IR35 is the shorthand name for the UK's off-payroll working rules — a set of tax provisions originally introduced in 2000 and substantially reformed in 2017 and 2021. The name comes from the Inland Revenue press release that first announced them: IR 35.

The core idea is straightforward: if a contractor would effectively be an employee of their client were it not for the presence of an intermediary — typically a personal service company (PSC) — then they should pay Income Tax and National Insurance Contributions at roughly the same level as a direct employee. Without the rules, operating through a limited company can offer tax advantages that HMRC considers unjustified when the working relationship is functionally indistinguishable from employment.

IR35 explained in full means understanding three things: the employment-status test that determines whether the rules apply, who is legally responsible for making that determination, and what happens when the answer is yes. Those three questions are what this guide works through.

The rules have changed significantly over the past decade, and further changes to umbrella company arrangements took effect in April 2026. If you're a contractor, a freelancer, a business that hires flexible workers, or an agency operating in the middle of a labour supply chain, this guide gives you the framework you need.

The off-payroll working rules apply to a given contract if, hypothetically, the contractor would have been treated as an employee had they been engaged directly rather than through their intermediary. This is sometimes called the hypothetical contract test, and it draws on decades of employment law case precedent.

The three core factors

No single factor is decisive, but three questions carry the most weight in any IR35 assessment:

• Substitution. Can the contractor send a substitute in their place, and is that right genuine rather than theoretical? A genuine, unrestricted right of substitution is strongly indicative of self-employment.
• Control. Who controls how, when, and where the work is done? The more control the client exercises over the day-to-day working method — not just the outcome — the more the relationship resembles employment.
• Mutuality of obligation. Is the client obliged to offer work, and is the contractor obliged to accept it? Where neither party has those obligations beyond the current contract, that points toward self-employment.

Secondary factors

Beyond the three core elements, assessors also consider: whether the contractor provides their own equipment; whether they carry genuine financial risk (including the possibility of making a loss); whether they are integrated into the client's organisation rather than working alongside it; and whether they work for multiple clients simultaneously.

No algorithm produces a definitive answer from these factors. Each contract is assessed on its own facts, which is exactly why IR35 disputes have been so frequent and so expensive over the years — even well-known cases involving household names have gone to tribunal on genuinely borderline facts.

The key takeaway for contractors is this: the contract wording matters, but so does the reality of how the engagement operates day to day. A contract that describes extensive substitution rights is worthless if those rights have never been used and the client would refuse them in practice.

One of the most important — and most misunderstood — changes introduced by the 2017 and 2021 reforms was the transfer of responsibility for determining employment status away from the contractor and onto the client. The rules now work differently depending on the type and size of the client organisation.

Medium and large private-sector and voluntary-sector clients

If the end client is a medium or large organisation in the private or voluntary sector, that client is legally responsible for deciding whether IR35 applies to each engagement. They must produce a written Status Determination Statement (SDS) setting out their conclusion and the reasons for it, and must pass that statement down the contractual chain — to the agency if one is involved, and ultimately to the contractor.

The 'medium or large' threshold follows the Companies Act definition: broadly, a company meets at least two of the following in a given financial year — more than 50 employees, turnover exceeding £10.2 million, or a balance sheet total exceeding £5.1 million. If a company meets two of those three, it is not a small company, and the off-payroll rules apply in full.

Public-sector clients

For all public-sector engagements, the client has been responsible for status determinations since April 2017 — the rules arrived in this sector four years ahead of the private sector.

Small private-sector clients

There is a deliberate carve-out for small businesses. Where the end client qualifies as a small company under the Companies Act definition, responsibility for determining IR35 status reverts to the worker's own intermediary — typically the personal service company itself. If you operate through a PSC and your client is a genuine small business, you are still responsible for assessing your own position and accounting for any tax liability accordingly.

This distinction matters enormously in practice. A contractor moving from one client to another needs to understand which regime they are operating under — because the administrative process, and the financial exposure if something goes wrong, are fundamentally different.

When IR35 applies to a contract, the income from that engagement is treated as a deemed employment payment. This has direct consequences for the tax and National Insurance position of both the contractor and, depending on who holds the responsibility, the fee-payer in the supply chain.

For the contractor's personal service company

Under the old IR35 regime (where the PSC made its own determination), the company was required to calculate a deemed payment at the end of the tax year and account for PAYE and National Insurance as if that sum were salary. In practice, this significantly eroded the tax efficiency of operating through a limited company for inside-IR35 work.

For medium and large clients (post-2021 reforms)

Where the client is responsible for making the determination and concludes that IR35 applies, the fee-payer — which is the entity that pays the PSC directly, usually an agency — becomes responsible for deducting Income Tax and employee National Insurance via PAYE before making payment. The fee-payer also becomes liable for employer's National Insurance.

This means the contractor receives a net payment rather than a gross fee, much like an employee receives net pay after deductions. The PSC still exists and still receives the money, but it arrives already taxed at source.

