UK HMRC Forms · 2026/27
What is the SA100 Self Assessment Form? UK 2026/27 Guide
What it is
The SA100 is the UK's main personal income tax return — equivalent in scope to the US 1040. It captures: employment income, self-employed income, partnership income, property income, dividends and interest, capital gains, foreign income, pension contributions, and any other income or relief claims. Supplementary pages (SA101, SA102, etc.) cover specific income types in more detail.
Who needs this
Anyone HMRC has registered for self-assessment must file an SA100: self-employed sole traders, partners, landlords with rental income above £1,000, company directors who need to file, anyone earning over £150,000, recipients of significant untaxed income, expatriates with UK obligations, and anyone HMRC has specifically asked to file (sometimes done randomly or after a query).
When you'll see it
Online: opens automatically once you log into your HMRC self-assessment account after April. Paper SA100s are no longer sent automatically by HMRC unless you've requested one — gov.uk online is the default. Filing window: 6 April to 31 January following year for online; 6 April to 31 October for paper.
How to get it
(1) File online via gov.uk/log-in-file-self-assessment-tax-return — the standard route since 2018 for most filers. (2) Paper form: request from HMRC if you specifically need it (rare — only used for those genuinely unable to file online). (3) Use a third-party tax software (FreeAgent, QuickBooks, GoSimpleTax, etc.) that submits to HMRC's API. Most accountants file SA100s through professional software too.
Common issues
Common SA100 issues: (1) missing the 31 January deadline (£100 fixed penalty + daily charges — file ASAP), (2) submitted but not signed (error code shown — re-submit with all sections complete), (3) wrong tax year (file for the year that ended 5 April; e.g. April 2026-April 2027 = 2026/27 return filed by 31 Jan 2028), (4) supplementary pages missing (e.g. SA101 for additional income — system usually flags this).
Recruiter pro tip
If you're newly self-employed or have a complex year (multiple income sources, foreign income, capital gains), consider an accountant for your first SA100 — typical cost £200-£500, and the time saved plus the relief from getting it wrong usually justifies the fee. After 1-2 years with the same income pattern, most people can file themselves using gov.uk online or commercial tax software. Never miss the 31 January deadline — penalties compound quickly.
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