IR35 is the shorthand name for the UK's off-payroll working legislation — a set of rules designed to ensure that contractors who work in a manner that closely resembles employment cannot avoid the Income Tax and National Insurance contributions that a direct employee would pay, simply by operating through their own limited company or personal service company (PSC). The IR35 rules explained in straightforward terms: if you would have been treated as an employee had you been hired directly, HMRC expects you to be taxed like one.

The rules have existed in various forms since 2000, but their practical reach changed significantly in April 2017 (public sector) and again in April 2021 (medium and large private-sector clients). These reforms shifted the responsibility for determining a contractor's employment status from the contractor to the end client, which is the source of much of the confusion that persists in the contractor market today.

This guide covers the key tests HMRC uses to assess employment status, who bears responsibility for making the determination, the financial consequences of an inside-IR35 finding, and where the rules currently stand in 2026 — including the ongoing controversy around blanket determinations and how contractors can protect themselves.

The off-payroll working rules exist because of a structural feature of the UK tax system: a worker who operates as a self-employed individual or through their own company pays considerably less tax and National Insurance than an equivalent employee on a payroll. Employer National Insurance, Apprenticeship Levy contributions, and the inability to extract income as dividends (which are taxed more lightly than salary) mean the gap between employed and self-employed taxation is material.

HMRC's concern is straightforward: some arrangements that look like genuine self-employment — with a contractor supplying services through their own limited company — are, in substance, disguised employment. The contractor works day-in, day-out for a single client, under the client's direction, at the client's premises, using the client's equipment. The only structural difference from a conventional employment relationship is the presence of a company in the middle.

Who does IR35 actually affect?

IR35 affects contractors, consultants, and freelancers who supply their services through an intermediary — most commonly their own limited company or personal service company (PSC). It also applies where services are supplied through an agency or partnership. The rules do not apply to sole traders, who are assessed for employment status under separate HMRC frameworks.

The types of workers most commonly caught by IR35 include:

• IT contractors supplying to large corporates or public bodies
• Engineering and construction project consultants
• Finance contractors placed on long-term engagements
• Interim managers and senior executives on fixed-term contracts
• Media and broadcasting professionals

That said, IR35 is not sector-specific — any contractor operating through a limited company and providing services to an end client falls within scope if the substance of the relationship resembles employment.

There is no single statutory definition of employment for IR35 purposes. Instead, HMRC applies a combination of factors drawn from employment case law, and decisions turn on the overall picture of the working relationship rather than any one element. The three primary tests are supervision, direction and control; mutuality of obligation; and the right of substitution.

Supervision, direction and control

This asks whether the client controls how, when, and where the contractor carries out their work. A contractor who turns up at a client's site each day, follows the client's processes, uses the client's systems, and takes instructions from the client's management team is exhibiting many of the hallmarks of employment. A contractor who determines their own methods, works to outputs rather than hours, and has genuine autonomy over how they deliver results points away from employment.

Mutuality of obligation

Genuine self-employment typically involves a series of discrete engagements. If a client is obliged to offer work and the contractor is obliged to accept it — or if the arrangement is effectively open-ended with no genuine ability for either party to walk away — that mutuality can indicate an employment relationship.

Right of substitution

Can the contractor send a suitable substitute to carry out the work, without the client's personal approval of the individual? A genuine and practical right of substitution is one of the strongest indicators of self-employment. If the client is in reality contracting for the services of a specific named individual and would not accept anyone else, that points strongly towards employment.

Other relevant factors

Beyond the three primary tests, tribunals and HMRC also consider:

• Whether the contractor provides their own equipment
• Financial risk — does the contractor bear the risk of cost overruns or defective work?
• Whether they work for multiple clients simultaneously
• Whether they are integrated into the client's organisation (attending team meetings, appearing on org charts, having a company email address)
• Whether there is a genuine business entity with its own branding, marketing and client base

No single factor is determinative. The overall picture is what matters, which is precisely why IR35 determinations are frequently contested.

This is where the 2017 and 2021 reforms made a fundamental change, and where a great deal of practical confusion continues to arise.

Medium and large private-sector clients — client decides

If the end client is a medium or large business — defined under the Companies Act 2006 as failing to meet two of the three small company thresholds (turnover below £10.2m, balance sheet below £5.1m, fewer than 50 employees) — it is the client, not the contractor, who is responsible for determining employment status. The client must produce a Status Determination Statement (SDS) and pass it down the supply chain to the contractor and any agency involved. If the client fails to issue a compliant SDS, the liability for unpaid tax stays with the client rather than passing to the contractor.

Small private-sector clients — contractor's intermediary decides

Where the end client qualifies as a small company under the Companies Act thresholds, the pre-2021 rules still apply: the contractor's own intermediary — typically their limited company — is responsible for making the employment status determination and accounting for any tax due. This exemption protects smaller businesses from the administrative burden of conducting status assessments, but it means contractors working with small clients must still conduct their own IR35 analysis rigorously.

Public sector clients

Public sector engagements have followed the client-decides model since April 2017. NHS trusts, local authorities, central government departments, and other public bodies are all responsible for issuing SDS documentation and ensuring correct tax treatment. Public sector contractors with engagements that pre-date 2017 should also confirm the basis on which their current status was determined — older arrangements sometimes carry assumptions that have not been revisited.

