IR35 is the UK tax legislation that determines whether a contractor working through their own limited company should be paying tax like an employee. If they should, the engagement is inside IR35. If they genuinely operate as an independent business, it's outside IR35.

That distinction has big financial consequences for both sides. And since April 2021, determining it isn't the contractor's problem anymore. It's your problem if you're a medium or large organization engaging contractors in the UK.

Below, we’ll break down what IR35 covers, how it works in practice, what changed in 2026, and what organizations engaging UK-based contractors need to do about it.

Key Takeaways

• IR35 is HMRC's off-payroll working legislation. It targets contractors who work through a Personal Service Company (PSC) but would effectively be employees if engaged directly.
• Since April 2021, medium and large private sector organizations are responsible for determining each contractor's IR35 status.
• An inside IR35 determination means the fee payer must deduct Income Tax and National Insurance before paying the contractor. An outside IR35 determination means the contractor handles their own tax.
• From 6 April 2026, expanded small company thresholds mean some organizations that were previously in scope are now exempt from the 2021 reforms. The practical effects for most contractors won't be felt until April 2027 when the previous year's accounts determine company size.

What Is UK IR35?

IR35 (formally the off-payroll working rules) is HMRC legislation designed to prevent disguised employment. It targets contractors who:

1. Set up a Personal Service Company (PSC)
2. Invoice clients through it
3. Pay themselves in dividends to minimize tax

And they do this despite working in a way that looks functionally identical to employment.

The rules don't prohibit contracting through a limited company. They require that contractors who would be employees if engaged directly pay Income Tax and National Insurance on the same basis as employees (through PAYE and not through dividend extraction).

IR35 applies on a contract-by-contract basis. A contractor can have one engagement inside IR35 and another outside it simultaneously. Status is determined by the nature of each specific working relationship instead of the contractor's business structure overall.

Inside IR35 vs. Outside IR35

This is the terminology you'll encounter, and here's what it means:

• Inside IR35 means the engagement resembles employment. The contractor works under the client's direction, has no meaningful substitution rights, and is economically integrated into the client's business. Tax is deducted before payment. The fee payer (the organization paying the contractor directly, whether that's the end client or an agency) operates PAYE and deducts National Insurance.
• Outside IR35 means the contractor operates as an independent business. They control how the work gets done, can send a substitute, take financial risk, and work for multiple clients. The contractor's PSC receives the full assignment rate and handles its own tax affairs.

Inside IR35, a contractor's effective take-home is closer to an employee's. Outside IR35, they retain more through efficient tax structuring via their limited company. That's why contractors care about status determinations so much, and it’s also why the 2021 reforms, which shifted responsibility for those determinations to the client, were so controversial.

Who Is Responsible for IR35 Determinations?

This changed in 2017 for the public sector and in April 2021 for medium and large private sector organizations.

Before April 2021, contractors determined their own IR35 status. Predictably, most concluded they were outside IR35 (makes sense). 

HMRC found this arrangement unsatisfactory.

From April 2021, medium and large private sector end users became responsible for assessing each contractor's status and issuing a Status Determination Statement (SDS). The SDS must state whether the engagement is inside or outside IR35, and give reasons. It goes to the contractor and any agency in the supply chain.

Small companies remain exempt. The contractor's PSC determines its own status for small client engagements.

The UK IR35 April 2026 Threshold Changes

From 6 April 2026, the size thresholds determining whether a company is small for IR35 purposes increased. Two of the three criteria changed:

A company is classified as small if it meets two of the three criteria. Meeting two means the organization falls outside the 2021 reforms. That means the contractors determine their own IR35 status for those engagements.

Organizations that previously fell within scope of the medium/large category may now qualify as small. If so, their IR35 compliance obligations shift back to the contractor's PSC.

Most contractors won't feel this change until April 2027. Company size for IR35 purposes is determined by the previous year's accounts. An organization that meets the new thresholds in their 2025-2026 accounts becomes a small company for IR35 purposes in the 2026-2027 tax year.

