HMRC letter codes · Self Assessment (SA)
SA100Main tax return
Last reviewed 14 October 2026 · How Tax Letters checks its facts
In short
GOV.UK's tax return forms page calls the SA100 the main tax return.
“The main tax return (SA100)”
GOV.UK: Self Assessment tax return forms · checked 14 October 2026
Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about SA100. Each fact has its source and the day it was checked beside it.
What it is
HMRC's Self Assessment manual forms index gives the SA100 the name 'Core Return (Individual)'.
“Core Return (Individual)”
GOV.UK: HMRC Self Assessment Manual, SAM002 Forms Index · checked 14 October 2026
What it asks
GOV.UK says you must send a Self Assessment return if HMRC asks you to, and that for a paper form you can download the SA100 tax return form.
“You must send a return if HMRC asks you to. … download the SA100 tax return form”
GOV.UK: Self Assessment tax returns · checked 14 October 2026
GOV.UK says you may need to fill in and send additional forms, known as supplementary pages, along with your SA100 tax return.
“You may need to fill in and send additional forms, known as supplementary pages, along with your SA100 tax return.”
GOV.UK: Complete your Self Assessment tax return for the last tax year · checked 14 October 2026
Time limits and rules
GOV.UK's tax return forms page, which names the main tax return (SA100), says you can send your tax return anytime on or after 6 April following the end of the tax year.
“You can send your tax return anytime on or after 6 April following the end of the tax year. … The main tax return (SA100)”
GOV.UK: Self Assessment tax return forms · checked 14 October 2026
How to send it or apply
GOV.UK says you can file your tax return online, on the page that covers the SA100 tax return.
“You can file your tax return online … along with your SA100 tax return”
GOV.UK: Complete your Self Assessment tax return for the last tax year · checked 14 October 2026
Free help and HMRC contacts
For a paper SA100, GOV.UK's forms ordering page says you can download the SA100 tax return form or call HMRC on 0300 200 3610, and says to find out about call charges before you call.
“download the SA100 tax return form … call HMRC … Before you call us, find out about call charges … Telephone: 0300 200 3610”
GOV.UK: Self Assessment: forms ordering · 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.
- GOV.UK: Self Assessment tax return forms (checked 14 Oct 2026)
- GOV.UK: HMRC Self Assessment Manual, SAM002 Forms Index (checked 14 Oct 2026)
- GOV.UK: Self Assessment tax returns (checked 14 Oct 2026)
- GOV.UK: Complete your Self Assessment tax return for the last tax year (checked 14 Oct 2026)
- GOV.UK: Self Assessment: forms ordering (checked 14 Oct 2026)
- GOV.UK: Get help from HMRC if you need extra support (checked 7 Oct 2026)
- GOV.UK: Appoint someone to deal with HMRC on your behalf (checked 7 Oct 2026)
- TaxAid: About us (checked 8 Oct 2026)
- Citizens Advice: About us (checked 7 Oct 2026)