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HMRC letter codes · Self Assessment (SA)

SA401Register a partner

Last reviewed 14 October 2026 · How Tax Letters checks its facts

In short

GOV.UK says form SA401 is used to register for Self Assessment and Class 2 National Insurance when you have joined a partnership.

“Register for Self Assessment and Class 2 National Insurance when you've joined a partnership using form SA401.”

GOV.UK: Register a partner for Self Assessment and Class 2 National Insurance · checked 14 October 2026

Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about SA401. Each fact has its source and the day it was checked beside it.

What it asks

GOV.UK's list of what you need before you start to register with form SA401 includes your National Insurance number, your Unique Taxpayer Reference (UTR) if you have one, the date your self employment started, the date you joined the partnership, and the name, address and UTR of the partnership.

“You’ll need: … your National Insurance number … your Unique Taxpayer Reference (UTR) if you have one … the date your self employment started … the date you joined the partnership … the name, address and UTR of the partnership”

GOV.UK: Register a partner for Self Assessment and Class 2 National Insurance · checked 14 October 2026

Time limits and rules

GOV.UK says that after you send the SA401 form, HMRC will usually contact you within 15 days of receiving it, but may take longer during busy periods.

“Registering a partner for Self Assessment and Class 2 NICs (SA401) … We’ll usually contact you within 15 days of receiving your form but may take longer during busy periods.”

GOV.UK: Register a partner for Self Assessment and Class 2 National Insurance · checked 14 October 2026

How to send it or apply

GOV.UK says only the nominated partner can register online, and that anyone else joining a partnership must register by post, using the form called 'Registering a partner for Self Assessment and Class 2 NICs (SA401)'.

“Only the nominated partner can register online. If you’re not the nominated partner and are joining a partnership, you must register by post. … Registering a partner for Self Assessment and Class 2 NICs (SA401)”

GOV.UK: Register a partner for Self Assessment and Class 2 National Insurance · checked 14 October 2026

Free help and HMRC contacts

GOV.UK's Self Assessment general enquiries page says you can ask HMRC's digital assistant for help with registering for Self Assessment, and gives 0300 200 3310 for queries about Self Assessment returns, payments, replacements or notices. The page does not name SA401.

“You can ask our digital assistant for help with: … registering for Self Assessment … Call HMRC if you have a query about: … your Self Assessment returns, payments, replacements or notices … Telephone: 0300 200 3310 … Find out about call charges”

GOV.UK: Self Assessment: general enquiries · checked 14 October 2026

Free help first

These routes are described by GOV.UK, TaxAid and Citizens Advice themselves.

  • HMRC's extra supportHMRC offers extra support if your health condition or personal circumstances make it difficult to contact HMRC, for example a mental health condition, financial hardship or being treated in hospital.
  • Someone to deal with HMRC for youYou can authorise a friend, relative or voluntary organisation to talk to HMRC for you and help you fill in forms.
  • TaxAidTaxAid helps people with tax problems who cannot pay for advice. Tax Help for Older People merged with TaxAid in July 2025.
  • Citizens AdviceCitizens Advice gives confidential advice online, over the phone and in person, for free.