If you operate through a limited company and provide services to clients, IR35 will affect you — or at the very least, it deserves your attention. The rules determine whether HMRC views your working arrangement as genuine self-employment or, in effect, disguised employment. Get the classification wrong and the tax consequences can be significant.

The legislation has been around since 2000, but it became much more disruptive for private-sector contractors after the April 2021 reform shifted responsibility for making the determination away from the worker and on to the client business. That change reshaped the contracting market and, according to HMRC’s own analysis, affected around 120,000 workers in the private and voluntary sectors alone.

The good news is that IR35 is not as impenetrable as it can seem. Once you understand the core tests and know how your contract measures up, you are in a much stronger position — whether you are negotiating a new engagement or reviewing one you are already on.

IR35 is shorthand for the off-payroll working rules. The legislation targets what HMRC calls a disguised employee: someone who works through their own limited company (typically called a personal service company, or PSC) but who, if you stripped away the company structure, would look and feel like an employee of the end client.

The rules do not mean that operating through a limited company is inherently suspect. Plenty of contractors work genuinely outside IR35 — they take on multiple clients, bear financial risk, control their own working methods, and substitute colleagues when they cannot attend. The company structure in those cases reflects a real commercial arrangement.

What IR35 does is prevent someone from sitting at the same desk, doing the same job, under the same supervision as the client’s employees, while paying significantly less tax by routing income through a company. HMRC’s position — broadly reasonable, in our view — is that if it walks and quacks like employment, it should be taxed like employment.

So IR35 is not a penalty for contracting. It is a test that asks a straightforward question: does this engagement look more like employment than self-employment? The answer shapes how your income is taxed.

If a contract is deemed inside IR35, the income from that engagement is treated as employment income. You still receive it via your company, but the fee-payer (usually your agency or the end client) must deduct PAYE income tax and National Insurance contributions before paying you. The tax treatment broadly mirrors what an employee would pay — which means losing the flexibility to take a low salary and draw the rest as dividends, the approach most contractor companies are built around.

If a contract is outside IR35, you retain that flexibility. You can structure your income tax-efficiently through salary and dividends, claim legitimate business expenses, and operate the company in the way it was intended.

HMRC’s own research found that, across the contractors affected by the 2021 private-sector reform, the average annual tax increase per worker was around £10,000. That is not a rounding error — it is a material difference in take-home pay that makes getting the determination right genuinely important.

It is also worth understanding that being inside IR35 does not make you an employee. You do not gain employment rights — holiday pay, sick pay, redundancy entitlement — you simply pay tax as if you were one. Many contractors find that the worst of both worlds.

Before April 2021, private-sector contractors were responsible for assessing their own IR35 status on each engagement. Many did so carefully and honestly; others, it is fair to say, took an optimistic view. HMRC’s enforcement was patchy and the legislation had limited teeth in practice.

The 2021 reform — which had already applied to the public sector since 2017 — moved that responsibility to the end client for medium and large businesses. If you contract with a company that meets two or more of the following criteria (over 50 employees, annual turnover above £10.2 million, balance sheet above £5.1 million), it is now their job to issue a Status Determination Statement (SDS) and determine whether your engagement sits inside or outside IR35.

For contractors working with small businesses (those that do not meet the size thresholds), the old rules still apply — you remain responsible for your own assessment.

The immediate market effect was pronounced. Many large clients, unwilling to take on the liability risk, blanket-determined all contractors as inside IR35 or stopped engaging PSCs altogether. HMRC’s data shows approximately 45,000 fewer new personal service companies were formed in the period around the reform compared with the preceding trend. The contracting market adapted, but it was a significant disruption.

HMRC provides a free online tool called Check Employment Status for Tax — CEST — which asks a series of structured questions about your contract and working practices. The tool covers things like whether you can send a substitute, who controls how and where work is done, whether you bear financial risk, and whether there is an obligation to offer or accept ongoing work.

HMRC has committed to standing by CEST determinations, provided the information entered is accurate and complete. That commitment matters — if you run the tool honestly and it returns an outside IR35 result, you have a documented basis for your position.

That said, CEST has its critics. Tax professionals (ourselves included) sometimes find it blunt around edge cases, and it has historically been criticised for not adequately capturing the concept of mutuality of obligation — which is one of the key common-law tests courts have used to determine employment status.

The three tests that have consistently shaped IR35 case law are:

• Substitution — can you genuinely send someone else to do the work?
• Control — does the client direct how, when, and where you work?
• Mutuality of obligation — is there an expectation of ongoing work and an obligation to accept it?

A contract that scores well on all three — genuine substitution rights, limited client control, no guaranteed pipeline of work — is in a strong outside IR35 position. The issue is that the contract wording must reflect the reality of how the engagement actually operates, not just what looks good on paper.

The single biggest mistake contractors make with IR35 is treating it as a one-time question answered at the start of an engagement and never revisited. Working practices drift. Contracts renew. The way you actually operate on site may bear little resemblance to what was agreed in writing twelve months ago.

Our advice to contractors is straightforward:

• Review the contract language carefully before signing. Vague substitution clauses and broad client control provisions are red flags.
• Make sure how you work in practice matches what the contract says. If the contract says you can substitute but your client would never actually allow it, the clause offers little protection.
• Run CEST honestly — not optimistically. If the result comes back as unable to determine, that is a signal to take further advice, not a green light.
• Keep records. If HMRC ever investigates, contemporaneous evidence of how the engagement actually operated is your strongest defence.

If your client issues a Status Determination Statement that you believe is wrong, you have the right to challenge it through the client-led disagreement process. Exercising that right — with proper reasoning — is entirely legitimate.

And if you are working with multiple clients, some inside and some outside, the IR35 position on each contract is assessed separately. A clean outside IR35 determination on one engagement does not insulate you on another.