IR35 explained: what self-employed contractors need to know in 2026

IR35 is one of the most misunderstood pieces of tax legislation affecting self-employed people in the UK. Get it wrong and you could face a substantial tax bill — potentially thousands of pounds in backdated income tax and national insurance. Get it right and you continue operating as a legitimate independent contractor with no problems.

This guide cuts through the jargon and explains how IR35 works, who determines your status, and what you can do to ensure your contracts hold up to scrutiny.

What is IR35?

IR35 is informal shorthand for the off-payroll working rules, introduced in 2000. The legislation targets what HMRC calls "disguised employment" — situations where a worker provides services through a limited company intermediary but, in practice, works like an employee of the client company.

The core principle is this: if you would have been an employee of the client had your limited company not existed, HMRC expects you to pay broadly the same income tax and national insurance contributions as an employee would. The tax saving you get from operating through a limited company (taking dividends at a lower tax rate, for example) is not available to you in that case.

IR35 only applies to contractors who operate through an intermediary — typically their own personal service company (PSC). If you are a sole trader, IR35 does not directly apply to you in the same way. However, the underlying employment status tests are relevant to anyone providing services.

Inside IR35 vs outside IR35

If your contract is determined to be inside IR35, your income from that contract is treated as employment income for tax purposes. You pay income tax and national insurance on it. Your take-home pay typically drops by around 20% compared to being outside IR35.

If your contract is outside IR35, you are treated as a genuine independent contractor. You can pay yourself through your company in whatever mix of salary and dividends is most tax-efficient. This is the standard position for most genuine independent tradespeople and contractors.

Who decides your IR35 status?

This depends on who your client is:

If your client is a medium or large private sector company or any public sector body

The client (the company you work for) is responsible for determining your IR35 status and issuing you with a Status Determination Statement (SDS). They must assess the contract and working practices and tell you whether they consider you inside or outside IR35. If they determine you are inside IR35, the fee payer (often the agency or the client themselves) deducts income tax and national insurance before paying you.

A company is "medium or large" if it meets two or more of: turnover over £10.2 million, balance sheet over £5.1 million, more than 50 employees.

If your client is a small company

If the client company is small (below two of those thresholds), responsibility for determining IR35 status falls back to you — specifically to your limited company. You are responsible for deciding whether you are inside or outside IR35 and accounting for any additional tax if you are inside.

The three key IR35 tests

Whether a contract is inside or outside IR35 comes down to three main factors established through case law:

1. Substitution

Can you send a substitute to do the work in your place? If your contract includes a genuine right of substitution — meaning you can send another qualified person to carry out the work if you are unavailable — that points strongly towards being outside IR35. An employee cannot send a substitute; a contractor can.

The key word is "genuine." A client who would refuse any substitute in practice, regardless of what the contract says, means substitution is not genuine, and HMRC will see through it.

2. Mutuality of obligation (MOO)

Is the client obliged to offer you work, and are you obliged to accept it? In an employment relationship, there is a mutual obligation: the employer provides work and the employee turns up and does it. An independent contractor has no obligation to accept future work from the client, and the client has no obligation to keep offering it.

If your contract includes clauses obliging both parties to offer and accept ongoing work, that suggests employment rather than self-employment.

3. Control

Who controls how, when, and where you do the work? If the client dictates your hours, tells you exactly how to do the job, requires you to work on-site every day, and supervises you closely, that looks like employment. A genuine contractor decides how to do the work and is only accountable for the result.

If you set your own hours, decide your own methods, and work across multiple clients, those are strong indicators of independence.

Other factors HMRC considers

Beyond the three main tests, HMRC also looks at:

• Part and parcel: Are you treated like an employee day-to-day (attending the company Christmas party, listed on the org chart, given a company email address)?
• Financial risk: Can you make a profit or loss? Do you quote a fixed price and absorb cost overruns?
• Equipment: Do you supply your own tools and equipment, or use the client's?
• Exclusivity: Do you work for other clients at the same time, or are you exclusively committed to one client?

HMRC's CEST tool

HMRC provides a free online tool called Check Employment Status for Tax (CEST) that you and your client can use to assess IR35 status. The tool asks a series of questions about the working arrangements and gives a verdict of "employed," "self-employed," or "unable to determine."

HMRC has committed to standing by CEST results that are accurate and completed honestly. However, the tool has been criticised for not adequately weighing mutuality of obligation. It is a useful starting point but should not be your only check.

What contractors in the construction sector should know

Most genuine tradespeople and subcontractors working on construction sites are outside IR35 — they supply their own tools, work for multiple clients, take financial risk on jobs, and are clearly not employees. The Construction Industry Scheme (CIS) already captures much of the tax that IR35 is designed to address in this sector.

IR35 becomes more relevant when a tradesperson moves into a "specialist consultant" role — for example, a project manager or site supervisor working exclusively for one main contractor over an extended period, using the client's equipment, under close direction.

Protecting yourself

Review your contracts to ensure they include genuine substitution rights, specify you are responsible for the outcome of the work rather than how it is done, and do not imply ongoing mutual obligation. Make sure the actual working practices match the contract — a contract that says "substitution is permitted" but a working relationship where you have always worked alone and exclusively counts for little.

If in doubt, an employment law or tax specialist can review your contracts for IR35 risk. The cost of a review is usually far less than the cost of an HMRC investigation.

Summary

IR35 catches contractors who work through a limited company but function as employees in practice. The three tests — substitution, mutuality of obligation, and control — are the main determinants. For medium and large clients (and all public sector bodies), the client determines your status and issues an SDS. For small companies, you determine it yourself. Most genuine independent tradespeople working across multiple clients, supplying their own tools and taking financial risk, are outside IR35. Review your contracts, ensure working practices match what is on paper, and get specialist advice if you are uncertain.