What is the IR35? Meaning, rules & legislation explained

What IR35 means

IR35 is UK tax legislation, also known as the intermediaries or off-payroll working rules. In simple terms, it exists to catch “disguised employment”.  

That means when someone works like an employee but operates through an intermediary, such as an IR35 limited company, mainly to benefit from lower tax.

What is an IR35 limited company?

An IR35 limited company usually refers to a personal service company (PSC) that a contractor sets up to provide their services. Instead of being paid as an employee, the contractor invoices through this company.

It becomes “IR35 relevant” when HMRC assesses whether the setup is genuinely self-employed or simply a way to work like an employee while paying less tax.

Who IR35 applies to

IR35 isn’t aimed at just one party. It applies to everyone involved in a contractor arrangement, depending on how the setup works. That includes the contractor, the client, any agency or fee-payer, and the intermediary used to deliver the work.

At the heart of most arrangements is a personal service company (PSC). The client pays the company, and the contractor then pays themselves through it.

This structure is perfectly legal, but it’s exactly what IR35 legislation is designed to assess. The key question is simple: is the contractor genuinely self-employed, or are they working like an employee in disguise?

Who makes that decision depends on the client’s size:

• Small businesses → The contractor usually decides their own IR35 status.  

• Medium and large businesses → The client must assess and confirm the status.  

So, the same contractor could fall inside IR35 in one role and outside IR35 in another. It all comes down to the working relationship, not just the contract.

Common IR35 scenarios

To bring this to life, here are the most common situations where IR35 rules apply:

• Contractor working through a limited company for a UK client - The classic setup. The contractor invoices through their company, and IR35 checks whether the working arrangement looks like employment.  

• Contractor engaged through an agency or fee-payer chain - There’s a middle layer involved. If the role is inside IR35, the fee-payer (often the agency) becomes responsible for deducting tax and National Insurance.  

• Small-business client vs medium or large client - Responsibility shifts here. Small businesses leave the decision to the contractor, while medium and large businesses must assess IR35 status themselves.

Who decides the IR35 status?

This is where many businesses get caught out. The responsibility for deciding IR35 status doesn’t always sit in the same place. It shifts depending on who the client is.

In simple terms, if you’re a contractor, you don’t always get to decide your own IR35 status. And if you’re a business hiring contractors, you may now carry more responsibility than you realise. Here’s the rule of thumb:

• Public sector and most medium or large UK businesses → The client decides IR35 status.

• Small businesses or certain overseas clients → The contractor may still decide their own status.  

This change came in with IR35 reforms to make sure tax responsibility sits closer to the organisation benefiting from the work. So, before you engage a contractor, it’s worth knowing exactly where you stand because getting it wrong can be costly.

Responsibility by business type

Let’s break it down properly:

• Public sector - The client is responsible for determining IR35 status. This has been the case since the IR35 reforms were introduced to public bodies.  

• Medium and large private-sector businesses - The client must assess whether a contractor falls inside or outside IR35. They also need to communicate this decision clearly and take responsibility for it.  

• Small businesses or certain overseas-client situations - The responsibility may still sit with the contractor. In these cases, the contractor assesses their own IR35 status and handles their own tax accordingly.  

The key takeaway? Don’t make assumptions. The responsibility highly depends on the structure. And in IR35, structure is everything.

Inside IR35 vs outside IR35 – And what they mean for UK businesses?

This is the part most people care about, and for good reason. Whether a role sits inside IR35 or outside IR35 directly affects tax, take-home pay, and how the working relationship is viewed.

In simple terms, inside IR35 means the role is treated like employment for tax purposes, while outside IR35 means the contractor is operating as a genuine business.

Here’s a quick side-by-side comparison to make things crystal clear:

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So, while the label might sound technical, the impact is very real. It changes how people are paid, taxed, and even how they work day to day.

Inside IR35

If a role falls inside IR35, HMRC sees it as employment in everything but name.

That means:

• Payments are processed through PAYE, just like a regular employee  

• Income Tax and National Insurance are deducted before the contractor gets paid  

• The business may also need to pay employer’s National Insurance  

In practice, the contractor works like an employee but without the usual employee benefits. Not exactly the best of both worlds.

