SA100 Form 2026: Everything You Need to Know for Your Self Assessment

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The SA100 Form is the main Self Assessment tax return document used by HM Revenue and Customs (HMRC). You use it to declare the money you earned during the 2025/2026 tax year (which ran from 6 April 2025 to 5 April 2026) so HMRC can calculate how much tax you owe.

In short: if your taxes aren’t entirely handled by an employer automatically, the SA100 is the primary form you must submit to square up with the taxman.

Whether you are filing for the first time or just want to make sure you have the right dates for this year, here is a simple guide to getting it done.

Who Needs to Fill Out the SA100?

You must complete an SA100 form for the 2025/2026 tax year if any of the following applied to you between 6 April 2025 and 5 April 2026:

• You were self-employed as a sole trader and earned more than £1,000 before deducting expenses.
• You were a partner in a business partnership.
• You earned more than £1,000 from renting out a property.
• You had untaxed income over £2,500 (such as from savings, investments, or dividends).
• You earned over £100,000 in the year.
• You need to pay the High Income Child Benefit Charge.

The Key Deadlines for Your 2026 Return

Missing HMRC deadlines usually results in automatic fines starting at £100, even if you don’t actually owe any tax. Keep these dates in mind for your 2025/2026 return:

Register for Self Assessment

5 October 2026

If this is your first time filing, you must tell HMRC by this date. They will send you a 10-digit Unique Taxpayer Reference (UTR) number, which you need to file the SA100.

Paper Form Deadline

31 October 2026

If you prefer to fill out a physical paper version of the SA100, HMRC must receive it by midnight. (Note: Most people file online now).

PAYE Tax Code Deadline

30 December 2026

File your return online by this date if you also have a regular job, owe less than £3,000, and want HMRC to collect what you owe in manageable chunks through your PAYE tax code next year.

Online Filing and Payment Deadline

31 January 2027

The absolute final cut-off. You must have submitted your online SA100 form and paid any tax you owe for the 2025/2026 tax year by midnight.

What You Need Before You Start

Before you sit down to fill out the SA100 online, gather your paperwork. Scrambling for numbers halfway through makes the process incredibly frustrating. Have these ready:

• Your 10-digit Unique Taxpayer Reference (UTR) and your National Insurance number.
• P60 or P45 forms if you also had employed work during the year.
• P11D form if you received any workplace benefits (like a company car).
• Records of all your self-employed income and allowable business expenses.
• Details of any rental income, interest on bank savings, or dividends.

Note: Depending on your income type, you may also need to fill out supplementary pages alongside the main SA100. For example, the SA103 for self-employment or the SA105 for property income. The online HMRC system usually adds these automatically based on how you answer the first few questions.

Should You Use an Accountant?

While the SA100 is designed so that individuals can fill it out themselves, handing it over to a professional accountant is often a smart move—especially if your income is slightly complex.

Here is how an accountant can help:

• Finding tax savings: An accountant knows exactly what business expenses you are legally allowed to claim, which can often reduce your tax bill by more than the accountant’s fee.
• Avoiding costly errors: Putting the wrong number in the wrong box can trigger HMRC investigations or fines. An accountant ensures the math and the rules are spot on.
• Saving time and stress: They handle the actual filing and the frustrating software systems so you don’t have to spend your evenings deciphering tax jargon.
• Dealing with HMRC: If HMRC has a question about your return, your accountant can speak to them on your behalf.

If your income is very straightforward (for example, a single small freelance gig on the side), you can likely manage the online form yourself. But if you have multiple income streams, property, or high business expenses, an accountant is highly recommended.

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