If you're close to the old thresholds, it's worth reviewing whether your company now qualifies as small. If it does, your IR35 compliance process changes meaningfully.

How IR35 Status Is Determined

There's no single checklist. HMRC evaluates the substance of the working relationship through a combination of factors. The three most important:

1. Substitution. Can the contractor send someone else to do the work? A genuine right of substitution points toward outside IR35. Engagements where the client effectively requires the specific individual point inside.
2. Control. Does the client control how the work gets done, or just what gets delivered? Direction over method, hours, and working practices points inside IR35. Outcome-based arrangements where the contractor decides how to meet the brief point outside.
3. Mutuality of obligation. Is there an expectation that the client will continue offering work and the contractor will continue accepting it? That ongoing expectation resembles employment. Discrete project-based engagements with no such expectation point outside.

HMRC's CEST (Check Employment Status for Tax) tool can help with determinations, but it has real limitations practitioners should know about. It has been criticized for underweighting Mutuality of Obligation — one of the three primary IR35 tests — and the PGMOL Supreme Court ruling reinforced MOO's significance in ways the tool hasn't caught up to. CEST returns 'unable to determine' in a meaningful share of cases. For complex engagements or any engagement that might receive HMRC scrutiny, document your analysis beyond what CEST captures.

What Happens If You Get It Wrong

The liability sits with the fee payer: the organization making the payment to the contractor's PSC. Get the determination wrong in HMRC's view and your organization becomes liable for the unpaid Income Tax, employee and employer National Insurance contributions, interest, and potential penalties.

HMRC actively pursues IR35 non-compliance. They issue compliance questionnaires, conduct investigations, and have pursued high-profile cases against organizations across financial services, media, and technology. 

The reputational and financial exposure is real.

Reasonable care means you can reconstruct the determination: what information you gathered about the working arrangement, which factors you weighed, why you reached the conclusion you did — not just 'we ran CEST and it said outside.' The SDS must give reasons, not just the conclusion. Organizations that issue SDSs with boilerplate language or no reasoning don't satisfy the standard.

What This Means for Your Business

For organizations engaging UK contractors at scale, IR35 compliance should be an ongoing program.

That means not a one-time, set-it-and-forget it setup.

• Every new engagement needs a status determination before work starts. That determination needs to be documented in an SDS and shared with the contractor and fee payer. 
• The working arrangement needs to match the determination. A contractor determined to be outside IR35 needs to be operating independently and not attending weekly team meetings as a de facto team member.
• Long-running engagements need periodic review. If the nature of the work changes, the status can change too.

For organizations with large contractor populations or contractors across multiple countries, this is where manual processes break down. UK contractors are often part of a broader global workforce that includes contractors in other jurisdictions with entirely different classification frameworks. 

Your compliance infrastructure needs to handle all of it.

How Worksuite Supports IR35 Compliance

For organizations managing UK contractors at scale, the manual IR35 process — status determination, SDS issuance, fee payer PAYE compliance, periodic re-screening — compounds quickly. Worksuite's classification engine handles this operationally.

Worksuite's classification engine applies the relevant local frameworks for each contractor's jurisdiction, and that includes IR35 for UK-based contractors. Classification recommendations are documented and backed by indemnification: if Worksuite's own recommendation turns out to be wrong due to an error on Worksuite's part, Worksuite covers the actual monetary losses that result. Coverage applies to engagements run through the platform using Worksuite's agreements. 

Coverage requires the engagement to run through the complete Worksuite workflow — including Worksuite's IC agreement templates and all onboarding steps. Engagements that bypass the platform or modify the standard agreements outside of Worksuite's review aren't covered.

Onboarding workflows, localized contractor agreements, and global payments handle the operational layer. For organizations managing UK contractors alongside contractors in other countries, everything sits in one platform. That means one source of truth for Finance, Legal, and Operations, regardless of where in the world the contractor is based.

Book a live demo to see how Worksuite handles UK contractors and your global program.