What Is IR35? Understanding the UK's Contractor Tax Rules

IR35 determines how a UK contractor’s earnings are taxed when they provide services through an intermediary such as a personal service company. If the engagement would look like employment without that intermediary, it is generally treated as inside IR35 for tax purposes; if it is genuinely a business-to-business arrangement, it may be outside IR35. The client’s size and sector affect who is responsible for assessing the engagement, while the fee-payer may have duties to apply the decision and account for tax and National Insurance. The contract is relevant, but the working relationship in practice also matters. A decision applies to the particular engagement, so having a limited company or an earlier outside-IR35 decision does not settle the status of a different assignment.

What Is IR35 Designed to Address?

IR35 is the common name for the UK’s intermediaries legislation. It addresses “disguised employment,” where someone provides services through an intermediary but the underlying relationship resembles employment for tax purposes. The rules are intended to prevent someone from gaining a tax advantage solely by providing services through a company or partnership rather than working directly as an employee.

The framework is associated with the Income Tax (Earnings and Pensions) Act 2003 and the Social Security Contributions (Intermediaries) Regulations 2000. These provisions address income tax and National Insurance contributions connected with intermediary arrangements. The label in a contract and the existence of a limited company do not, by themselves, determine the result. For additional background, see a general overview of IR35.

Who Determines IR35 Status?

For public-sector engagements and engagements with medium or large private-sector clients, the client is generally responsible for deciding whether IR35 applies. The client must communicate its decision in a Status Determination Statement. The fee-payer in the supply chain, often an agency, may have responsibility for applying the determination and accounting for the relevant tax and National Insurance. The client’s size and the nature of the engagement therefore affect who must make and communicate the decision.

Small private-sector clients are generally treated differently. If a client qualifies as small under the relevant rules, responsibility for determining status can remain with the contractor’s intermediary. The applicable rules and thresholds determine whether the exception applies, so a client should not assume that it qualifies. The allocation of responsibility also affects who may be liable if HMRC challenges how the rules were applied.

What Do Inside and Outside IR35 Mean?

Inside IR35

An inside-IR35 outcome means the engagement is treated as employment-like for tax purposes. The contractor may continue to provide services through an intermediary, but the tax and National Insurance treatment reflects a deemed employment relationship. This does not automatically make the contractor an employee for every legal purpose. Employment rights and status can involve different tests depending on the issue.

Outside IR35

An outside-IR35 outcome means the engagement is treated as a genuine business-to-business relationship for tax purposes. The contractor’s intermediary remains part of the arrangement, and the actual working practices should support the conclusion that the contractor operates independently. Outside IR35 is not a permanent status attached to a person or company. Each engagement is assessed on its own facts.

How Is IR35 Status Assessed?

The assessment considers the whole working relationship, including the written terms and how the engagement operates in practice. Relevant factors include control over the work, the possibility of substitution, mutuality of obligation, integration into the client’s organization, and the contractor’s financial risk and independence. No single factor determines the result.

For example, a contract might allow a contractor to send a suitably qualified substitute. That provision carries less weight if the client would not realistically accept a substitute. Flexible hours alone also do not establish independence if the client otherwise directs the work in a way that resembles an employer’s control. The written agreement and everyday working arrangements should tell a consistent story. For a broader explanation of worker classification, see what determines worker status.

• Control: Who decides what work is done and how it is done? Consider who sets the timing and location when relevant.
• Substitution: Can the contractor provide a suitably qualified substitute? Would the client genuinely accept one?
• Mutuality of obligation: Is the client expected to offer ongoing work? Is the contractor expected to accept it?
• Integration: Is the contractor treated as part of the client’s workforce?
• Financial risk and independence: Does the contractor bear business risk or have a genuine opportunity to profit through how the work is managed? The contractor’s provision of equipment may also be relevant.

How Can an IR35 Assessment Be Managed?

Consider IR35 when designing the engagement rather than treating it as a box to tick after the contract is signed. Before work begins, define the deliverables and expected duration. Clarify reporting lines and whether the contractor is providing an independent service or filling an employee-like role.

The contract should accurately describe the intended relationship, and managers should follow its terms in practice. Keep the status assessment and statements of work along with records that show how the arrangement operates. Reassess the position if responsibilities, supervision, or working practices change materially. A defined project can evolve into ongoing work that looks more like employment, so an earlier determination may no longer reflect the engagement.

Contractors should understand the determination before agreeing to terms or setting a rate because the tax treatment affects the economics of an assignment. Clients should document their reasoning and ensure the relevant parties in the payment chain receive and apply the decision. For practical guidance on managing contractor relationships, see how to manage independent contractors.

IR35 and the UK Contracting Workforce

IR35 affects how organizations engage contractors through intermediaries and who in the payment chain must handle the associated tax responsibilities. For contingent-workforce planning, this means the status assessment should be considered alongside the engagement’s scope and working practices. A determination is specific to the arrangement; it does not establish the status of other contractors or assignments.

*This article is for general informational purposes only and is not legal advice.