Outside IR35

If a role sits outside IR35, the contractor is treated as a genuine business providing services.

That means:

• The contractor’s company is paid gross, without PAYE deductions  

• The contractor manages their own tax through their limited company  

• The working relationship reflects true independence i.e. control, flexibility, and autonomy  

But here’s the catch: it’s not just about what the contract says. The actual working practices must match. If they don’t, HMRC may still pull the role inside IR35.

How IR35 status is assessed

This is where IR35 moves from theory to reality. On paper, a contract might say “independent contractor”, but HMRC doesn’t stop there. They look at the actual working relationship, not just the wording.

That’s why IR35 assessments focus on a few key tests that reveal whether someone is genuinely self-employed or effectively working as an employee.

The main status tests

These are the core areas HMRC and advisors use to assess IR35 status:

1. Control

This looks at who is in charge of the work.

• Does the client decide how, when, and where the work is done?  

• Or does the contractor have the freedom to choose their own approach?  

The more control the client has, the more the relationship starts to look like employment.

2. Substitution

This focuses on whether the contractor must do the work themselves.

• Can they send someone else (a substitute) to complete the work?  

• Or are they personally required to deliver it?  

A genuine right of substitution points towards being outside IR35. But it needs to be real, not just a clause that never gets used.

3. Mutuality of obligation

A bit of a mouthful, but the idea is simple.

• Is the client expected to keep offering work?  

• Is the contractor expected to accept it?  

If both sides are locked into an ongoing obligation, it starts to resemble an employer-employee relationship rather than a project-based engagement.

4. Other supporting indicators

These don’t decide IR35 on their own, but they help build the full picture:

• Financial risk - Does the contractor risk making a loss, or can they increase profit through efficiency?  

• Provision of equipment - Who provides the tools needed for the job?  

• Integration - Is the contractor treated like part of the internal team?  

• Working practices - Do day-to-day behaviours reflect independence or employment?  

How businesses can stay compliant

Think of it this way: if your contractor setup changes, your IR35 position might change too. So staying compliant is really about building simple habits that keep everything aligned and well documented.

Recommended compliance steps

• Review every contractor engagement individually - Don’t rely on blanket decisions. Each role should be assessed on its own working arrangement.  

• Document the reasoning behind each decision - Keep a clear record of how you reached your IR35 status. This becomes essential if HMRC ever asks questions.  

• Make sure contracts and working practices match - It’s not enough for the contract to say “outside IR35”. The day-to-day work must reflect genuine independence too.  

• Use HMRC’s CEST tool carefully - It can help guide decisions, but it’s not perfect. Use it alongside your own judgement and supporting evidence.  

• Reassess status if the role changes - If responsibilities, control, or working patterns shift, revisit the IR35 decision. What was compliant before may not be anymore.  

• Seek professional advice when needed - For complex setups, getting expert input can save you from costly mistakes later on.  

Done properly, IR35 compliance becomes less about ticking boxes and more about running a clean, well-structured contractor model that stands up to scrutiny.

Common IR35 mistakes to avoid

IR35 isn’t usually where businesses start; it’s where they trip up later. And more often than not, it’s down to a few avoidable mistakes rather than anything complicated.

If you’re hiring contractors, these are the ones worth watching closely:

• Relying only on contracts - A well-written contract helps, but it’s not the full picture. HMRC looks at how the work actually happens, not just what’s written down.  

• Ignoring working practices - If a contractor is managed like an employee day to day, the role may fall inside IR35, regardless of the contract.  

• Treating all contractors the same - Blanket decisions might feel efficient, but they don’t hold up. Each contractor engagement should be assessed individually based on how they work.  

• Not documenting decisions - Even a correct IR35 decision can become a problem if you can’t explain it. Clear records show you’ve taken reasonable care and followed a proper process.  

• Misunderstanding responsibility - If you’re a medium or large business, the responsibility likely sits with you, not the contractor.

