.

The 5 October Deadline

You must register the partnership by 5 October in the second tax year of trading. A partnership that started at any point during the 2025 to 2026 tax year (6 April 2025 to 5 April 2026) has to register before 5 October 2026.³ Missing the date does not remove the requirement. You still have to register, and you may also pick up a failure-to-notify penalty tied to the tax that went unpaid while the partnership was off HMRC’s books.⁴

What to Have Ready Before You Start

SA400 asks for thirteen pieces of information plus a signed declaration. Getting them together first avoids a half-finished form and a returned application.

The full legal name of the partnership.
The main business address and a phone number.
A trading name, if the business trades under something different.
A trading address, if it differs from the main partnership address.
A short description of what the partnership does. The form’s own examples include “plumbers,” “investment business,” and “electrical engineers.”
The exact date the partnership began trading or receiving income.
The partnership type (general, LLP, or LP — although LLPs and LPs register through Companies House).
A Company Registration Number, which is only needed for LLPs and LPs.
The partnership’s accounting date (the date its accounting year ends).
The nominated partner’s name and correspondence address.
The nominated partner’s UTR, if they are already registered for Self Assessment. This is the ten-digit reference on their existing HMRC Self Assessment correspondence.

The nominated partner then signs a declaration that the information is complete and correct and that HMRC will be told if things change.²

Registering Online

HMRC recommends the online service, and it is the faster route. The paper SA400 exists as a fallback for anyone who cannot use the digital form.³ To register online you need a Government Gateway user ID and password (you can create one during the process) and either the nominated partner’s UTR or a VAT reference number from the business tax account.¹

One practical benefit: the partnership’s UTR may become visible through the HMRC app or the business tax account before a letter arrives in the post.¹

Filing the Paper Form

If you cannot register online, download the SA400 PDF from GOV.UK, complete it on your computer or print it and fill it in by hand in black ink. Check the business start date and accounting date carefully. Errors in either field, and missing information generally, are common reasons the form is returned unprocessed.

Post the completed form to:

HM Revenue and Customs
National Insurance Contributions and Employer Office
BX9 1AN²

You can put partner registration forms (SA401 and SA402) and agent authorisation forms (64-8) in the same envelope. There is a box on the form for noting how many additional forms are enclosed. Do not attach anything else.

Registering Each Partner Individually

Registering the partnership only sorts out the entity. Every partner also has to register personally for Self Assessment, even if they already file their own tax returns.⁵ Filing SA400 does not carry the partners across, and assuming it does is a common way to end up with missed filings and penalties.

SA401 covers individual partners. It registers the person for Self Assessment and Class 2 National Insurance contributions. Each individual partner completes their own SA401.
SA402 covers partners that are not individuals — a company, trust, or another partnership joining as a partner. Where the non-individual partner is itself a partnership or trust, the form is completed by that entity’s nominated partner or trustee.⁶

The 5 October deadline applies to individual partner registrations as well. Sending the SA401 or SA402 forms in the same envelope as the SA400 is the cleanest approach: it reduces the chance of a partner being missed and lines up the UTRs at roughly the same time.

After You Register: UTR and the First Return

HMRC typically writes to the nominated partner within 15 days of receiving the registration, though busy periods stretch that out. If nothing has arrived after three weeks, you can check expected reply times on the HMRC website.¹ The partnership’s UTR comes by post and is required on every future return and every piece of correspondence with HMRC.

From there the partnership has to file an annual return (Form SA800) covering each tax year it was active. The paper deadline is 31 October after the end of the tax year; online filing runs until 31 January. Preparing and submitting SA800, handling any HMRC enquiry into it, and keeping the other partners informed about that enquiry all fall to the nominated partner.⁷

The partnership return does not calculate each partner’s tax bill. It allocates income and expenses among the partners, who then report their share on their own Self Assessment returns and pay tax individually.

What the Nominated Partner Signs Up For

Because the nominated partner is HMRC’s contact for the partnership, the choice matters. The role covers:

Filing the SA400 or completing the online registration.
Submitting the SA800 partnership return each year.
Dealing with any HMRC enquiry into that return and keeping the other partners updated.⁷
Receiving all formal HMRC correspondence at the address on file.

If the nominated partner dies or steps down, the partnership must tell HMRC as soon as possible. If the partnership does not put someone forward, HMRC will pick a partner itself and notify the partnership in writing.⁸

Penalties If You Miss the Deadline

Late registration and late filing are treated separately. Registering the partnership after 5 October can trigger a failure-to-notify penalty based on the tax that was unpaid during the period the partnership was unregistered.⁴

Late filing of the annual SA800 escalates the longer it drifts:

Up to 3 months late: an initial fixed penalty.
3 to 6 months late: HMRC can charge £10 per day for up to 90 days, adding up to £900.
6 months late: a further 5% of the tax that would have been shown on the return, or £300, whichever is greater.
12 months late: another 5% (minimum £300). Where HMRC considers the delay deliberate, this can rise to 70% or 100% of the tax liability.

Late payment of the tax itself carries its own charges: 5% of unpaid tax if payment is more than 30 days overdue, a further 5% at six months, and another 5% at twelve months. Filing penalties and payment penalties stack, so a partnership that is late on both can rack up charges quickly.

1
GOV.UK. Register a Partnership for Self Assessment
2
GOV.UK. SA400 – Registering a Partnership for Self Assessment
3
GOV.UK. Set Up a Business Partnership – Register the Partnership
4
GOV.UK. Self Assessment Tax Returns – Penalties
5
GOV.UK. SA401 – Registering a Partner for Self Assessment and Class 2 NICs
6
GOV.UK. SA402 – Registering a Partner for Self Assessment if They Are Not an Individual
7
GOV.UK. EM7507 – Partnerships: Nominated, Representative or Reporting Partner
8
GOV.UK. Tell HMRC About a Change to Your Business