The SA100 is HMRC's main Self Assessment tax return: the core form you complete each year to report income that isn't fully taxed through PAYE, with supplementary pages for things like self-employment or property. For the 2025 to 2026 tax year, a paper SA100 must reach HMRC by 31 October 2026 and an online return is due by 31 January 2027, which is also when the tax must be paid.
Most people never see the paper form, because filing online is the same return in a different format: GOV.UK says 97% of people already do it that way. Everything below comes from GOV.UK's Self Assessment guidance and HMRC's 2026 SA150 notes, checked in October 2026.
SA100 deadlines for 2025 to 2026
| What | Deadline |
|---|---|
| Tell HMRC you need to file (if you're new to Self Assessment) | 5 October 2026 |
| Paper SA100 received by HMRC | 11:59pm, 31 October 2026 |
| Online return | 11:59pm, 31 January 2027 |
| Pay the tax you owe | 11:59pm, 31 January 2027 |
The tax year covered started on 6 April 2025 and ended on 5 April 2026. If you register after 5 October 2026, HMRC sends you a letter or email with a deadline 3 months from that date, but you still have to pay by 31 January 2027. Filing online gives you the extra 3 months and lets you save and finish later.
The main form and its supplementary pages
The SA100 itself covers items such as UK interest and dividends, UK pensions and state benefits, and Gift Aid payments. Anything else goes on supplementary pages. If you tick a Yes box on page TR 2 and don't send the matching pages, HMRC treats the return as incomplete and sends it back.
| If you had | Supplementary page |
|---|---|
| Employment or a company directorship | SA102 Employment (one per job) |
| Self-employment | SA103S short, or SA103F full |
| Partnership income | SA104S or SA104F |
| UK property income | SA105 UK property |
| Foreign income or gains | SA106 Foreign |
| Income from a trust or estate | SA107 Trusts etc |
| Capital gains and losses | SA108 Capital Gains summary |
| Non-UK residence or dual residence | SA109 |
| Less common income, deductions and reliefs | SA101 Additional information |
The SA150 notes say to use the short self-employment pages if your business is straightforward and turnover was less than £90,000, and the full pages if it's more complex, turnover was £90,000 or more, or you need to adjust your profits, as of October 2026.
The £1,000 trading allowance detail
If your combined self-employment receipts and certain miscellaneous income were £1,000 or less, they're exempt and don't need to go on a return. The SA150 lists reasons you might still choose to fill in the self-employment pages: to claim a loss, because you expect turnover over £1,000 next year, to pay voluntary Class 2 National Insurance for State Pension entitlement, or to support a Maternity Allowance or Tax-Free Childcare claim.
Late filing penalties
| How late | Penalty |
|---|---|
| 1 day | £100 |
| 3 months | £10 a day, up to £900 |
| 6 months | 5% of the tax due or £300, whichever is greater |
| 12 months | Another 5% or £300, whichever is greater |
They're separate from late payment penalties of 5% of unpaid tax at 30 days, 6 months and 12 months, plus interest, as of October 2026. If you miss the paper deadline, GOV.UK's advice is to file online instead to avoid a penalty.
Filing a paper SA100
- Download the SA100 and any supplementary pages from GOV.UK, or ask HMRC for paper copies.
- Read the first 2 pages and notes 1 to 9 of the SA150 before you start.
- If you make a mistake, strike through the error and write the correct figure next to the box.
- Sign the form. If someone else fills it in, you're still responsible for what it says.
- Post it to Self Assessment, HM Revenue and Customs, BX9 1AS (from abroad: HM Revenue and Customs, Benton Park View, Newcastle Upon Tyne, NE98 1ZZ).
You can correct a return within 12 months of the Self Assessment deadline. You'll need your UTR number for any return or amendment. If you're unsure whether you need to file at all, start with whether you need to do a Self Assessment, and for the full calendar see the Self Assessment deadline guide.
If you have foreign income or live abroad
Foreign income or gains go on the SA106 Foreign pages, but the SA150 notes a shortcut: if your only foreign income was untaxed foreign interest up to £2,000, or foreign dividends up to £500, you can put these straight onto the SA100 instead, as of October 2026. If you're non-resident or dual resident, you'll need the SA109 pages, and paper returns from outside the UK go to the Newcastle address.
The verdict
Unless you have a reason to use paper, file online: you get until 31 January rather than 31 October, you can save as you go, and you don't have to post anything. Whichever route you take, the £100 penalty starts the day after the deadline, so file even if you can't pay in full.
Want AI doing the heavy lifting in your marketing?
I build the systems that handle the boring 80 percent, so you get your week back. Done properly, with the human kept in.
Frequently asked questions
What is an SA100?
It's HMRC's main Self Assessment tax return form, completed with any supplementary pages for income such as employment, self-employment or property.
When is the SA100 paper deadline?
For the 2025 to 2026 tax year, HMRC must receive a paper return by 11:59pm on 31 October 2026.
Is the online return the same as the SA100?
Yes, it's the same return filed online, with a later deadline of 31 January 2027.
What is the penalty for filing an SA100 late?
An initial £100, then £10 a day after 3 months up to £900, then the greater of 5% of the tax due or £300 at 6 months and again at 12 months.
Where do I send a paper SA100?
Self Assessment, HM Revenue and Customs, BX9 1AS, United Kingdom. If you live outside the UK, send it to the Benton Park View address in Newcastle.
Do I need the SA106 pages for a small amount of foreign interest?
Not always. The SA150 notes say untaxed foreign interest up to £2,000 or foreign dividends up to £500 can go straight onto the SA100.
Can I amend my SA100 after filing?
Yes. GOV.UK says you can correct a tax return within 12 months of the Self Assessment deadline, online or by sending another paper return.
