IR35 case law: why tribunal decisions — not tools — decide your status
Direct Answer
There's no tidy statutory checklist for IR35. Employment status is decided by case law — principles the courts have built up over decades and apply to the facts of each engagement. Three tests run through all of it: mutuality of obligation, control, and personal service, weighed alongside whether you're genuinely in business on your own account. HMRC's CEST tool is only their reading of that case law — when a dispute reaches a tribunal, the case law wins. This hub explains why the cases matter and links to our worked case breakdowns.
Why case law, not a rulebook, decides IR35
IR35 asks a single hard question: if you stripped away your limited company, would the relationship between you and the client look like employment? Parliament never wrote a definitive test for that. Instead the courts decide it, drawing on a line of cases stretching back to Ready Mixed Concrete (1968), which first set out the framework still used today.
That's the crucial point for contractors: HMRC's guidance and its CEST tool are interpretations of the case law, not the law itself. When a status dispute is actually litigated, the tribunal applies the case-law tests to what really happened — which is why the leading decisions, and how they came out, matter more than any online checker.
The three tests that run through every case
| Test | The question it asks | Points to self-employment when… |
|---|---|---|
| Mutuality of obligation | Must the client offer work, and must you accept it? | Neither side is obliged to provide or take ongoing work |
| Control | How far can the client direct what, how, when and where you work? | You decide how the work is done, with limited client direction |
| Personal service | Must you do the work yourself? | You have a genuine, usable right to send a substitute |
No single test is decisive on its own. Tribunals stand back and ask whether, taken together and alongside the wider picture, the worker is genuinely in business on their own account. For the mechanics of how HMRC's tool handles these, see what is CEST.
Case breakdowns
We publish plain-English breakdowns of the landmark decisions — the facts, what the tribunal actually reasoned, and the practical takeaway for your own contracts:
More case breakdowns are on the way. We're expanding this series to cover further significant tribunal decisions. Each new page will follow the same format — facts, tribunal reasoning, and what it means for contractors — and will be linked here as it's published.
How to use the case law to protect your position
The consistent lesson from the tribunals is that they look at the reality of an engagement, not just the wording of the contract. To sit comfortably outside IR35, both should point the same way:
- A genuine right of substitution that isn't fettered in practice.
- Limited client control over how you deliver the work.
- No mutuality of obligation — no expectation of a continuous flow of work on either side.
- Clear signs you're in business on your own account — your own equipment, financial risk, other clients, and marketing.
Start with the IR35 questions guide and the full IR35 guide for how these apply to a contract review.
Outside IR35? Make sure your contracts can stand up to the case-law tests.
Autobooks works with contractors to keep their outside-IR35 position defensible — from £89+VAT/month.