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What IR35 actually is

IR35 is a tax rule, not a status. Officially called the Intermediaries Legislation, it was introduced in 2000 to address a specific situation: workers leaving permanent employment, setting up limited companies, and returning to the same role for the same employer — but paying significantly less tax because dividends are taxed more efficiently than salary.

HMRC called this disguised employment. The argument: if you're doing the same job in the same way for the same employer, you're effectively still an employee and should be taxed like one.

That argument has some merit. But it also sweeps in genuine contractors — people who work for multiple clients, carry their own financial risk, use their own equipment, and can send someone else to do the work — who bear no resemblance to disguised employees. IR35 is supposed to catch the former without penalising the latter. In practice, it's complicated.

Inside vs outside: what it means financially

Outside IR35

Your engagement is considered genuine self-employment through your limited company. You pay yourself a salary (typically around £12,570 to use your personal allowance) and take remaining profit as dividends taxed at 8.75% basic rate. You can claim business expenses. You pay employer's NI only on your salary, not your dividends.

Inside IR35

Your engagement looks enough like employment that you're taxed as if you're an employee. Income tax at your marginal rate, employee's NI, and employer's NI all apply. You can't use dividends. You can't claim most expenses. If you're working through an umbrella company, they also take a weekly margin.

The financial difference is significant. At a £500/day rate on 220 working days, the gap between outside and inside IR35 is typically £14,000–£16,000 per year in take-home pay. See the exact figures at your day rate using the free calculator.

Who decides if you're inside or outside

The answer changed in 2021 and confuses many first-time contractors.

Medium or large private sector clients (or any public sector body)

They decide. They must issue you a Status Determination Statement giving their view and the reasons. If they get it wrong and HMRC investigates, the liability falls on them.

Small company clients

Under £10.2m turnover, under £5.1m balance sheet, under 50 employees — meeting at least two of three — you decide, and you carry the liability if you get it wrong.

For most contractors working with medium-to-large businesses, your client's view of your status is what matters. If they say inside, you're inside — unless you successfully dispute it through the formal disagreement process.

What the determination is actually based on

IR35 status is a multi-factor assessment of the real nature of your working relationship. The key tests, drawn from decades of employment law case law, are:

Personal service

Are you required to do the work yourself, or could you send a suitably qualified substitute? A genuine right of substitution — not just in theory but in practice — is one of the strongest outside IR35 indicators.

Control

Who decides how, when, where, and what you work on? The more your client controls the method of your work (not just the deliverable), the more employment-like it looks.

Mutuality of Obligation

Is the client obliged to offer you work and are you obliged to accept it? Ongoing mutual obligation is a characteristic of employment. Each contract being discrete, with no obligations once it ends, points toward genuine contracting. This test comes from Ready Mixed Concrete (SE) Ltd v Minister of Pensions [1968].

Financial risk

Do you carry genuine financial risk? Can you profit or lose depending on how efficiently you work? Employees are insulated from financial risk; genuine contractors bear it.

Equipment and facilities

Do you use your own equipment and provide your own working environment, or does the client provide everything?

Integration

Are you recognisably an external resource brought in for a specific purpose, or are you embedded in the client's organisation, attending all-hands meetings and appearing on their org chart?

No single factor is decisive. It's the overall picture that matters and that tribunals weigh.

The CEST problem you need to know about

Most clients use HMRC's CEST tool to make their determinations. You should know its limitations before accepting their answer as final.

CEST ignores Mutuality of Obligation entirely. As explained above, MoO is one of the foundational tests — established in case law since 1968 — and CEST doesn't ask about it. HMRC's position is that MoO exists in all contracts, but tribunals assess the type of obligation, not merely its presence.

Freedom of Information data shows CEST produces an undetermined result in approximately 22% of cases — a rate that has risen since the tool launched. When CEST returns undetermined, that means the tool doesn't know, not that you're inside. If your client issues an inside determination based on CEST, you have the right to dispute it and they must respond within 45 days.

