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

IR35 is often one of the trickiest tax rules that self-employed folks in the UK have to get their heads around. Mistakes can lead to a hefty tax bill, possibly involving backdated income tax and National Insurance. But if you understand it, you can keep trading as a genuine independent contractor without a hitch.

This article breaks down how IR35 works, who decides your status, and what steps you can take to ensure your contracts stand up to inspection.

What is IR35?

IR35 refers to the off-payroll working rules introduced in 2000. The legislation targets what HMRC terms "disguised employment" — where someone provides services through a limited company but essentially works as an employee of the client company.

The main idea is this: if you'd be an employee of the client without your limited company, HMRC expects you to pay roughly the same income tax and National Insurance as an employee. The tax benefits of a limited company, like taking dividends at a lower tax rate, aren't available in that case.

IR35 only impacts contractors using an intermediary, usually their own personal service company (PSC). Sole traders aren't directly affected by IR35, but the overall employment status tests still apply to anyone providing services.

Inside IR35 vs outside IR35

If your contract is deemed inside IR35, your income from that contract is treated as employment income for tax. You'll pay income tax and National Insurance on it, and your take-home pay could drop by around 20% compared to being outside IR35.

If your contract is outside IR35, you're recognised as a genuine independent contractor. You can pay yourself through your company in the most tax-friendly mix of salary and dividends. This is the usual situation for most genuine independent tradespeople and contractors.

Who decides your IR35 status?

This depends on your client's size:

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

The client you're working for is tasked with determining your IR35 status and issuing a Status Determination Statement (SDS). They need to assess both the contract and working practices to decide if you're inside or outside IR35. If inside, the fee payer, usually the agency or the client themselves, deducts income tax and National Insurance before paying you.

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

If your client is a small company

For small companies (below two of the thresholds), the responsibility to determine IR35 status falls to you — specifically, your limited company. You're responsible for deciding your status and handling any additional tax if you're inside IR35.

The three key IR35 tests

Whether a contract is inside or outside IR35 depends on three main factors established through case law:

1. Substitution

Can you send a substitute to do the work instead of you? If your contract genuinely allows substitution — meaning you can send someone else qualified to do the work if you're unavailable — that strongly indicates being outside IR35. An employee can't send a substitute, but a contractor can.

The key is "genuine." If a client would always refuse any substitute, regardless of what the contract says, substitution is not genuine, and HMRC will spot this.

2. Mutuality of obligation (MOO)

Is the client required to offer you work, and are you required to accept it? In employment, there's a mutual obligation: the employer provides work, and the employee shows up to do it. An independent contractor has no obligation to accept future work from the client, and the client isn't obliged to keep offering it.

If your contract obliges both parties to offer and accept ongoing work, this suggests employment rather than self-employment.

3. Control

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

If you set your own hours, choose your methods, and work for multiple clients, these are strong signs of independence.

Other factors HMRC considers

Besides 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 offers 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 provides a verdict of "employed," "self-employed," or "unable to determine."

HMRC has promised to stand by CEST results that are accurate and completed honestly. However, the tool has faced criticism for not adequately weighing mutuality of obligation. It's a good starting point but shouldn't be your only check.

What contractors in the construction sector should know

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

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

Protecting yourself

Review your contracts to ensure they include genuine substitution rights, clearly state you're responsible for the outcome of the work rather than how it's done, and don't 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 always work alone and exclusively counts for little.

If you're uncertain, 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 targets contractors who work through a limited company but operate like employees. The three tests — substitution, mutuality of obligation, and control — are the main factors. For medium and large clients (and all public sector bodies), the client determines your status and issues a SDS. For small companies, you determine it yourself. Most genuine independent tradespeople working across multiple clients, providing their own tools, and taking financial risk, are outside IR35. Review your contracts, ensure working practices match what's on paper, and seek specialist advice if you're unsure.