Understanding IR35 and Off-Payroll Working Rules
IR35 and the off-payroll working rules rank among the most misunderstood — and most anxiety-inducing — areas of UK tax law. Since April 2021, when HMRC extended these rules to private-sector businesses, every medium and large company engaging contractors through personal service companies has had to wrestle with employment status determinations, fee-payer obligations, and the constant threat of getting it wrong. This guide cuts through the jargon and explains what you actually need to do to stay compliant.
What is IR35?
IR35 is the shorthand name for the intermediaries legislation introduced in 2000. Its job is simple on the surface: prevent workers from dodging income tax and National Insurance by routing themselves through a personal service company (PSC) when the working arrangement is effectively employment. HMRC calls this "disguised employment" — and they take it seriously.
The core question is: if you stripped away the intermediary, would the arrangement look like a traditional employment? If yes, the worker is "inside IR35" and should be taxed as an employee — meaning income tax and National Insurance through PAYE rather than dividends. HMRC provides a free online tool called CEST (Check Employment Status for Tax) to help make determinations, though it has earned a reputation for producing dodgy results in borderline cases.
Who's responsible for the determination?
Here's where it gets tricky. Before April 2021, the contractor's own company decided whether IR35 applied. The off-payroll working rules flipped that responsibility: now the end client has to decide — but only if you're a medium or large business.
You're considered medium or large if you meet two or more of these thresholds:
- Annual turnover above £10.2 million
- Balance sheet total above £5.1 million
- More than 50 employees
If you fall short on all three, you're a small company, and the responsibility stays with the contractor's PSC. That said, you should still understand the rules. They affect who you can safely engage and on what terms.
The three tests: how HMRC decides if someone is an employee
Employment status comes down to three tests. None of them is decisive on its own — it's the overall picture that matters.
Personal service. Can the contractor genuinely send someone else to do the work in their place? If they must show up personally, that screams employment. If they can send a qualified substitute without asking your permission, that screams self-employment. The critical word: genuine. A substitution clause that's theoretically there but never used in practice won't fool HMRC. They look at what actually happens, not what the contract prettifies says.
Mutuality of obligation. Is there a mutual obligation for you to provide work and for the contractor to accept it? Employees have that — you must offer work, they must do it. Genuine contractors don't. If someone's been with you continuously for two years, getting steady assignments with no formal engagement process, the mutuality test points to employment.
Control. Who decides how, when, and where the work gets done? Employees get told. Contractors should have autonomy over their methods, hours, and location. If your "contractor" must work 9-to-5 in your office, follow your processes, report to a line manager, and can't take other clients during the engagement, those are employment indicators — no matter what the contract says.
How to assess an engagement (if you're medium or large)
The process is straightforward in theory, messy in practice.
Step 1: Run the facts through the three tests. Use CEST as a starting point — it's free, it's official, and it'll give you something to point to if HMRC asks. But don't stop there. CEST doesn't capture everything, and it's produced incorrect results in enough borderline cases that you shouldn't rely on it alone. If the engagement looks murky, get professional advice.
Step 2: Issue a Status Determination Statement (SDS). Put your conclusion in writing — inside IR35 or outside — along with the reasons why. Give a copy to the contractor and the fee-payer (if there's an agency in the middle). This isn't bureaucracy for its own sake. The SDS is your evidence that you took reasonable care. Without it, you're exposed.
Step 3: Document your working practices. What you write in the contract is one thing. What actually happens is what HMRC cares about. If your contractor can theoretically send a substitute but you've never accepted one and they've never offered, that mismatch will flag in an enquiry. Make sure your actual practice matches your contractual terms.
Step 4: Keep records. File your assessments, the evidence you considered, and any disagreements the contractor raised. You may need to defend these decisions years later if HMRC opens an enquiry.
What "inside IR35" actually means for your cashflow
If you decide an engagement is inside IR35, the fee-payer (you, or an agency acting on your behalf) must deduct income tax and employee National Insurance from the payment before it reaches the contractor's PSC. You also pay employer National Insurance on top. The result: the contractor gets significantly less than they would under an outside-IR35 arrangement, which is why many will either negotiate higher rates or walk away. Managing these deductions requires proper payroll infrastructure — the same Real Time Information (RTI) reporting you'd do for an employee.
The disagreement process: when contractors push back
If a contractor disagrees with your determination, they can challenge it in writing. You then have 45 days to either revise your decision or defend the original one with reasons. Ignoring a disagreement isn't just rude — it can shift the tax liability to you. Treat every pushback seriously and respond on time.
