IR35 comes up in almost every serious conversation about hiring fractional or interim marketing leadership, and it's usually asked about vaguely — "are you inside or outside IR35?" — as if it's a fixed property of the person rather than the engagement. It isn't. Status is determined by how the work is actually structured, not by a job title or a day rate.
What genuinely outside-IR35 work looks like
Control. The contractor decides how and when the work gets done, within the agreed scope — not managed day-to-day like an employee.
Multiple clients. Genuine fractional work means working across more than one business, not being embedded full-time in one as if employed, even if the title says "fractional."
No mutuality of obligation beyond scope. The business isn't obliged to keep offering work, and the contractor isn't obliged to accept whatever comes up outside the agreed engagement.
A right of substitution, in principle. Even if rarely exercised, genuine contracts typically allow the contractor to send a suitably qualified substitute in some circumstances.
Who's actually responsible for the determination
Under current off-payroll working rules, the hiring business is generally responsible for making the status determination for most engagements — commonly via HMRC's Check Employment Status for Tax (CEST) tool, considered alongside how the engagement is genuinely structured in practice, not just what the contract says on paper.
Why this matters more for interim than fractional
Interim engagements are typically full-time and dedicated to one business for a defined period — which sits closer to the line and deserves a proper status check every time. Fractional and advisory work, spread across multiple clients with genuine autonomy over delivery, more naturally sits outside IR35, though "naturally" still isn't a substitute for an actual determination.
What I'd actually recommend
Run the specific engagement through HMRC's CEST tool rather than relying on a general answer from anyone, including this page. If the result is genuinely ambiguous, a short conversation with an accountant who deals with contractor status regularly is worth more than any amount of general reading.
This is general information, not tax advice specific to your situation. Full detail on how engagements here are actually structured — fractional, interim, project or advisory — is on the services page.