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The Complete Guide to IR35: What It Is, How It Works & How to Stay Compliant

IR35 is one of the most frequently misunderstood pieces of tax legislation in the UK for contractors, freelancers and the businesses that engage them. Get it wrong and you risk unexpected tax bills, penalties or disputes with HMRC. Get it right and you’ll have clarity over your status and reduce your tax risk. In this guide, we’ll walk you step by step through everything you (or your clients) need to know about IR35.

What is IR35?

“IR35” is shorthand for the UK’s intermediaries legislation (originally introduced in 2000) intended to prevent “disguised employment” — i.e. situations where someone works like an employee but uses an intermediary (often a limited company) to gain a tax advantage.

Off-payroll working rules

These rules ensure that, for tax and National Insurance purposes, a “deemed employee” pays broadly similar tax to an actual employee.

IR35 can affect:

• A worker providing services to a client via an intermediary (such as their own limited company)

• The client / engager who receives the service

• Agencies or suppliers who arrange for workers to provide services via intermediaries

Over time, IR35 rules have evolved. Two major reforms are especially significant:

From 6 April 2017, public sector bodies became responsible for deciding whether a contractor engagement is inside or outside IR35. They must issue a Status Determination Statement (SDS) and handle deductions for Income Tax and National Insurance if inside IR35.

From 6 April 2021, similar rules were extended to medium and large private sector organisations. Now, for applicable clients, the client (not the contractor) must make the IR35 status decision.

Important exception:

If the client is a small company under the size criteria (per the Companies Act), the responsibility to determine status remains with the contractor/intermediary.

Inside IR35 vs Outside IR35 — What Do They Mean?

Inside IR35

If a contract is inside IR35, it means HMRC considers that, for tax purposes, the contractor is effectively an employee for that engagement. That leads to:

  • Deductions of Income Tax and employee National Insurance contributions from payments.
  • The “deemed employer” (often the client or agency) may also be liable for employer National Insurance contributions and Apprenticeship Levy.
  • Contractors lose some of the tax advantages of using a limited company.

Outside IR35

When a contract is outside IR35, the contractor is treated as genuinely self-employed (for that engagement). Key implications:

  • No tax or National Insurance deductions taken at source by the client.
  • Contractor is responsible for their own tax obligations through self-assessment, corporation tax, dividend tax, etc.
  • More flexibility and potential tax efficiency — but also more responsibility and risk.

How to Determine IR35 Status

Because IR35 is not black-and-white, the decision is fact- and contract-specific. Some of the major considerations:

  • Client dictates how, when, and where work is delivered - Indicator pointing inside IR35
  • Contractor retains control over methods, working hours, location - Indicator pointing outside IR35
  • Client is obligated to offer work; contractor obligated to accept - Indicator pointing inside IR35
  • No obligation on either side outside agreed deliverables - Indicator pointing outside IR35
  • Contractor cannot send a substitute - Indicator pointing inside IR35
  • Right to send a substitute worker - Indicator pointing outside IR35
  • Contractor is paid regardless and bears minimal risk- Indicator pointing inside IR35
  • Contractor bears risk (e.g. needing to correct mistakes, cover costs) - Indicator pointing outside IR35
  • Client supplies most tools, equipment - Indicator pointing inside IR35
  • Contractor provides own tools, materials - Indicator pointing outside IR35
  • Contractor is integrated with staff (attends meetings, uses staff systems) - Indicator pointing inside IR35
  • Contractor is separate, independent - Indicator pointing outside IR35

You can also use HMRC’s Check Employment Status for Tax (CEST) tool as guidance, though it’s not perfect.

What If You Disagree with the Status Determination?

Clients must provide a Status Determination Statement (SDS) with reasons for the decision.

Contractors have the right to challenge or appeal the SDS.

The client must set up a disagreement process, often within 45 days to respond.

If unresolved, disputes might escalate (e.g. tribunal), but there’s no guarantee a contractor’s appeal will succeed.

Practical Tips & Best Practices for Contractors

Draft robust contracts - Include clauses that reflect substitution, control, risk, and termination to support your outside IR35 status.

Ensure working practices align - Your actual day-to-day practice must align with your contract — otherwise HMRC may ignore wording and look at reality.

Avoid blanket “inside IR35” decisions by clients - Request a case-by-case assessment and push for clarity.

Get a contract review or legal advice - An independent review can help spot red flags and support your position.

Maintain records and evidence - Emails, project plans, invoices, timesheets — use them to demonstrate independence.

Use professional indemnity / IR35 insurance - If under risk, such insurance can help with legal costs or HMRC challenges.

Stay updated - The IR35 landscape evolves. Changes in legislation, court rulings, or HMRC guidance may impact your status.

Impacts on Clients / Businesses

For businesses engaging contractors, there are obligations and risks:

Responsibility for status decisions - Clients (if medium/large) must make and document IR35 status decisions (SDS).

“Reasonable care” requirement - Clients must use reasonable care in determining status, taking into account contracts, working practices, and relevant guidance.

Liability for employer NIC / tax - If they get it wrong, clients may face liability for unpaid taxes, National Insurance, and penalties.

Disagreement process - Clients must give contractors a formal route to challenge SDS decisions.

Operational impact - Some clients may avoid contractors or push for only inside IR35 roles, affecting flexibility and cost.

Common Myths & Pitfalls

“Getting a contract inside IR35 always means higher tax”

While inside IR35 generally reduces take-home pay, much depends on how deductions are made and the client’s approach.

“CEST gives definitive answers”

HMRC’s tool is a guide, not a guarantee. It may not fully reflect every situation.

“If the client says inside IR35, there’s nothing you can do”

You can challenge and negotiate, especially if the decision isn’t well reasoned or based on misleading contractual terms.

“IR35 only applies to UK-based clients”

Even non-UK clients may fall under IR35 if they have a UK connection or permanent establishment.

Summary & Final Advice

IR35 is complicated, and there’s no one-size-fits-all answer. Whether you're a contractor or a client, the key is clarity, consistency, and documented decision-making.

Contractors should ensure that both contracts and actual working practices support their desired status.

Clients must act with reasonable care, be transparent, and allow for challenge.

Seek professional advice where the risk and stakes are high — a small error can cost a lot.

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