IR35 legislation and key reforms

IR35 legislation has been around for a while, but the way it works today has changed quite a bit. It was first introduced in 2000 to tackle “disguised employment”, where contractors worked like employees but paid less tax through limited companies.

The real shift came later:

• 2017 (public sector) → Responsibility moved from the contractor to the client.  

• 2021 (private sector) → The same rules were extended to medium and large UK businesses.

These reforms changed the game. Instead of contractors deciding their own status, many businesses now need to assess IR35 themselves and take responsibility for getting it right.

In short, IR35 started as a contractor-focused rule. Today, it’s very much a business responsibility.

Penalties and risks of non-compliance

This is the bit most businesses don’t think about until it’s too late. Getting IR35 wrong can lead to serious financial and reputational consequences, especially if HMRC decides your assessments haven’t been handled properly.

Here’s what’s at stake:

• Backdated tax - If a contractor should have been inside IR35 but wasn’t treated that way, HMRC can recover unpaid Income Tax and National Insurance, and sometimes go back several years.  

• Interest on unpaid amounts - On top of the tax owed, interest is added. So, the longer it goes unnoticed, the more expensive it becomes.  

• Penalties - HMRC may apply penalties if they believe reasonable care wasn’t taken. This can increase the total liability significantly, depending on the severity of the mistake.  

• Reputational risk - Investigations and disputes don’t stay quiet. They can affect your brand, your ability to attract contractors, and even how clients and partners view your business.  

What does it mean? IR35 is about risk management. A clear, well-documented approach today can save you from a very uncomfortable (and expensive) conversation later.

IR35 checklist for businesses

If you just want the essentials, here’s a simple checklist to keep things on track. Think of it as your quick sanity check before and during any contractor engagement:

• Determine your business size - This decides who is responsible for IR35 status. Medium and large businesses usually carry the responsibility, not the contractor.  

• Assess contractor status - Review the role carefully using the key IR35 tests. Focus on how the work is actually carried out, not just what the contract says.  

• Issue a Status Determination Statement (SDS) if required - If you’re responsible, provide a clear decision and share it with the contractor and any agency involved.  

• Ensure correct tax treatment - Apply PAYE if the role is inside IR35. If it’s outside, pay the contractor’s company correctly without deductions.  

• Review regularly - Don’t set and forget. If the role or working arrangement changes, reassess the IR35 status.

Final thoughts

IR35 isn’t just a rule to be aware of; it’s something that quietly shapes every contractor decision you make for your business. From hiring to payroll to compliance, it all links back to getting the status right from the start.

But here’s the good news. You don’t have to carry all of that complexity on your own.

Black Piano helps businesses sidestep the usual IR35 stress with a fully managed, end-to-end Employer of Record (EOR) model. Instead of worrying about contractor status, tax treatment, or compliance risk, you can hire global talent through a structure that’s already built to be compliant.

It’s a simpler way to scale without second-guessing every decision.

Want to hire internationally without the IR35 headache? Take a look at Black Piano’s services and see how easy compliant global hiring can be. Contact us today to get started.

FAQs

1. What is a Status Determination Statement (SDS)?

A Status Determination Statement (SDS) is a formal decision issued by the client explaining whether a role is inside or outside IR35, along with the reasoning behind that assessment.

2. Can a contractor challenge an IR35 decision?

Yes, contractors can challenge an IR35 decision. The client must review the disagreement, respond within a set timeframe, and either confirm or update the original status decision.

3. Does IR35 apply if the client is overseas?

IR35 can still apply with overseas clients, but responsibility may shift. In many cases, the contractor may need to assess their own IR35 status instead of the client.

4. Does IR35 apply only to limited companies?

IR35 mainly applies to contractors working through intermediaries like limited companies or personal service companies, but it can also apply to other structures where similar arrangements exist.

5. What happens if HMRC says an IR35 decision was wrong?

If HMRC finds an IR35 decision incorrect, the responsible party may need to pay backdated tax, interest, and penalties, depending on whether reasonable care was taken.

6. Can IR35 apply differently from one contract to another?

Yes, IR35 can vary between contracts. Each engagement is assessed individually based on working practices, meaning the same contractor can be inside IR35 for one role and outside for another.