SA103 Expense Classifier
AI-powered classification for UK sole trader expenses. Map any expense to the correct HMRC SA103 category in seconds.
10 free classifications per month · 100/month with Pro
See how it works: example classifications
“Train ticket to client meeting £45”
Box 12: Car, van and travel expenses
“Accountant's annual fee £500”
Box 21: Accountancy, legal and other professional fees
“Vercel hosting subscription £20/month”
Box 16: Phone, fax, stationery and other office costs
“Replacement laptop screen £200”
Box 15: Repairs and renewals of property and equipment
These are pre-computed examples. Try your own classification below.
Classify your expense
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Start your free trial: 10 classifications per month, free
Sign up to start classifying your expenses. Free plan includes 10 SA103 classifications per month. Upgrade anytime for 100/month with Pro.
What is SA103?
SA103 is the Self Assessment supplementary page for self-employment income. UK sole traders and freelancers use it to report business income and expenses to HMRC. The short form (SA103S) is for businesses with turnover under the VAT threshold (£90,000 in 2025/26) and uses 13 numbered boxes (11–23) to classify deductible expenses.
Getting the right box matters for two reasons:
- MTD requires categorisation. From 6 April 2026, MTD for Income Tax requires quarterly digital updates that include expense categories. See our MTD Deadline Tracker to find out which phase applies to you.
- Misclassification can trigger HMRC enquiries. Putting an expense in the wrong box doesn't usually change the total tax you owe, but inconsistent categorisation across years can attract scrutiny.
How this tool helps
Describe any business expense in plain English (for example, "new laptop for client work £899" or "drove 120 miles to a trade show") and the AI classifies it into the correct SA103S box. You get:
- The recommended box (e.g. Box 16: Phone, fax, stationery and other office costs)
- Whether it's fully allowable, not allowable, or partially deductible
- A 2–3 sentence reason
- What records you should keep (receipts, mileage logs, etc.)
- Caveats and edge cases
The first 10 classifications per month are free. Pro raises this to 100/month.
Why we use AI for this
There are 13 SA103S boxes, and many real-world expenses sit on the line between two of them (is a coworking-space day pass Box 14 premises or Box 16 office costs? Probably Box 14, but cash-basis sole traders sometimes file it under Box 16, and the AI flags this kind of ambiguity).
A pre-coded lookup table can't handle the long tail. An LLM can read your description, weigh the context, and surface the right box with reasoning.
Limitations and when to consult an accountant
Katalyst is not on HMRC's MTD-recognised software list. This tool is a planning aid; it doesn't file anything on your behalf, and it isn't professional tax advice.
You should consult a qualified accountant when:
- The expense involves mixed business/personal use (a car, home office, phone bill)
- You're considering capital allowances on equipment over £1,000
- You have complex deductions (R&D, training expenses, foreign income)
- HMRC has contacted you about an enquiry or correction
For straightforward expenses, this tool will give you the right box almost every time. For edge cases, get a second opinion. Source: HMRC SA103S form guidance.
Frequently asked questions
What is SA103S?▼
SA103S is the short version of the SA103 Self-Employment supplementary page in HMRC's Self Assessment. It's for businesses with turnover under the VAT threshold (£90,000 in 2025/26) and has 13 numbered boxes (11–23) for classifying deductible business expenses.
Source: gov.uk SA103S form.
How accurate is the AI classification?▼
The AI uses HMRC's official SA103S category descriptions and is tuned for conservative, defensible classifications. For common expenses (travel, software subscriptions, professional fees), it's reliably accurate. For edge cases (mixed-use items, capital purchases, unusual business activities) it will flag the ambiguity in the caveats field rather than guess. Always confirm complex cases with a qualified accountant.
What categories does this cover?▼
All 13 SA103S boxes for self-employment expenses:
- Box 11: Cost of goods bought for resale
- Box 12: Car, van and travel expenses
- Box 13: Wages, salaries and other staff costs
- Box 14: Rent, rates, power and insurance
- Box 15: Repairs and renewals of property and equipment
- Box 16: Phone, fax, stationery and other office costs
- Box 17: Advertising and business entertainment
- Box 18: Interest on bank and other loans
- Box 19: Bank, credit card and other financial charges
- Box 20: Irrecoverable debts written off
- Box 21: Accountancy, legal and other professional fees
- Box 22: Depreciation and loss/profit on sale of assets (cash basis: not used)
- Box 23: Other business expenses
Why do I need to sign up?▼
The page is free to browse. Read the explainer, FAQ, and sample classifications without signing up. Submitting an expense to be classified runs an AI call, which costs us money per request. We require a free account so we can apply a fair monthly limit (10 free classifications per month) and so the tool isn't drained by automated abuse.
What happens after I hit 10 free classifications?▼
You'll see an upgrade prompt suggesting Pro, which raises the limit to 100 classifications per month. The counter resets on the 1st of each month. Pro also unlocks the rest of Katalyst's freelance toolkit (deadline tracking, penalty estimation, MTD-ready records, invoicing, expenses, and more).
Can I trust this for my Self Assessment?▼
For straightforward expenses, the classifications are reliable. For anything complex or unusual, confirm with a qualified accountant. The tool always includes a caveats note flagging when something needs a second opinion. We do not file anything to HMRC on your behalf. When you submit your Self Assessment or MTD quarterly update, you'll use MTD-recognised software from gov.uk's official list.
What if my expense doesn't fit a category?▼
Box 23 ("Other business expenses") catches genuinely uncategorisable items. The AI tries to find the closest fit among Boxes 11–22 first and only falls back to Box 23 when nothing else applies. If your expense genuinely belongs in Box 23, the response will explain why.
How is Katalyst different from MTD-recognised software?▼
Katalyst is not on HMRC's MTD-recognised software list. We're a planning tool. We help you keep MTD-ready records, estimate penalties, and classify expenses, but we don't submit to HMRC for you. When you're ready to file, you'll use compatible software from gov.uk's official list. Think of Katalyst as the planning layer; MTD-recognised software is the submission layer.