IR35 Rules Explained: A Plain English Guide for Contractors

What IR35 actually is — and why the name is misleading

IR35 takes its name from a 2000 Inland Revenue press release number — not from any statute title you'll find on gov.uk. That alone tells you something about how the rules evolved: as a policy response to a specific problem, not as a clean piece of legislation with a memorable name. The core idea, though, is straightforward enough. If the work you do through your limited company looks like employment in substance — same desk, same hours, managed day to day by someone on the client side — then HMRC's view is that you should pay roughly the same tax and National Insurance as an employee would, even though the money technically flows through your company.

What trips people up is treating "looks like employment" as a checklist you can tick off from a contract template. It isn't. It's a judgment call, and even HMRC staff and tribunal judges have disagreed over the years about where the line sits on near-identical fact patterns. Two contractors on the same client site, with similar day rates and similar job titles, can end up with different outcomes depending on how control, substitution and obligation actually work in practice. The name "IR35" makes it sound like a single rule with a single answer. In reality it's a framework for arguing about what kind of relationship you really have — and those arguments can run for years.

Inside vs outside, summarised

Status	What it means in practice
Outside IR35	Operate as a genuine business — salary plus dividends, corporation tax, full limited company efficiency.
Inside IR35	Taxed broadly as an employee — income tax and NI on your contract income regardless of company structure.

The gap between these two outcomes is routinely £5,000–£20,000 a year in take-home pay, depending on your day rate and total contract income. For many contractors, that's not a rounding error — it's the single largest tax decision they'll make all year, and it gets made contract by contract rather than once at incorporation. Getting it wrong doesn't just cost you this year's margin; it can leave you exposed to back-tax, interest and penalties if HMRC disagrees with a determination you accepted too easily.

For the full current rates and bands behind this calculation, see our complete UK tax rates guide for 2026/27.

Who actually decides your status now

Who carries the decision — and the liability if they get it wrong — has shifted several times since IR35 was introduced. Where you sit in the supply chain and how large your end client is now matters as much as the contract wording itself.

Medium and large private sector clients

Since April 2021, medium and large private sector clients must decide IR35 status for each engagement and issue a Status Determination Statement (SDS) setting out their conclusion and reasons. Liability for deducting tax and NI sits with whoever pays you in the supply chain — typically the fee payer — and the rules include a formal disagreement process if you challenge an inside determination. A client counts as medium or large if it meets two of three tests: turnover above £10.2m, balance sheet total above £5.1m, or more than 50 employees. If your end client clears those thresholds, their determination is the one that governs how you're paid, not your own assessment.

Small companies

Small companies are exempt from the 2021 reform. If your end client qualifies as small under the same tests, you — as the contractor operating through your limited company — remain responsible for assessing your own IR35 status, much as under the original rules. This exemption gets overlooked constantly: plenty of contractors assume every client decides for them, when in fact a significant share of engagements still sit with the intermediary to assess. If you're working for a small business, your own judgment and evidence matter more, not less.

Public sector

The public sector has operated under client-side determination since April 2017 — four years before the private sector reform. Public authorities and agencies paying your company are generally responsible for deciding status and operating PAYE where the rules apply. If you contract into government or NHS frameworks, you're already familiar with this model even if your private-sector colleagues only encountered it after 2021.

The 2021 reform was supposed to shift risk onto end clients who were best placed to assess working arrangements. The predictable result, in our view, is that many large companies became risk-averse and issue blanket "inside IR35" determinations for whole categories of work rather than assessing each engagement individually. That's arguably not what the legislation intended, but it's the reality most contractors now navigate. Arguing your specific case rarely moves a client who's decided that blanket-inside is simpler to administer than nuanced individual review — even when your contract and working practices would support an outside determination on the merits.

The three tests that actually matter

HMRC and tribunals look at the whole picture, but three themes recur in almost every serious IR35 dispute. If you can't answer these honestly in your favour, contract wording alone won't save you.

Could you send someone else to do the work?

Genuine substitution rights — the ability to send a suitably qualified substitute to deliver the work, without the client having an unlimited veto — are a strong indicator that you're in business on your own account rather than selling personal service. The clause in your contract matters less than whether substitution could actually happen in practice. A right you've never exercised, or one the client would never realistically accept, carries little weight. Evidence that you've sent a colleague, associate or subcontractor to cover an engagement, with the client's knowledge and consent, is worth more than any boilerplate substitution paragraph.

Who controls how the work gets done?

Being told what outcome is needed — deliver this system, complete this audit, hit this milestone — is normal supplier behaviour. Being told how to do it every day, by someone who functions as your line manager, is the employment signal. Control is about the day-to-day reality: can you choose your methods, your hours within reason, and your location? Or are you embedded in the client's processes, attending their stand-ups, using their tooling exclusively, and taking direction on task sequencing? The more the latter looks like your week, the harder an outside argument becomes regardless of what the contract says about autonomy.

Is there an ongoing expectation of work?