What to check in your contract before you sign

Before you sign your first contract, read it against these specific points:

The substitution clause

Does your contract give you the right to send a substitute? A clause saying "subject to client approval" is weaker than one that's genuinely unilateral. Check what it actually says rather than assuming it's there.

The control clauses

Who sets your working hours? Where are you required to work? "9-5 at the client's premises, directed by their manager" is employment language. "The contractor shall deliver defined outputs within agreed timescales" is contractor language.

The termination clause

How much notice does either party need to give? Short notice periods are more consistent with a business engagement than an employment relationship.

The exclusivity clause

Are you prohibited from working for other clients? Exclusivity is an employment indicator. A genuine contractor should be free to take other work.

What IR35 insurance is and whether you need it

IR35 insurance covers your legal costs and any tax liability if HMRC investigates your status and concludes you should have been inside. Specialist providers include Qdos and Kingsbridge, typically at £200–£500/year depending on your rate and contract type.

If you're working entirely inside IR35 through an umbrella, you don't need it — the risk sits with the engager. If you're outside IR35 through your Ltd Co, it's worth considering, particularly if your contract has any grey areas.

The practical checklist for your first contract

1. 1

   Set up your limited company — or decide to use an umbrella. Your accountant can advise which is right for your circumstances.

2. 2

   Ask your client for their IR35 determination and SDS before you start work.

3. 3

   Read the contract against the six IR35 tests — or use the IR35 Verdict contract checker.

4. 4

   Check what you'll actually take home before you accept the rate — the calculator shows real figures at your day rate.

5. 5

   Consider IR35 investigation insurance if you're outside IR35 with any ambiguity.

6. 6

   Set up a separate business bank account from day one.

The first contract is the hardest. Once you've been through it once, the IR35 assessment becomes routine — another thing to check before signing, not something to fear.

Frequently asked questions

What does IR35 mean for a first-time contractor?

IR35 is a tax rule that determines whether you're treated as an employee or a genuine contractor for tax purposes. If you're outside IR35, you can operate tax-efficiently through a limited company using dividends. If you're inside IR35, you're taxed broadly like an employee even though you're not one, which significantly reduces your take-home pay.

Who decides if I'm inside or outside IR35?

It depends on your client's size. If they're a medium or large company, they decide and issue you a Status Determination Statement. If they're a small company (under £10.2m turnover, under 50 employees), you decide and carry the liability yourself.

What is a Status Determination Statement?

An SDS is a written record of your client's IR35 determination for your engagement, including the reasons for their decision. You're entitled to one from any medium or large client. You have the right to dispute it if you believe it's wrong.

How much does being inside IR35 cost me?

At typical contractor rates, being inside IR35 via umbrella costs approximately £12,000–£22,000 per year in take-home pay compared to operating outside IR35 through a limited company. The exact figure depends on your day rate, working days, and expenses.

Do I need an umbrella company if I'm inside IR35?

Not necessarily, but it's the most common arrangement. An umbrella company acts as your employer, handling PAYE, NI, and payroll on your behalf. The alternative is your own limited company operating inside IR35, which is more administratively complex and rarely more tax-efficient than umbrella for genuinely inside engagements.

What is the substitution clause and why does it matter?

A substitution clause in your contract gives you the right to send a suitably qualified alternative to do the work if you're unavailable. A genuine, unrestricted right of substitution is one of the strongest indicators that your engagement is outside IR35. A clause that requires client approval before substitution is significantly weaker.

Sources and further reading

• —HMRC: Understanding off-payroll working (IR35)
• —HMRC: Check Employment Status for Tax (CEST)
• —HMRC: Employment Status Manual
• —Ready Mixed Concrete (SE) Ltd v Minister of Pensions [1968]
• —Autoclenz Ltd v Belcher [2011] UKSC 41
• —IPSE: Guide to IR35
• —IR35 Verdict calculator — check your take-home at your specific day rate