The financial difference

The practical impact of an inside-IR35 determination can be significant. A contractor billing £700 per day who falls inside IR35 may find their effective take-home is materially lower than an equivalent contractor who is genuinely outside. The difference is not the existence of a limited company structure — it is the tax treatment of the income that flows through it. This is why accurate status determination matters: an incorrect inside determination costs the contractor real money, and an incorrect outside determination can trigger HMRC investigation and backdated liabilities.

If a medium or large client determines that IR35 applies, they are legally required to document that conclusion in a Status Determination Statement. The SDS must explain the reasoning — not simply state a conclusion — and must be provided to the next party in the contractual chain within a reasonable timeframe.

What an SDS must contain

There is no prescribed template, but HMRC expects the statement to identify the specific contractual and working arrangements that informed the determination, and to explain why those arrangements led to the conclusion reached. A blanket policy of declaring all contractors inside IR35 without individual assessment has been successfully challenged, and HMRC itself has acknowledged that blanketing is not a legally compliant approach.

The client-led disagreement process

Contractors who disagree with a determination made by their client have a formal right to challenge it. The client must respond within 45 days, either changing the determination and issuing a revised SDS, or maintaining the original determination and providing written reasons. If the client fails to respond within 45 days, they lose the benefit of the transfer-of-liability protection — meaning HMRC can pursue them rather than the contractor for any underpaid tax.

Keeping records

Both clients and contractors should retain documentation supporting their status determinations for at least six years. HMRC operates on a rolling basis when investigating employment status, and contracts that were in force several years ago can still be subject to enquiry. The SDS, any correspondence during a dispute, and contemporaneous evidence of how the engagement actually operated in practice are all relevant to any future challenge.

The CEST tool — covered in the next section — is one way to generate a documented determination, but it is not mandatory. Clients can commission specialist IR35 reviews through their advisors, or use a combination of the tool and professional judgment for borderline cases.

HMRC provides a free online tool called Check Employment Status for Tax, known as CEST. It asks a structured series of questions about the contractual arrangements and working practices of an engagement, and returns one of several possible outcomes: employed, self-employed, unable to determine, off-payroll rules apply, off-payroll rules do not apply, or unable to determine for the off-payroll rules.

What HMRC promises about CEST

HMRC has stated that it will stand behind determinations produced by the tool, provided the information entered was accurate and complete and consistent with how the engagement actually operates. This gives the tool a degree of evidential weight — if you run an engagement through CEST honestly and receive an 'outside IR35' result, and the facts on the ground match what you entered, that record provides meaningful protection in the event of an enquiry.

The tool is anonymous and does not collect identifying information about the user or the engagement, so it can be used speculatively before a contract begins.

The limitations of CEST

CEST has attracted persistent criticism since its introduction. The main complaints are that it does not adequately account for mutuality of obligation — which courts have consistently treated as a significant factor — and that the 'unable to determine' outcome, which the tool produces for a meaningful proportion of cases, provides no practical guidance for the parties involved.

For straightforward engagements at either end of the spectrum — a short, project-based contract with a genuine substitution clause and multiple concurrent clients, or a long-term, single-client engagement with daily supervision and set hours — CEST will usually produce a clear result. For genuinely borderline cases, the tool's limitations become more significant, and professional advice on the specific contractual wording and working practices is well worth the cost.

HMRC continues to update the tool periodically. Always use the current version available at gov.uk/guidance/check-employment-status-for-tax rather than any cached or third-party version.

A separate but related change took effect on 6 April 2026, affecting contractors who work through umbrella companies rather than their own personal service company. These changes do not alter the off-payroll working rules themselves, but they do significantly shift who is responsible for ensuring PAYE is operated correctly in labour supply chains that include umbrella companies.

The new liability framework

Under the rules in force from April 2026, where a worker is employed by an umbrella company and placed with an end client via a recruitment agency, that agency — as the entity with the direct contractual relationship with the end client — becomes responsible for ensuring PAYE is deducted correctly. If there is no agency in the chain, that responsibility falls on the end client directly.

HMRC can recover any underpaid PAYE from the agency or the end client under this framework. This is a substantial change in exposure for agencies and end clients that use umbrella arrangements: previously, much of the compliance risk sat with the umbrella company itself, and non-compliant or fraudulent umbrella operators created significant unpaid tax liabilities that were difficult for HMRC to recover. The new rules close that gap.

Who is affected — and who is not

These changes apply to new and existing labour supply chains, and cover payments made to workers on or after 6 April 2026 regardless of when the underlying contract was signed. Agencies and businesses that use umbrella companies should have reviewed their supply chains before this date and confirmed that their umbrella partners operate compliantly.

Critically, these rules do not apply to workers operating through their own personal service company where the off-payroll working (IR35) rules apply instead. The two regimes — off-payroll working for PSCs, and the new umbrella PAYE rules — sit side by side and address different parts of the flexible labour market. If you work through your own limited company, the April 2026 umbrella changes are not directly relevant to your IR35 position.

Whether you're a contractor reviewing an engagement or a business that has just been asked to issue a status determination statement, the following steps provide a practical framework for getting to the right answer.

These are the errors that come up repeatedly in practice — and that generic content rarely covers with sufficient specificity.