HMRC's CEST tool

HMRC provides a free online tool called Check Employment Status for Tax (CEST), which many clients use as part of their SDS process. CEST has faced persistent criticism from contractors and tax professionals for failing to properly account for mutuality of obligation and producing indeterminate results in borderline cases. A CEST result is not legally binding — it simply reflects HMRC's own interpretation. It is advisable to document the basis of any determination thoroughly, regardless of whether you use CEST or conduct a more detailed assessment.

Being assessed as inside IR35 — or accepting an inside determination without challenge — has a significant financial impact. Understanding the precise mechanics helps explain why contractors and their advisers treat the rules so seriously.

How the tax is collected

Where a contractor is deemed to fall inside IR35, the entity responsible for making the determination and operating the deemed employment is the deemed employer — this is typically the fee-payer in the supply chain (often the agency, or the end client if there is no agency). The deemed employer must deduct Income Tax and employee National Insurance contributions from the contractor's fees before paying them on, treating the amounts as if they were employment income.

On top of that, employer National Insurance contributions are also due on the deemed employment income, along with Apprenticeship Levy where applicable. These costs are borne by the deemed employer — but in practice, many engagements see clients or agencies renegotiate rates to reflect the additional cost, meaning the contractor often absorbs the impact indirectly.

The 5% expenses allowance is gone

Prior to 2021, contractors caught inside IR35 could deduct a flat 5% from their gross fees to notionally cover the cost of running their limited company before calculating deemed employment income. That allowance was removed for engagements where the medium- or large-client rules apply. It remains available — marginally — for small-client engagements where the contractor is still making the determination, though its practical value is limited.

The net financial impact

A contractor billing £700 per day who is assessed as inside IR35 will typically see a meaningful portion of their income recharacterised as employment income, subject to PAYE Income Tax and both employee and employer NI. Compared with the tax efficiency of extracting income via a combination of salary and dividends outside IR35, the inside position can represent a material reduction in net take-home pay — often estimated at 20–30% of gross contract value depending on rate, personal circumstances, and the specific tax year. Every situation is different, which is why individual professional advice matters.

One of the most persistent criticisms of the reformed IR35 regime is the prevalence of blanket determinations. Rather than assessing each contractor individually, some large clients — particularly in financial services, the public sector, and large technology businesses — have adopted a policy of declaring all contractors inside IR35, regardless of the actual circumstances of each engagement. This is still a significant issue in 2026.

Why blanket determinations happen

Clients adopt blanket approaches for understandable commercial reasons: the administrative cost of conducting individual assessments across hundreds of contractors is significant, and the reputational and financial risk of getting an inside determination wrong (and the tax liability reverting to them) is real. Declaring everyone inside IR35 eliminates that risk at a stroke — but it does so by transferring an unwarranted tax burden onto contractors whose genuine self-employment status would likely survive proper scrutiny.

The right to request reasons and appeal

The legislation provides contractors with a formal disagreement process. If you receive an SDS that you believe is incorrect, you can raise a dispute with the client. The client must respond within 45 days with either a revised SDS or written reasons for maintaining the original determination. If the client fails to respond in time, the tax liability reverts to them — a lever that is worth using.

Building a strong outside-IR35 position

Regardless of who makes the determination, contractors benefit from actively managing their IR35 position. Practical steps include:

• Ensuring your contracts accurately reflect the working relationship — vague or generic contracts that replicate employment terms are an obvious risk
• Documenting genuine substitutions or the right to substitute in a practical, not theoretical, way
• Avoiding behaviours that signal integration — company email addresses, fixed desks, attending client staff events as a team member
• Working with multiple clients where commercially possible
• Obtaining a professional contract review from a qualified IR35 specialist before entering an engagement

An inside-IR35 determination is not always the end of the matter. Legal challenges at employment tribunal level have succeeded where contractors can demonstrate that the true nature of the working relationship does not meet the tests for employment — though tribunal proceedings are costly and time-consuming.

The IR35 landscape in 2026 is characterised by a broadly stable legislative framework, but ongoing practical and legal turbulence. The core rules — client determination for medium and large engagements, contractor determination for small clients — remain in place following the April 2021 reforms.

HMRC enforcement activity

HMRC has continued to invest in IR35 compliance activity, with investigations targeting both large clients who fail to operate the rules correctly and individual contractors (particularly in the small-client sector) where self-assessment of status is still required. HMRC has access to real-time PAYE data and Companies House filing history, which means unexplained shifts in a contractor's tax profile are more visible than they were a decade ago.

Ongoing legal challenges and tribunal decisions

There have been a number of significant tribunal decisions in recent years that have refined how the tests for employment status are applied — including cases turning on the relative weight given to mutuality of obligation, the practical (rather than contractual) right of substitution, and the significance of integration into the client's workforce. These cases are a reminder that IR35 outcomes are genuinely fact-specific: a contractor working in IT on a three-month engagement with documented substitution rights and multiple concurrent clients is in a very different position to a contractor who has been placed at the same site for three years under a series of rolled-over contracts.

The ongoing debate about reform

There has been continuing discussion in the contractor sector and among tax professionals about whether the 2021 reforms achieved their stated objectives without creating disproportionate compliance burdens — particularly for smaller businesses caught at the margins of the small-company threshold. As of May 2026, no material statutory changes to the IR35 framework have been announced, though government consultations and fiscal events can shift the position. Any contractor or finance director managing a contractor workforce should monitor HMRC guidance at gov.uk/guidance/understanding-off-payroll-working-ir35 for up-to-date guidance.

Whether you are a contractor reviewing a new engagement or a business assessing your obligation to issue status determinations, the following steps give you a clear action plan.

These are the errors that come up repeatedly in practice — and the ones most likely to attract HMRC scrutiny.