Genuine business relationships tend to be project-shaped: a defined scope, a clear end date, and no automatic assumption that the next piece of work will follow. Mutual expectation of indefinite, ongoing work — where the client assumes you'll be there next month because you've always been there — looks like employment regardless of whether the contract calls itself a "fixed term." This is mutuality of obligation, and it's one of the tests courts take seriously even when HMRC's own online tools don't.

No single factor decides status on its own. But if you answer "no" to all three of these honestly — no real substitution, heavy daily control, ongoing open-ended expectation — you have a real problem regardless of how well your contract was drafted. That's the point most template reviews miss: they fix the paper, not the practice.

Why I wouldn't rely on CEST alone

HMRC's Check Employment Status for Tax (CEST) tool is free, widely used, and often the first thing contractors reach for when a client asks for evidence. It can be a useful sanity check if you answer the questions honestly about how work actually happens. But it has a specific, serious gap: CEST does not assess mutuality of obligation at all, despite that being one of the three core tests courts and tribunals actually use in published decisions. HMRC has been criticised on this point directly in tribunal rulings where CEST's methodology was examined and found wanting.

An "outside" CEST result feels like a clean bill of health. It isn't a legal shield — it's evidence that you tried to assess status, not proof that you were right. If HMRC opens an enquiry, they'll look at contracts, emails, timesheets, and how the relationship operated day to day. CEST output may form part of the picture, but it won't end the conversation. Our view: a proper specialist review that weighs all three real-world tests — substitution, control, and mutuality — is worth more than CEST's green tick, especially when the contract has real financial stakes. Use CEST as a first pass if you want, but don't treat it as the end of your homework.

If you disagree with a client's determination

Since April 2021, you have a formal right to challenge an SDS you disagree with. The client must respond within 45 days — either confirming the original determination with reasons or issuing a revised one. If they miss that window, liability can shift back to them under the off-payroll rules. On paper, that's meaningful leverage.

In practice, most clients won't budge once they've made a blanket policy decision. Individual reassessment is more work than absorbing the occasional formal disagreement, especially when the supply chain includes agencies and multiple fee payers who'd rather standardise than negotiate. The leverage is real in law; whether it's worth the energy depends on the contract's value, how replaceable the client is, and whether an inside determination is a deal-breaker for you. Sometimes the rational move is to accept inside, price accordingly, or walk away. Sometimes a well-argued challenge with evidence of genuine outside working practices does shift the outcome. Know which situation you're in before you spend political capital.

Working inside IR35 through an umbrella

Many contractors working inside IR35 are paid through an umbrella company that becomes the employer of record. The umbrella invoices the agency or client, deducts income tax and employee NI properly through PAYE, may also deduct employer NI and apprenticeship levy from the assignment rate depending on the contract model, and retains a margin for its service — typically £15–30 per week, though this varies.

You should receive Key Information Documents before signing up and payslips showing gross pay and all deductions. Employment rights depend on your contract with the umbrella, not your old limited company arrangement.

What actually protects you if you're outside IR35

A well-written contract that nobody follows in practice is worth almost nothing. HMRC and tribunals look at how the relationship actually operates day to day — what emails show about control, whether substitution ever happened, whether the engagement had a genuine end. Paper outside status with employee-like working practices is one of the fastest routes to a painful enquiry.

What genuinely helps: a specialist contract and working-practices review before each engagement, not a reused template from three clients ago; real evidence of running a business — other clients, your own equipment where appropriate, business insurance, professional invoices; and IR35 enquiry insurance that covers defence costs separately from the cost of losing an argument. Build contemporaneous evidence as you work, not a reconstruction folder after HMRC writes. Screenshots, timesheets, scope changes in writing, and proof of autonomy matter more than a certificate from an online questionnaire that skipped half the relevant tests.

Frequently asked questions

Does IR35 apply to sole traders?

No, not directly — IR35 targets people operating through limited companies. Sole traders are already taxed as individuals through Self Assessment. HMRC can still challenge whether you're genuinely self-employed at all, but that's a different and rarer fight than an IR35 determination.

Can I be inside IR35 on one contract and outside on another?

Yes, and it's more common than people expect — status is assessed per engagement, not per person. Don't assume your status on one contract tells you anything about the next.

Is a CEST "outside" result enough protection on its own?

We wouldn't treat it that way. CEST doesn't assess mutuality of obligation, which courts consistently treat as central. Use it as a first pass, then get a proper review if the contract matters financially.

How far back can HMRC investigate IR35 status?

Four years for an innocent mistake, six for carelessness, and up to twenty years if HMRC concludes the non-compliance was deliberate — exactly why contemporaneous evidence built at the time, not reconstructed afterwards, matters so much.

Does my pension change if I'm inside IR35?

Being inside IR35 doesn't automatically give you employer pension contributions unless whoever pays you offers them. Outside IR35, employer pension contributions through your own Ltd remain one of the most tax-efficient moves available to a contractor.