The self-employment pages of the Self Assessment return come in two versions. SA103S, the short form, is for businesses with turnover under £90,000 (the VAT registration threshold) and no complications; it lets you enter one total for expenses. SA103F, the full form, asks for a breakdown. If you keep records by category anyway, the full breakdown is no extra work and answers HMRC’s questions before they’re asked.
Income
| Box (SA103F) | What goes in it |
|---|---|
| 15 | Turnover: everything you invoiced or were paid, depending on whether you use traditional or cash basis accounting |
| 16 | Any other business income, such as grants (interest the business earns is savings income, declared on the main return) |
| 16.1 | The trading income allowance, if you claim the £1,000 allowance instead of expenses |
If you claim the allowance in box 16.1, the expense boxes below stay empty. It’s one or the other. Trading allowance or actual expenses explains how to choose.
The expense boxes
| Box | Description on the form | Typical examples |
|---|---|---|
| 17 | Cost of goods bought for resale or goods used | Stock, raw materials, components |
| 18 | Construction industry subcontractor costs | Payments to subcontractors under CIS |
| 19 | Wages, salaries and other staff costs | Employees’ pay, employer NI, pension contributions for staff (not your own drawings) |
| 20 | Car, van and travel expenses | Mileage at the flat rate, parking, tolls, train and bus fares, hotel stays for business |
| 21 | Rent, rates, power and insurance costs | Workshop rent, business rates, electricity, working-from-home costs, business insurance |
| 22 | Repairs and maintenance of property and equipment | Fixing tools, servicing machinery, repairs to premises |
| 23 | Phone, fax, stationery and other office costs | Business phone and internet, postage, printing, software subscriptions |
| 24 | Advertising and business entertainment costs | Website, adverts, samples, marketing. Entertaining clients goes here but is not allowable |
| 25 | Interest on bank and other loans | Interest on a business loan or overdraft |
| 26 | Bank, credit card and other financial charges | Bank charges, card processing fees, PayPal fees |
| 27 | Irrecoverable debts written off | Invoices you’ve given up on collecting |
| 28 | Accountancy, legal and other professional fees | Accountant, solicitor, professional subscriptions |
| 29 | Depreciation and loss or profit on sale of assets | Not allowable for tax, so it never reduces your profit; capital allowances are claimed separately |
| 30 | Other business expenses | Anything genuinely for the business that fits nowhere else: marketplace and platform selling fees, training, small tools and equipment under the cash basis, protective clothing, trade magazines |
| 31 | Total expenses | The sum of boxes 17 to 30 |
Two boxes deserve a second look. Box 24 takes advertising and business entertainment, but entertaining customers is disallowed, so the form asks you to add it back in the disallowable column. Box 29, depreciation, is never allowable: the tax system replaces it with capital allowances, which have their own boxes further down the form.
From records to return
The tidy way to do this is to give every expense a category when you record it, with each category mapped to a box. The Sole Trader Bookkeeping spreadsheet has eighteen categories in a dropdown, each already mapped, and an SA103 Summary tab that totals boxes 17 to 28 and 30 for the tax year (box 29, depreciation, is never allowable), adds the mileage claim to box 20 and the working-from-home claim to box 21. The Tax Estimate tab tells you when the trading allowance would beat the lot. Filing the return becomes copying a column.
A note on box numbers
The numbers above follow the 2025/26 SA103F. HMRC publishes each year’s form the following April and occasionally renumbers, so check the 2026/27 pages against your figures when you file in early 2028. The descriptions are stable even when the numbers move.
Questions people ask
Where does my mileage claim go?
Box 20, car, van and travel expenses, on the full SA103F. Parking, tolls and train fares go in the same box.
Where do working-from-home costs go?
Box 21, rent, rates, power and insurance costs, whether you use HMRC's flat rate or a share of actual household bills.
Do I have to break my expenses down at all?
If your turnover is under £90,000 you can use the short form SA103S and enter a single total for expenses. The full breakdown is compulsory above that; below it you can still give the breakdown, and it makes any query easier to answer.
Will the box numbers be the same for 2026/27?
Usually, but HMRC occasionally renumbers. The numbers here follow the 2025/26 SA103F. Check them against the 2026/27 form when HMRC publishes it in April 2027.
Sources
- HMRC: Self-employment (full) notes, SA103F, tax year 6 April 2025 to 5 April 2026.
- HMRC: Self-employment (short) notes, SA103S, tax year 6 April 2025 to 5 April 2026.
- gov.uk: Self Assessment tax return forms, self-employment supplementary pages.
This is general information, not tax advice. Figures are as published on gov.uk on the date shown and can change. Check them, or ask an accountant, before you rely on them.