What Is an SA103 Form?
An SA103 form is the supplementary page self-employed individuals attach to their Self Assessment tax return to report business income and expenses. HMRC requires it alongside the main SA100 return whenever someone has self-employment income to declare. It captures turnover, allowable expenses, and net profit for the tax year, which HMRC then uses to calculate the tax due.
This guide explains the two versions of the form, who must use each one, and how to complete the sections correctly.
SA103 Short Versus SA103F: What Is the Difference?
Two versions of this supplementary page exist, and the correct choice depends on turnover.
- SA103 Short: for self-employed individuals with annual turnover below £90,000. It asks for simplified totals rather than a full breakdown of every expense category.
- SA103F (Full): required when turnover exceeds £90,000, or when the business has more complex circumstances, such as claiming certain capital allowances or reporting a change in accounting basis. It requests a detailed breakdown across specific expense categories rather than combined totals.
Filing the wrong version does not automatically trigger a rejection, but HMRC may request the correct one, which delays processing.
Who Needs to File an SA103?
Filing this page becomes necessary for anyone with self-employment income that must go through Self Assessment. This includes:
- Sole traders running an ongoing trade or business
- Self-employed individuals with multiple trades, who complete a separate SA103 for each distinct trade
- CIS subcontractors, who report their Construction Industry Scheme income and any tax already deducted at source through this form
- Anyone starting self-employment partway through the tax year, even with modest turnover, who will also need a UTR number before filing
A director who only receives PAYE salary and dividends does not need this form, since that income falls under different sections of the main return.
What Information Goes on the SA103?
The form asks for figures across several categories rather than a single profit total.
- Business details: trading name, business description, and the accounting period covered
- Turnover: total income from sales or services before any expenses
- Allowable expenses: costs such as materials, travel, and business premises, broken into categories on the SA103F, or combined on the SA103 Short
- Capital allowances: claims for equipment, vehicles, or other business assets
- Net profit or loss: turnover minus expenses and allowances, which feeds into the overall Self Assessment tax calculation
Basis period reform, which took full effect from the 2024-25 tax year, also affects how profit gets allocated to a specific tax year rather than an accounting period, so figures should align to the tax year rather than a business’s own year-end.
How Do I Know Which Expenses Are Allowable?
An expense qualifies as allowable when HMRC accepts it as incurred wholly and exclusively for the business. Common allowable categories include:
- Cost of goods sold or materials
- Business travel, excluding ordinary commuting
- Use of home as office, calculated on a reasonable basis
- Professional fees, including accountancy costs
- Business insurance and subscriptions
Personal expenses, even when partly business-related, generally cannot be claimed in full. A mixed-use cost, such as a mobile phone used for both business and personal calls, needs an apportioned figure rather than the total amount.
Do I Need to File an SA103 If My Business Made a Loss?
Yes. A loss still requires reporting on this form. Declaring a loss allows it to be carried forward against future profits from the same trade, or in some cases set against other income in the same tax year. Skipping the form because no tax is due removes the ability to use that loss later.
What Happens If I File the Wrong Version?
Submitting the SA103 Short when turnover exceeds the £90,000 threshold, or vice versa, does not usually result in a penalty on its own. HMRC generally still processes the return, but a mismatch can prompt a query or a request for the correct form, which extends the time it takes to finalise the return.
For anyone unsure which version applies, especially where turnover sits close to the threshold, reviewing this each year before starting the return avoids the back-and-forth. Our Self Assessment accountants in Slough check which version applies and complete the SA103 as part of your return.
Frequently Asked Questions
Can I file an SA103 online? Yes. Both versions are available through HMRC’s online Self Assessment service, which selects the correct one automatically based on the turnover figure entered.
Do I need a separate SA103 for each business? Yes, if someone runs more than one distinct self-employed trade, each one needs its own SA103 supplementary page within the same tax return.
Is the SA103 the same as the SA100? No. The SA100 is the main Self Assessment return, while the SA103 is a supplementary page attached to it specifically for self-employment income.
What if my turnover changes partway through the year? The version required is based on total turnover for the full tax year, not the turnover at any single point, so this is only known once the year’s figures are complete.