SA400 Form Explained for New Partnerships

The SA400 is a form used by a new business partnership to register for Self Assessment with HMRC. The nominated partner completes and signs it.

Once HMRC processes the form, the partnership gets its own Unique Taxpayer Reference. Every partner then registers separately as well.

Setting up a partnership can feel informal: just two people, a shared idea and a handshake. HMRC sees it very differently.

A partnership isn’t taxed as a single entity, so the business and every partner have to be registered separately. That’s the gap the SA400 fills.

It’s the HMRC partnership registration form, and it tells HMRC that a new partnership exists and needs to file. Plenty of people search what is SA400 before they begin, and the short answer is that it’s only the first step.

The nominated partner is the one who completes the form to register a partnership for self assessment. Filing the SA400 doesn’t register the individual partners, because each of those is a separate task.

This guide explains how to register a partnership with HMRC, and what the form asks for along the way. It also covers the SA400 form itself and when to submit SA400 to stay penalty-free.

Who Completes the SA400 Form

Every partnership has to pick one person to deal with HMRC directly. That person is the nominated partner.

The nominated partner self assessment role means signing the SA400 now and, later, sending the partnership’s annual return to HMRC each year on time.

It’s a genuine responsibility, and it’s one worth taking seriously from day one. The SA400 partnership form runs to just two pages.

Its questions about the business are straightforward enough.

A partnership isn’t a separate legal entity in the way a limited company is. For tax filing, though, HMRC treats it as one taxpayer with its own reference.

So the SA400 sets up the partnership’s record, while each partner’s own tax sits elsewhere.

You don’t need to be the senior partner to take this on. Pick someone organised, because the deadlines and the paperwork land squarely on their desk.

Get that choice right and everything downstream runs more smoothly.

What the SA400 Asks You to Provide

Before you start, gather everything in one place. The form moves quickly when you’ve got the details ready.

A common question is simply: what do I need to complete SA400? It asks for the partnership’s name and address, its trading details, and the nominated partner’s information.

You’ll need the following information to hand:

• Give the partnership’s registered name and main trading address.
• State a business phone number and the type of business.
• Note the date the partnership started trading.
• Add the partnership’s accounting date for its books.
• Include the nominated partner’s full name and home address.

You can fill in the SA400 form online through HMRC’s service, or print it and post it. The online route needs a Government Gateway user ID, plus the partnership’s UTR or VAT reference (both found in the business tax account).

There’s one extra detail for some structures.

Limited partnerships and LLPs need their Company Registration Number from Companies House. Official SA400 HMRC guidance on GOV.UK lists the current requirements in full.

Which Partnership Type Applies to You

Not every partnership fills in this form, so it’s worth knowing which type you’ve set up. There are three types of business partnership UK law recognises:

• An ordinary partnership is the simplest, and for an ordinary partnership HMRC needs the SA400 to open the tax record.
• A limited partnership mixes general and limited partners, so its limited partnership registration goes through Companies House instead.
• A limited liability partnership (LLP) is a corporate body, and limited liability partnership registration also starts at Companies House.

That distinction matters more than it looks. Limited partnerships and LLPs are registered for Self Assessment automatically, so they don’t file the SA400 at all.

An ordinary partnership has no legal existence separate from the partners who run it. If a partner resigns, dies or goes bankrupt, the partnership itself ends, though the underlying business can usually carry on under a new arrangement.

Limited partners are different again. Their liability is capped at what they put into the business, plus any personal guarantees they have signed.

An LLP must have at least two designated members, who take on extra filing and reporting duties. It files annual accounts with Companies House as well as a Self Assessment tax return each year.

Worried you picked the wrong structure? For most new small partnerships, the ordinary route applies, and that’s the one the SA400 was built for.

Registering Partners as Well as the Partnership

Filing the partnership form is only half the job.

Each partner has to register in their own right too. That second layer is the partnership self assessment registration that people forget.

Miss it, and individual partners can face their own penalties later, even when the partnership itself has been set up perfectly correctly.

An individual partner completes the SA401 form. It tells HMRC they’ve joined the partnership and need to pay tax through their own return.

A partner that isn’t an individual, like a company or a trust, uses the SA402 form instead.

