Last updated: October 2026
The SA103 form is the supplementary page self-employed people add to their Self Assessment tax return to report business income and expenses. You file it alongside the main SA100 return whenever you have self-employment income to declare. It records your turnover, allowable expenses and profit for the tax year, and HMRC uses these figures to work out your Income Tax and Class 4 National Insurance.
This guide explains the two versions of the SA103, who needs each one and how to fill in the key sections for your 2025/26 return.
SA103S vs SA103F: What Is the Difference?
| SA103S (Short) | SA103F (Full) | |
|---|---|---|
| Turnover | Below £90,000 | £90,000 or more |
| Expenses | Can enter one total figure | Must split into categories |
| Best for | Most sole traders with simple affairs | Larger businesses or more complex claims |
You must also use the full SA103F in some other situations, even with lower turnover. These include changing your accounting date, making certain capital allowance claims, or carrying a loss back to earlier years.
If you file online, HMRC’s service asks a few questions and shows you the right version automatically.
Who Needs to File an SA103?
You need an SA103 if you have self-employment income that goes through Self Assessment. This includes:
- Sole traders running an ongoing business
- People with more than one trade, who complete a separate SA103 for each trade
- CIS subcontractors, who report their income and any tax already deducted by contractors
- Anyone who started self-employment during the tax year
You also need a UTR before you can file. Our guide explains how to get a UTR number. If you are unsure whether you need to file at all, read do I need to file a Self Assessment tax return?
A company director who only receives salary and dividends does not need an SA103. That income goes in other sections of the main return.
What Goes on the SA103 Form?
- Business details: your business name, a short description and the dates your accounts cover.
- Turnover: your total income from sales or services before expenses.
- Allowable expenses: one total on the SA103S, or split into categories on the SA103F.
- Capital allowances: claims for equipment, vans and other business assets.
- Net profit or loss: turnover minus expenses and allowances.
- CIS deductions: tax that contractors have already taken from your payments.
Cash Basis Is Now the Default
From the 2024/25 tax year, sole traders use the cash basis automatically. This means you record income when you receive it and expenses when you pay them. You can opt out and use traditional accounting instead, but most small businesses find the cash basis simpler.
Basis Period Reform
Since 2024/25, you report profits for the tax year itself (6 April to 5 April), not for your own accounting year. If your accounts end on a different date, you need to apportion your profits to match the tax year.
Which Expenses Can You Claim?
An expense is allowable when you incur it wholly and exclusively for your business. Common examples include:
- Stock, materials and cost of goods sold
- Business travel, but not your normal commute
- Phone, internet and software used for the business
- Accountancy and other professional fees
- Business insurance and subscriptions
If you use something partly for personal reasons, such as a mobile phone, you can only claim the business share.
Simplified Expenses: Flat Rates
Instead of working out actual costs, you can use HMRC’s simplified expenses flat rates for some costs:
| Cost | Flat rate |
|---|---|
| Car or van mileage | 45p per mile for the first 10,000 miles, then 25p |
| Motorcycle mileage | 24p per mile |
| Working from home (25 to 50 hours a month) | £10 a month |
| Working from home (51 to 100 hours a month) | £18 a month |
| Working from home (101 hours or more a month) | £26 a month |
The £1,000 Trading Allowance
If your expenses are small, you can claim the £1,000 trading allowance instead. You deduct £1,000 from your income rather than listing your actual costs. You cannot claim both, so choose whichever gives you the lower profit.
Do I Need to File an SA103 If I Made a Loss?
Yes. You should still report a loss. Declaring it lets you carry it forward against future profits from the same trade, or in some cases set it against your other income. If you skip the form, you may lose the right to use that loss later.
What If I File the Wrong Version?
Filing the SA103S when your turnover is over £90,000, or the other way round, does not usually lead to a penalty on its own. However, HMRC may query your return or ask for the correct form, which delays things. Check your turnover each year before you start.
Does Making Tax Digital Replace the SA103?
For your 2025/26 return, you still complete the SA103 as normal. From the 2026/27 tax year, sole traders with qualifying income over £50,000 send quarterly updates through software instead, followed by a year-end final declaration. Our Making Tax Digital guide explains who is affected and when.
When Is the SA103 Due?
The SA103 is due with your main tax return. For 2025/26, the online deadline is 31 January 2027. Our Self Assessment deadlines guide lists every key date.
If you would rather not deal with it yourself, our Self Assessment accountants in Slough choose the right version, claim every allowable expense and file your SA103 as part of your return. CIS subcontractors can also use our CIS service to reclaim overpaid tax.
Frequently Asked Questions
Can I file an SA103 online?
Yes. Both versions are part of HMRC’s online Self Assessment service, which shows you the right one based on your answers.
Do I need a separate SA103 for each business?
Yes. If you run more than one self-employed trade, each one needs its own SA103 within the same tax return.
Is the SA103 the same as the SA100?
No. The SA100 is the main tax return. The SA103 is a supplementary page you add to it for self-employment income.
What if my turnover changes during the year?
The version you need depends on your total turnover for the full tax year, so you only know for sure once the year ends.
Do I pay Class 2 National Insurance through the SA103?
Not usually. Since April 2024, self-employed people with profits above the Small Profits Threshold no longer pay Class 2. Your Class 4 National Insurance is still worked out from your SA103 profit.