IR35, also known as the off-payroll working rules, is one of those topics that makes business owners and contractors groan. It is genuinely complex, the rules have changed several times, and getting it wrong can be expensive. But the core idea is not as scary as the reputation suggests, and understanding it helps you stay on the right side of the line.
This guide explains what the rules are, the difference between inside and outside IR35, who decides status, and how to reduce your risk. It is written in plain English for small businesses and the contractors they work with.
This article is general information only and is not legal advice. Employment rules can change, so always confirm the details that apply to you on GOV.UK or speak to a qualified employment adviser.
What the off-payroll working rules are
The off-payroll working rules exist to stop people working like employees while being paid like a business to save on tax. Imagine someone who turns up every day, does the same job as the staff around them, and reports to the same manager, but invoices through their own limited company. To the tax system, that can look like disguised employment.
IR35 is designed to test whether a contractor working through their own company is genuinely running a business, or whether the working relationship really looks like employment. If it looks like employment, the arrangement should be taxed roughly like employment.
The rules do not say contracting is wrong. Plenty of people run real, independent businesses serving many clients. The rules simply ask: is this particular engagement employment in all but name?
Inside versus outside IR35
You will hear contracts described as inside or outside IR35. The difference is about how the work is taxed.
- Outside IR35 means the engagement is treated as a genuine business-to-business arrangement. The contractor is running their own business and is responsible for their own taxes in the usual way.
- Inside IR35 means the engagement looks like employment for tax purposes. Tax and National Insurance are generally handled more like they would be for an employee, even though the person is not entitled to employment rights from that arrangement.
The uncomfortable truth is that someone can be inside IR35 for tax without gaining the rights of a real employee. That is why getting the status right, and structuring genuine arrangements properly, matters so much to both sides.
Who decides status and the small-company question
Who is responsible for deciding a contractor's status depends on who the client is and how big they are. In many cases the responsibility for assessing status, and any resulting tax, sits with the organisation engaging the contractor rather than the contractor themselves.
There has long been an idea that smaller private-sector clients are treated differently from larger ones, with the responsibility falling differently when the client is a small company. The definitions of what counts as small, and exactly where responsibility lands, are the kind of detail that changes, so do not take any figure here as current. Check the position on GOV.UK or with an adviser before you rely on it.
What does not change is the underlying test. Whoever is responsible, status is decided by looking at how the work is really carried out, not just what the contract says on paper.
The key status factors
Three factors come up again and again when status is assessed. None decides the answer alone, but together they paint a picture.
- Control. How much say does the client have over how, when and where the work is done? The more they direct the contractor like an employee, the more it looks like employment.
- Substitution. Could the contractor send a qualified substitute to do the work in their place? A genuine right to substitute points towards being in business on your own account.
- Mutuality of obligation. Is the client obliged to offer work and the contractor obliged to accept it, on an ongoing basis? Strong mutual obligation looks more like employment.
Other factors matter too, such as who provides the equipment, whether the contractor takes financial risk, and whether they work for other clients. The overall picture is what counts.
HMRC's CEST tool and getting it wrong
HMRC provides a free online tool called Check Employment Status for Tax, usually shortened to CEST. You answer questions about the working arrangement and it gives a view on the likely status. It is a useful starting point and creates a record of the answers you gave.
CEST is not the final word, though. It is only as accurate as the information you put in, and it does not cover every situation neatly. Treat it as one input, keep the result, and consider professional advice for anything borderline or high value.
The risks of getting status wrong are real. If an engagement is treated as outside IR35 when it should have been inside, the party responsible can face demands for unpaid tax and National Insurance, plus interest and possible penalties. Across several contracts over several years, that can add up to a serious sum.
How to stay on the right side
You cannot make an engagement outside IR35 just by wording the contract cleverly. What matters is reality. The contract and the day-to-day working practices need to line up and genuinely reflect an independent business relationship. A few sensible habits help:
- Make sure written contracts match how the work is actually done.
- Assess each engagement on its own facts rather than assuming.
- Keep records of your status assessments and the reasons behind them.
- Avoid treating contractors exactly like employees, with the same hours, line management and perks.
- Get advice when an engagement is long, high value or unclear.
If you also take on genuine employees, it is worth being clear on the difference. Our guides on what to include in employment contracts and PAYE payroll basics for small employers cover the employment side. And whoever you take on, do not forget your right to work checks.
IR35 is decided by how the work is really done, not how the contract is dressed up, so make sure the paperwork and the practice tell the same story.
Frequently asked questions
Does IR35 apply to sole traders?
The off-payroll rules are aimed at people working through an intermediary such as their own limited company, rather than sole traders billing a client directly. Sole traders have their own employment status questions to consider, but they are assessed under different rules. If you are unsure which applies to your situation, check on GOV.UK or take advice.
Can a clever contract make me outside IR35?
No. Wording alone cannot determine status. What matters is the reality of the working relationship, including control, substitution and mutuality of obligation. If the contract says one thing but the day-to-day work looks like employment, the reality usually wins. Make sure your paperwork genuinely reflects how the work is carried out.
What happens if I get an IR35 decision wrong?
The party responsible for the decision can face a bill for unpaid tax and National Insurance, plus interest and possibly penalties, often going back over the period the wrong treatment applied. This is why it is worth assessing each engagement carefully, keeping records, and getting advice on anything borderline rather than guessing.
IR35 rewards businesses that keep their arrangements genuine and well documented. For more plain-English guides on employing people and staying compliant, join our newsletter and we will send the practical ones your way.
