IR35 has been a source of confusion, anxiety, and costly mistakes for UK contractors and small businesses since its introduction back in 2000. Yet despite over two decades on the statute books, many sole traders and hirers still do not fully understand when it applies, who is responsible for determining employment status, or what the financial consequences of getting it wrong can be. Whether you operate through a personal service company (PSC), regularly engage contractors, or handle payroll compliance for clients, this guide gives you the practical clarity you need.
What Is IR35 and Why Does It Exist?
IR35 — formally known as the off-payroll working rules — was introduced by HMRC to tackle what it called "disguised employment." The concern was straightforward: a worker leaves a company on a Friday as an employee, returns on Monday through their own limited company, does exactly the same job, but pays significantly less tax and National Insurance because they pay themselves in dividends rather than salary. HMRC viewed this as tax avoidance, and IR35 was designed to close that loophole.
At its core, IR35 asks a single question: if the intermediary — typically the contractor's limited company — did not exist, would the worker be classed as an employee of the end client? If the answer is yes, the contractor is "inside IR35" and must pay Income Tax and National Insurance Contributions (NICs) broadly equivalent to those of an employee. If the answer is no, they are "outside IR35" and can continue to benefit from the tax efficiencies of operating through a limited company.
The rules do not affect sole traders operating directly, since there is no intermediary involved — but if you are a sole trader who regularly subcontracts work to limited company contractors, understanding IR35 is still very much your concern.
The Two Regimes: Private Sector vs. Public Sector Rules
IR35 does not operate as a single, uniform set of rules. There are two distinct regimes depending on where the contractor works.
Public sector (from April 2017): The responsibility for determining whether a contractor is inside or outside IR35 shifted from the contractor's own company to the public authority engaging them. NHS Trusts, local councils, and government departments must assess each contractor's status and, if inside IR35, deduct tax and NICs through PAYE before paying the contractor's company.
Private sector (from April 2021): Medium and large private sector businesses now carry the same responsibility as public bodies. Since April 2021, if you are a medium or large company engaging contractors, you must issue a Status Determination Statement (SDS) for each engagement, stating whether the contractor is inside or outside IR35 and giving reasons. Crucially, small companies are exempt. A company qualifies as small if it meets at least two of these three criteria: annual turnover of no more than £10.2 million, a balance sheet total of no more than £5.1 million, and no more than 50 employees.
If your business is genuinely small under those thresholds, the contractor's own company remains responsible for its IR35 determination — the old rules still apply to you as the hirer.
The Employment Status Tests: What HMRC Actually Looks At
Whether you are a contractor assessing your own status or a medium-sized hirer issuing an SDS, the same underlying employment status tests apply. HMRC's Check Employment Status for Tax (CEST) tool is the official starting point, though many tax professionals treat it as one indicator rather than a definitive answer.
The key factors HMRC and the courts examine include:
- Substitution: Can the contractor send a substitute to do the work, and has this actually happened in practice? A genuine, unfettered right of substitution is a strong indicator of being outside IR35.
- Control: Does the hirer dictate how, when, and where the work is done? High levels of control point toward employment and IR35 applying.
- Mutuality of obligation (MOO): Is the hirer obliged to offer work and the contractor obliged to accept it? If yes, this suggests an employment relationship.
- Financial risk: Does the contractor bear real financial risk — for example, having to fix defective work at their own cost?
- Part and parcel of the organisation: Is the contractor integrated into the client's business, with a company email address, an office, and a line manager? Integration suggests employment.
No single factor is conclusive. HMRC looks at the overall picture, which is why accurately drafted contracts — ones that genuinely reflect working practices — are essential. A contract that says "right of substitution exists" but where a substitute has never been permitted in reality will not hold up under scrutiny.
Responsibilities for Hirers: Practical Steps to Stay Compliant
If your business engages contractors and you are not small under the Companies Act definition, here is what you must do in practice.
- Run a CEST assessment for every new contractor engagement and keep a record of the result and your reasoning.
- Issue a written SDS to the contractor and any agency in the labour supply chain before the engagement begins. The SDS must state your conclusion and the reasons behind it.
- Create a disagreement process. Contractors have the right to challenge your SDS. You must have a formal process to consider representations and respond within 45 days.
- Deduct PAYE and NICs for any contractor you determine to be inside IR35 before making payment. Remember: employer's NICs (currently 13.8%) become a cost to you, not the contractor.
- Review engagements periodically. A status determination is not permanent. Changes in working practice can alter IR35 status, so review contracts whenever the nature of the engagement changes significantly.
Maintaining thorough records is non-negotiable. HMRC can open an IR35 enquiry years after an engagement ends, and you will need documentary evidence to defend your position. Platforms like BizHub365 can help small businesses and accountants keep contractor records, invoices, and engagement histories organised in one place — which proves invaluable when correspondence from HMRC arrives unexpectedly.
Responsibilities for Contractors: Protecting Yourself
If you operate through a personal service company and your hirer is a small private sector business, the determination of IR35 status still rests with you. That is a significant responsibility, and the consequences of getting it wrong can be severe — HMRC can claim unpaid Income Tax, NICs, interest, and penalties going back years.
Start by ensuring your contract accurately reflects your actual working arrangements. Work with a specialist IR35 contract reviewer — organisations such as IPSE (the Association of Independent Professionals and the Self-Employed) provide resources and guidance. Consider taking out IR35 insurance, which covers the cost of an HMRC investigation and any resulting tax liability. Premiums are generally modest relative to the potential exposure.
Also, keep evidence of your outside-IR35 position: copies of contracts, evidence of substitution, records of working from your own premises, invoices to multiple clients, and any financial risk you have genuinely borne. If HMRC investigates, contemporaneous evidence is far more convincing than reconstructed paperwork.
Common Mistakes and How to Avoid Them
A few errors come up repeatedly in IR35 disputes.
- Treating all contractors the same: Blanket determinations — deciding all your contractors are inside IR35 to avoid the administrative burden — are explicitly criticised by HMRC and can expose you to legal challenge from contractors.
- Ignoring working practices: A carefully drafted outside-IR35 contract means nothing if the contractor sits in your office five days a week, uses your equipment, and reports to your management hierarchy.
- Forgetting the supply chain: If there is an agency between you and the contractor, you must pass the SDS to the agency. Responsibility for deducting PAYE falls on the fee-payer — usually the party making the payment to the contractor's company.
- Assuming small business status: Companies close to the thresholds should check their size classification each financial year, since growth can tip you into medium-sized territory and change your obligations.
Conclusion: Clarity Is Your Best Defence
IR35 is not going away. HMRC has consistently signalled that off-payroll compliance is a priority, and its investment in compliance activity has grown accordingly. The good news is that the rules, once understood, are manageable — provided you approach them with honesty and rigour.
For hirers, that means proper status determinations, well-drafted SDSs, and a clear audit trail. For contractors, it means ensuring your contracts and working practices genuinely reflect a business-to-business relationship. For accountants advising either group, staying current with tribunal decisions — cases such as HMRC v Atholl House Productions Ltd and Kickabout Productions Ltd continue to refine how the tests are applied — is an ongoing professional duty.
Get the fundamentals right, document everything, and take professional advice when an engagement sits in the grey area. That approach will protect both contractors and hirers far better than hoping an HMRC enquiry never lands on the doorstep.