Who decides your IR35 status?
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Short answer: usually the client — but not always, and "deciding" is not the same as "being right". Who holds the determination duty depends on the client's size and where you sit in the chain.
The default: medium and large clients decide
Since the off-payroll rules extended to the private sector (April 2021), a medium or large engager must assess your role for IR35, make a "reasonable care" determination, and record it in a Status Determination Statement. Their conclusion passes down the chain: the agency you contract through applies it, and so do you. A client that fails to make a determination must apply the inside-IR35 treatment by default — the risk sits with the fee-payer, not you.
The exception: small clients
If the client qualifies for the small-company exemption (two of three: turnover ≤ £15m, balance sheet ≤ £7.5m, ≤ 50 employees), the pre-2027-style rules apply and your own limited company makes the determination. That flips both the freedom and the liability back to you — use the CEST guide and the evidence checklist to work your position.
You can disagree — here's the mechanism
A determination you receive isn't final. You can challenge it: write to the client within 45 days, they have 45 days to respond, and a compliant client must give reasons and a final conclusion. Grounds that work: the role genuinely substitutes, control in practice is minimal, or the SDS misdescribes the actual working arrangements (compare it to your contract and your timesheets).
What an agency can and can't tell you
An agency must pass on the client's SDS. It can also run its own assessment where it's the fee-payer. What no party can do is hand-wave: "our client says inside" with no SDS behind it is exactly the pattern the 2026/27 enforcement guidance targets — and a role advertised inside that operates outside in practice is the classic misclassification dispute waiting to happen.
Source: ITEPA 2003 Chapter 10 (off-payroll working rules for clients), gov.uk guidance on SDS duties and the status disagreement process, verified October 2026.
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More in this cluster: evidence checklist · how we classify · the live board