IR35 Explained: A Complete Guide for Contractors and Finance Professionals

IR35 is a set of UK tax rules that has caused a great deal of discussion — and some confusion — particularly among contractors, businesses that engage them, and the accountants who advise both. Understanding what IR35 is and how it works is important for anyone affected. This guide explains IR35 in clear, plain language: what it is, why it exists, how it works in principle, and why it matters. Because the rules are detailed and have changed over time, always check the current position with HMRC or a qualified adviser for specific situations. It complements our broader taxation guides.

What is IR35?

IR35 refers to UK tax rules designed to address what's sometimes called "disguised employment". The core issue it targets is this: where an individual provides their services through an intermediary — typically their own personal service company — but works in a way that, were it not for that company, would look like employment, the rules aim to ensure that broadly the right amount of tax is paid. In other words, IR35 is concerned with situations where someone is, in substance, working like an employee but is engaged through a company structure in a way that could otherwise reduce the tax paid. The rules seek to tax such arrangements more like employment. Understanding this underlying purpose helps make sense of how IR35 works.

Why IR35 exists

IR35 exists because of a perceived fairness and tax issue. Employees and employers pay certain taxes (such as income tax and National Insurance) on employment income. If someone doing essentially the same work as an employee could instead provide their services through a company and be taxed more favourably, that creates an inconsistency — and a potential loss of tax revenue. IR35 was introduced to counter this, ensuring that people working in a manner equivalent to employment are taxed broadly accordingly, regardless of the intermediary structure. Whether and how the rules achieve this fairly has been much debated, but the underlying rationale is to align the tax treatment of genuinely employment-like arrangements with that of employment.

How IR35 works in principle

At the heart of IR35 is the question of whether a particular engagement is, in substance, more like employment or genuine self-employment — often framed as whether the individual would be an employee if the intermediary company were removed from the picture. This is assessed by looking at the reality of the working relationship, considering factors that distinguish employment from self-employment (such as control, the obligation to provide work and to do it personally, and other indicators). If an engagement is found to be "inside IR35" (employment-like), it's taxed broadly like employment; if "outside IR35" (genuine self-employment), it isn't. Importantly, the rules around who is responsible for assessing status and accounting for tax have changed over time and can differ depending on the situation (for example, the type and size of the engaging organisation). Because of this complexity, the current rules should always be checked.

Why IR35 matters

IR35 matters because it can significantly affect the tax position and arrangements of contractors and the businesses that engage them. Getting the status assessment wrong — or failing to apply the rules correctly — can lead to unexpected tax liabilities, penalties and disputes. The rules have also influenced how businesses engage contractors and how contractors operate. For contractors, businesses and their advisers, understanding IR35 — at least well enough to know when careful assessment and advice are needed — is important. Because IR35 is complex, has changed over time, and depends heavily on the specifics of each engagement, this guide is a general overview only. Always check the current rules with HMRC, and seek qualified advice for specific situations, rather than relying on general information.

Frequently asked questions

What is IR35?

UK tax rules targeting "disguised employment" — where someone provides services through an intermediary (such as their own company) but works in a way that would otherwise look like employment — aiming to ensure broadly the right tax is paid.

Why does IR35 exist?

To address a fairness and tax issue — ensuring that people working in a manner equivalent to employment are taxed broadly accordingly, rather than gaining a more favourable treatment purely through an intermediary structure.

How does IR35 work?

By assessing whether an engagement is, in substance, more like employment or genuine self-employment — looking at the reality of the working relationship. "Inside IR35" is taxed like employment; "outside" is not. Responsibility for assessment can vary and has changed.

Why does IR35 matter?

It can significantly affect the tax position of contractors and engaging businesses, and getting it wrong can lead to unexpected liabilities, penalties and disputes — so careful assessment and current advice matter.

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