The rule still applies even to partners who already file returns. Here’s the catch that surprises people.

You must register partnership for self assessment at the business level and register each partner separately.

Both layers have to happen. Think of it as partnership tax return registration in two parts: the firm, then the people.

Get both done and nobody gets a surprise letter from HMRC later.

When to Submit the SA400 Form

Timing is where partnerships can slip up most. It has a real deadline, and missing it can cost money.

Knowing when to submit SA400 matters because registration is a legal duty, not a nicety.

You must register by 5 October following the end of the tax year in which the partnership started trading. Leave it late and HMRC can charge a failure-to-notify penalty.

The penalty is worked out as a percentage of the tax left unpaid, meaning the tax you’d have paid had you registered on time.

There’s a let-off worth knowing. If you register after 5 October but pay everything you owe by the 31 January payment deadline, the penalty should come to nothing.

So the safest move is to send the SA400 early.

Early registration also gives HMRC time to issue your reference before the return is due. Aim to register online where you can, because it’s quicker than posting the paper form.

How Long the SA400 Takes to Process

After you file, the waiting begins.

HMRC processes the SA400 and sets up the partnership’s tax record. Wondering how to get a partnership UTR?

You don’t apply for one separately, because it’s issued automatically once the SA400 is processed.

Your partnership UTR number has ten digits, and that’s the number you quote on every return. It arrives by post to the partnership’s registered address.

If you registered online, you may be able to see your UTR sooner through the HMRC app or your business tax account.

Keep that UTR safe. You’ll need it every single time the partnership files its return, and chasing up a lost reference can waste several working days.

The SA400 opens the record, while the partnership return fills it in each year. That return is the SA800, and every partner reports their share on their own tax return too.

HMRC usually replies within about two weeks, though busy periods run longer. Plan around that, and don’t leave registration until the deadline looms.

Your Next Step With the SA400

A partnership registration comes down to two moves. Register the business with the SA400, then register every partner individually.

Get the nominated partner sorted, gather your details, and send the form well before the 5 October deadline.

The partnership UTR follows, and from there the annual return becomes routine. There’s no fee to register, so the only cost of waiting is a possible penalty.

If you’d like a fuller walkthrough of the whole process, the partnership tax return guide covers exactly what happens after you register.

Check your own business tax account on GOV.UK to confirm what’s outstanding. The SA400 is only the start, but it’s the step that sets everything else up properly.

Key Takeaways

Here’s a quick recap of the main SA400 points to take away:

• The SA400 registers a new business partnership for Self Assessment with HMRC.
• The nominated partner completes and signs the SA400 on the partnership’s behalf.
• Each partner also registers individually, using the SA401 form or the SA402 form.
• Limited partnerships and LLPs register through Companies House and skip the SA400.
• You must register by 5 October after the partnership’s first trading tax year.
• HMRC issues a ten-digit partnership UTR once the SA400 has been processed.

Common SA400 Questions

A few practical questions tend to crop up once the SA400 itself is filed:

Can the partnership change its nominated partner later?

Yes, you can. If the nominated partner leaves or the partners agree on someone new, you tell HMRC and update the record.

The new nominated partner then takes over filing the partnership return. It’s worth doing promptly, so HMRC writes to the right person.

Does a new partner joining mean registering the partnership again?

No. The partnership keeps its existing UTR, so there’s no need to register the business a second time.

The joining partner does register themselves, though. They complete their own SA401 so HMRC knows they share the profits.

When is the partnership tax return itself due?

The partnership return, the SA800, has its own filing deadline. Online returns are due by 31 January after the tax year ends.

Paper returns come earlier, by 31 October. Each partner’s personal return follows the same January online deadline.

What happens if the partnership stops trading?

You tell HMRC the partnership has ceased, so it can close the record and stop expecting returns.

A final partnership return covers the last trading period. Leaving the record open risks penalty notices for returns nobody owes.

Does registering the partnership also handle VAT?

No, the SA400 deals only with Self Assessment. VAT registration is a separate process with its own threshold and form.

If turnover crosses the VAT threshold, you register for VAT on top. The two registrations don’t overlap.