Common mistakes that get businesses fined
"Inside IR35 for everyone" policies. Some businesses blanket-classify all contractors as inside IR35 to dodge the complexity. This isn't taking reasonable care — it's not taking care at all. It also drives away skilled contractors who genuinely operate outside IR35.
Trusting CEST too much. CEST is a useful starting point, but it's not the whole picture. Supplement it with professional advice for anything borderline.
Ignoring what actually happens. The contract might claim the contractor can send a substitute, but if they've never done it and you wouldn't accept it, HMRC will look at reality, not paperwork.
Failing to reassess. Engagements drift over time. A contractor hired for a defined project might gradually become a permanent fixture. Reassess your determinations regularly, especially when scope changes.
Staying compliant: record-keeping and payroll integration
IR35 compliance gets easier when contractor engagements are tracked alongside your regular payroll. You need to know which contractors are inside IR35, when determinations were made, and when reassessments are due. Good HR and payroll software integrates this tracking with your standard PAYE obligations, ensuring deductions are correct and RTI reporting is on time.
If you're running payroll manually across spreadsheets and separate contractor files, you're making this harder than it needs to be.
The penalties for getting it wrong
If HMRC decides you failed to take reasonable care, or your determinations are simply wrong, the tax liability can land on you as the end client. That's income tax, employee and employer National Insurance, plus interest and penalties. In serious cases, HMRC can pursue careless or deliberate inaccuracy penalties on top, which can reach 100% of the underpaid tax.
Frequently Asked Questions
Q: Can I use CEST as my sole evidence of reasonable care? A: CEST is a good starting point and gives you something to point to, but it's not foolproof. HMRC expects you to supplement it with consideration of actual working practices, especially in borderline cases. If you can't explain why you reached your determination beyond "CEST said so," you're undercooked on reasonable care.
Q: What should I do if a contractor disagrees with my SDS? A: Take it seriously. Respond in writing within 45 days with either a revised determination or a detailed explanation of why you're standing by the original. Ignoring a disagreement can shift tax liability to you. Document everything and consider seeking advice if the disagreement points to a grey area.
Q: Do I need to reassess contractors regularly? A: Yes. Engagements change — scope, hours, line-management structure, location. If any of those shifts materially, reassess your determination. At minimum, review your assessments annually or when the contract is renewed.
Q: If I'm a small business, can I ignore IR35? A: No. If you meet all three size thresholds below, the contractor's PSC bears the determination responsibility, but you should still understand the rules. They affect who you can safely engage and on what terms. You could still be caught up in an HMRC enquiry if something goes wrong.
Q: What counts as "reasonable care" in making a determination? A: Reasonable care means considering the actual working practices, not just contract language. It means using CEST but supplementing it with professional judgment or advice. It means documenting your decision. It means reassessing when circumstances change. In short: treat it like you mean it, not like a box-ticking exercise.
Q: If an engagement is inside IR35, does the contractor get paid less? A: Yes. The fee-payer deducts income tax and National Insurance before the money reaches the contractor's PSC, and the contractor also owes employer National Insurance on top. The net result is a significant reduction in take-home pay compared to an outside-IR35 arrangement, which is why many contractors will renegotiate their rates or decline the work.
Q: Who pays the tax if an engagement is inside IR35? A: The fee-payer (you, or an intermediary agency) is responsible for operating PAYE and making the deductions. This is the same obligation as for a traditional employee. If you engage the contractor directly, you bear the obligation. If an agency sits in the middle, the obligation typically falls on the agency, though contractual language may shift it.
Q: Can IR35 status change mid-engagement? A: Yes. If the scope of work changes materially — for example, a project-based contractor gradually becomes permanent, or they start working more hours and more directly under your management — the status determination can shift. Regular reassessment helps catch these shifts before they become compliance problems.
The bottom line
IR35 compliance isn't about finding loopholes or treating contractors as expenses to minimize. It's about assessing the working arrangement honestly, documenting your reasoning, and making sure your actual practice matches your contractual terms. Take time on the first assessment, reassess when things change, and keep records. If you're a medium or large business, this is now a core compliance obligation — getting it right protects you from both tax liability and the cost of HMRC enquiries.
For detailed guidance, consult HMRC's official IR35 information and consider specialist advice if you're working in grey-area industries.