IR35 sends a lot of good contractors into a spin, mostly because it’s explained in legalese. Stripped back, it asks one question: are you genuinely running a business, or an employee in all but name?
What IR35 is actually testing
The rules look past your contract to how you really work. Three things matter most: control (who decides how, when and where you work), substitution (could you send someone else?), and mutuality of obligation (must they offer work, and must you take it?). The more you look like staff, the more likely you’re “inside”.
Who decides your status
For most medium and large clients, the end client makes the determination, not you, and not your agency. They must take “reasonable care” and give you a Status Determination Statement. If something looks wrong, you’re entitled to challenge it.
Questions to ask before you sign
Ask for the determination in writing, ask whether the role is inside or outside and why, and ask how a genuine substitution would be handled. None of these are awkward questions. A well-run client will have clear answers ready.
This is general information, not tax advice. For your own position, speak to a qualified accountant.