IR35 is the off-payroll working rule HMRC uses to decide whether you're genuinely running a business or are, for tax purposes, a disguised employee. Get on the wrong side of it and the tax bill climbs sharply, because an "inside IR35" contractor is taxed much like a permanent member of staff. For anyone moving into contracting, understanding which side of the line you sit on matters as much as the day rate itself.
Inside versus outside
Outside IR35 means you're operating as a true business: you control how the work gets done, you can send a substitute, you take on real commercial risk, and you're not woven into the client's organisation like an employee. Inside IR35 means the relationship looks like employment in all but name - fixed hours, close direction, a desk and a manager - and you're taxed accordingly. The difference in take-home pay is not small, so it's worth knowing where you stand before you sign anything.
Since April 2021, for medium and large private-sector clients, it's the client - not you - who decides your status and issues a Status Determination Statement (SDS) explaining the reasoning. You're entitled to challenge an SDS you think is wrong, and you should, with evidence, if the determination doesn't match how you actually work.
Umbrella versus limited company
If you're inside IR35, working through an umbrella company is often the simplest route: the umbrella employs you, runs PAYE, and deals with the deductions, though it takes a fee for the privilege. If you're outside IR35, your own limited company usually leaves you better off and gives you more control, at the cost of more admin and an accountant's bill.
- Keep evidence that supports your status - contracts, working practices, and anything showing genuine business risk.
- Read the actual contract and check it matches reality, because a beautifully worded "outside" contract means nothing if you behave like staff.
- An umbrella that promises unusually high take-home pay may be a tax-avoidance scheme in disguise, so treat those claims with deep suspicion.
Where to get a straight answer
Commit to getting professional advice before you accept a role with an uncertain status - a specialist accountant or an IR35 contract review is cheap next to a retrospective HMRC bill. The gov.uk guidance and the CEST tool give you a starting point, though CEST is imperfect and its result isn't gospel.
IR35 rewards substance over wording. If you genuinely run a business and can show it, you've little to fear. If you're an employee wearing a contractor's lanyard, the rules will eventually notice.