The SA100 is HMRC's main Self Assessment tax return for individuals. It is the core form on which you report income, gains and reliefs for a tax year. The current version, SA100 (2026), covers the 2025/26 tax year (6 April 2025 to 5 April 2026). A paper SA100 must reach HMRC by 11:59pm on 31 October 2026; filed online, the same return is due by 31 January 2027. The SA100 itself is short. Most of the detail goes on supplementary pages, such as SA102 for employment, SA103 for self-employment and SA105 for property. This guide explains what the form covers, which pages you need, and when paper still makes sense.
What the SA100 is, and what it is not
The SA100 is the "core" return. The 2026 form covers taxed and untaxed UK interest and dividends, and UK pensions, annuities and other state benefits. It also covers payments into registered pension schemes, Gift Aid, Student Loan and Postgraduate Loan repayments, the High Income Child Benefit Charge, Marriage Allowance, and your declaration. Anything more involved goes on separate supplementary pages filed with it. Filing online through HMRC's service covers the same ground.
You complete an SA100 because HMRC has issued you a notice to file, usually after you registered for Self Assessment. HMRC is clear that you must send a return if it asks you to. If you believe you no longer need to file, for example because a rental or a business has stopped, contact HMRC and explain. Do not simply ignore the notice, because late filing penalties run on an outstanding notice.
It is not the only Self Assessment return. HMRC's other forms are:
- SA200. The short tax return. You can only use it if HMRC sends it to you, and it cannot be downloaded;
- SA800. The partnership return, filed by the nominated partner alongside each partner's own SA100;
- SA900. The trust and estate return;
- SA700. The return for non-resident companies;
- SA970. The return for trustees of registered pension schemes. It is paper only, with a 31 January deadline.
The supplementary pages
| Form | Use it for |
|---|---|
| SA101 | Additional information: less common income, reliefs, losses, pension charges, tax avoidance schemes |
| SA102 | Employment, including company directors |
| SA103S / SA103F | Self-employment (short or full version) |
| SA104S / SA104F | Partnership income (short or full) |
| SA105 | UK property income |
| SA106 | Foreign income or gains |
| SA108 | Capital gains |
| SA109 | Residence: non-UK residents and dual residents |
Each page has its own guidance notes, and the SA100 has SA150, the main notes. Forms are year-specific, so use the form and notes for the year you are reporting: the 2026 versions are for 2025/26.
Deadlines for the 2025/26 SA100
- Paper return: must reach HMRC by 11:59pm on 31 October 2026. That date is a Saturday, so post it with enough time to arrive. Proof of posting does not move the deadline;
- Online return: by 11:59pm on 31 January 2027;
- To have tax under £3,000 collected through your PAYE code: file on paper by 31 October 2026, or online by 30 December 2026;
- Payment: any tax due, plus any first payment on account, by 31 January 2027.
These dates come from section 8 of the Taxes Management Act 1970, which gives paper returns until 31 October and electronic returns until 31 January. If HMRC's notice to file was issued late, after 31 July, the deadline shifts. A paper return is then due within 3 months of the notice, and if the notice came after 31 October, so is an online return. If you miss the paper deadline, HMRC's advice is to file online instead, because the online deadline is still open until 31 January. The initial late filing penalty is £100, and further penalties follow at 3, 6 and 12 months.
Filing on paper: when it still makes sense
Online filing is the norm. It gives you a later deadline than paper, and you can print your SA302 tax calculation 72 hours after filing. Paper remains necessary or sensible in a few situations:
- Required by HMRC. SA970 pension scheme trustee returns, and non-resident company returns, must be sent on paper;
- Commercial software cases. HMRC's guidance directs some returns to commercial software for online filing. Its list includes partnerships, trusts and estates, non-residents, Lloyd's underwriters, ministers of religion, and returns reporting gains on more than one asset;
- Returns for someone who has died. HMRC tells the personal representative if a return is needed and sends a form to complete and return.
Paper returns go to Self Assessment, HM Revenue and Customs, BX9 1AS. Non-UK residents use the Newcastle address shown on HMRC's SA100 page.
Practical points when completing an SA100
- Use provisional figures if you must. If a figure is not final, such as a valuation, enter your best estimate, flag it as provisional, and amend it when known. Do not miss the deadline waiting for it;
- Amend rather than leave errors. You can amend within 12 months of the filing deadline. Online, wait 72 hours after filing first. On paper, write "amendment" on each corrected page with your name and UTR;
- Get the State Pension figure right. HMRC expects your annual State Pension entitlement, not the payments that happened to reach your bank. Its method is explained in our guide to how much a pensioner can earn tax-free;
- Do not forget the payment side. Filing and paying are separate. See our guide to paying Self Assessment tax for methods and the weekend deadline issue in 2027.
If you have never filed an SA100 before, you first need a UTR. Our guide to registering for Self Assessment explains how.
Acumon completes SA100 returns and supplementary pages for individuals, directors, landlords and partners through our Self Assessment tax return service, with more complex cases handled by our private client tax team.