IR35 has been a thorn in the side of UK contractors and the businesses that hire them since its introduction in 2000. Yet despite more than two decades on the statute books — and two major reforms — it remains widely misunderstood. HMRC collected an estimated £1.4 billion in additional tax through IR35 compliance activity in 2023/24 alone, which tells you just how seriously the taxman takes it. Whether you are a sole trader building a contractor client base, a small business regularly engaging freelancers, or an accountant advising both, this guide will give you a clear and practical foundation.
What Is IR35 and Why Does It Exist?
IR35 — formally known as the off-payroll working rules — exists to tackle what HMRC calls "disguised employment". The concern is straightforward: a worker who functions day-to-day as an employee but invoices through a limited company (often called a Personal Service Company, or PSC) pays significantly less tax than a salaried employee doing the same job. IR35 closes that gap by asking a single key question: if the intermediary company did not exist, would this worker be an employee of the engager?
If the answer is yes, the worker is said to be inside IR35. Their income from that engagement is treated as employment income for tax purposes, meaning Income Tax and National Insurance Contributions (NICs) must be accounted for — even though the worker receives no employment rights such as holiday pay or redundancy entitlement. It is a notoriously uncomfortable position to be in.
If the answer is no — the worker is genuinely in business on their own account — they are outside IR35 and can continue to be paid via dividends and salary in the normal way.
Who Is Responsible for Determining IR35 Status?
This is where the rules get more nuanced, and where many small businesses trip up. The responsibility for making a Status Determination Statement (SDS) depends on the size of the business doing the hiring.
- Public sector engagers (NHS trusts, local councils, government departments): Responsible for determining status since April 2017.
- Medium and large private sector businesses: Responsible since April 2021. A business is medium or large if it meets two or more of the following in a financial year — annual turnover above £10.2 million, balance sheet above £5.1 million, more than 50 employees.
- Small private sector businesses: The responsibility remains with the contractor's own PSC. A small business is one that does not meet the medium/large thresholds above.
For accountants advising owner-managed businesses, it is worth flagging that sole traders cannot engage contractors through a PSC arrangement in the same way — the IR35 rules specifically concern intermediaries such as limited companies. However, sole traders who operate their own PSC must still assess their own engagements carefully.
How Is Employment Status Actually Determined?
There is no single checklist HMRC uses. Status is assessed by weighing up a range of factors drawn from decades of case law. The key tests include:
- Control — Does the engager dictate how, when, and where the work is done? High levels of control point towards employment.
- Substitution — Can the contractor send a substitute to do the work? A genuine, unfettered right of substitution is one of the strongest indicators of self-employment.
- Mutuality of Obligation (MOO) — Is the engager obliged to offer work, and is the contractor obliged to accept it? If yes, this points towards employment.
- Financial risk — Does the contractor risk their own money? Providing their own equipment, correcting faulty work at their own cost, or working for multiple clients all point to genuine self-employment.
- Integration — Is the worker treated as part of the organisation — attending company away days, using a company email address, appearing on the org chart? Integration suggests employment.
HMRC's free CEST (Check Employment Status for Tax) tool at gov.uk can help both contractors and hirers assess status. It is a sensible starting point, but it is not infallible — courts have overturned CEST results, and HMRC itself does not guarantee to stand by the outcome if the facts entered are incorrect. Always back up a CEST result with a thorough written contract review.
Practical Steps for Contractors and Hirers
Knowing the theory is one thing. Putting it into practice is another. Here is what each party should be doing.
For Contractors (PSC owners)
- Review every contract before signing. Look specifically for substitution clauses, control language, and any provisions that imply ongoing obligation. If the contract reads like an employment contract, it probably is one in HMRC's eyes.
- Document your working practices. A contract can say one thing while day-to-day reality says another. Keep a contemporaneous record — emails, project briefs, invoices — that demonstrates you genuinely work as an independent business.
- Avoid the "Friday to Monday" trap. Leaving an employer on a Friday and returning as a contractor on Monday is a red flag HMRC knows well. Demonstrate genuine breaks, multiple clients, and independent business activity.
For Hirers (Medium/Large Private Sector and Public Sector)
- Issue a Status Determination Statement for every engagement. This is a legal requirement. Failing to issue one means the liability for unpaid tax falls on you, the engager.
- Give contractors the right to disagree. You must have a disagreement process in place. If a contractor challenges your SDS, you must respond within 45 days.
- Audit your contractor base regularly. A determination made two years ago may not reflect current working practices. Review status when contracts are renewed or working arrangements change significantly.
For businesses managing multiple contractor relationships alongside payroll, PAYE obligations, and client invoicing, keeping all of this organised in one place makes a tangible difference. BizHub365 supports full RTI payroll including FPS and EPS submissions direct to HMRC, which means that when an engagement is correctly deemed inside IR35 and tax needs to be processed through payroll, there is no need for separate bridging software or manual workarounds.
What Happens If You Get It Wrong?
The consequences of incorrect IR35 determinations are serious on both sides of the engagement.
For contractors who have incorrectly treated income as outside IR35, HMRC can raise assessments covering unpaid Income Tax and NICs going back up to six years (20 years in cases of deliberate non-compliance). Interest and penalties are applied on top. Several high-profile cases — including TV presenters and IT contractors — have resulted in six-figure tax bills.
For hirers who fail to make an SDS or make a careless determination, the liability for the contractor's unpaid PAYE and NICs transfers to them. HMRC has been actively opening compliance checks on medium and large businesses since 2022, with particular focus on financial services, construction, and the public sector.
One important point often overlooked: even if an engager makes a genuine, reasonable determination that a contractor is outside IR35 and it later turns out to be wrong, demonstrating that the process was thorough and documented can significantly reduce penalties. Process matters as much as outcome.
Conclusion: Clarity Beats Complacency
IR35 is not going away. In fact, HMRC has consistently signalled that off-payroll compliance is a priority, and the shift of responsibility to engagers in the private sector has only increased scrutiny across the board. The good news is that the rules, whilst complex, are navigable with the right approach.
Contractors should treat every engagement as a standalone assessment — never assume outside IR35 because the previous contract was. Hirers should build status determination into their onboarding process as standard, not as an afterthought. And accountants advising both should ensure their clients have documented evidence to back up whatever determination has been reached.
Getting IR35 right is ultimately about substance over form. If a working arrangement genuinely reflects independent contracting, the paperwork should reflect that too. If it does not, fixing the paperwork alone will not help you in an HMRC enquiry — but fixing the working practices will.