IR35 — formally called the off-payroll working rules — is tax legislation designed to ensure that contractors who work in a way that resembles employment pay roughly the same Income Tax and National Insurance as a permanent employee would. The name comes from the 2000 Budget press release that announced it: Inland Revenue 35.

The core idea is straightforward: if you work through a limited company or other intermediary but your day-to-day working arrangement looks and feels like employment — same hours, same supervision, same exclusive commitment to one client — HMRC argues you should be taxed accordingly. Your company structure should not, in HMRC’s view, be the reason you pay significantly less tax than someone doing the same role on a payroll.

In practice, IR35 changes explained simply means: the rules have been updated several times since 2000, most significantly in 2017 and 2021, and a further significant change affecting umbrella companies came into effect in April 2026. This guide takes you through each of those changes, explains how the current rules work, and sets out what contractors should be doing right now.

When IR35 was first introduced in 2000, the responsibility for deciding whether the rules applied sat entirely with the contractor and their personal service company (PSC). If your contract was caught by IR35, you were supposed to calculate a deemed payment, account for the extra tax, and pay it yourself. In reality, many contractors simply didn’t — and HMRC found it very difficult to enforce the rules at scale.

The 2017 public sector reform

The first major overhaul came in April 2017, targeted at the public sector. HMRC shifted the responsibility for making the IR35 determination away from the contractor’s PSC and onto the public-sector organisation engaging them. If the end client decided the engagement was inside IR35, the fee-payer (the agency or client paying the PSC) became responsible for deducting Income Tax and National Insurance at source — just as an employer would through PAYE.

This was a significant shift. It meant contractors working for NHS trusts, local councils, central government departments, and other public bodies suddenly found IR35 determinations were made by the client, not by them. Many experienced blanket determinations — clients deciding that all contractor roles were inside IR35 regardless of the actual working arrangements.

The 2021 extension to the private sector

From April 2021, the same framework extended to medium and large private-sector businesses. The so-called Chapter 10 rules now apply to any end client that meets two or more of the following criteria: an annual turnover above £10.2 million, a balance sheet above £5.1 million, or more than 50 employees. Where those thresholds are met, the end client must issue a Status Determination Statement (SDS) to the contractor and the agency setting out its IR35 decision, and the fee-payer must operate PAYE accordingly if the determination is inside IR35.

Small private-sector clients remain exempt from these rules. Where the end client is small, the contractor’s PSC is still responsible for determining its own IR35 status — the original Chapter 8 rules continue to apply to those engagements.

Understanding how the current rules operate in practice is more useful than memorising the legislation. Here is how it works for most contractors in 2026.

The Status Determination Statement

Where the off-payroll rules apply (medium or large private-sector client, or any public-sector body), the end client must produce a Status Determination Statement before the engagement begins or before any change in working arrangements. The SDS must state whether the engagement is inside or outside IR35, and it must give reasons. A blanket determination that applies to all contractors without individual assessment does not meet the legal requirement.

The contractor has the right to disagree with the determination and trigger the client’s disagreement process. The client then has 45 days to respond with either an upheld or revised determination. If they do not respond within that window, they become the fee-payer and inherit the tax liability.

What happens if you are inside IR35

If a determination is inside IR35, the fee-payer (usually the agency) must deduct Income Tax and employee National Insurance from the payment before it reaches the contractor’s PSC. The fee-payer also pays employer National Insurance on top. The contractor’s PSC receives a net payment after these deductions, similar to a payslip net amount. The PSC still needs to handle its own corporation tax and accounts, but it cannot claim the standard small salary/dividend split that makes contracting tax-efficient outside IR35.

What happens if you are outside IR35

If the engagement is determined to be outside IR35 — or if the end client is small, meaning the PSC determines its own status — the PSC receives its fee gross. Tax is managed through the company in the usual way: salary, dividends, pension contributions, and allowable expenses. The director/contractor pays their personal tax through self-assessment. The contractor must be genuinely self-employed in practice, not just on paper, for this to hold up to scrutiny.

One of the most significant recent IR35 changes — and one that affects a large number of contractors who thought IR35 didn’t apply to them — came into effect on 6 April 2026. These are the new PAYE rules for umbrella companies and labour supply chains.

What changed and why

Many contractors who are inside IR35 choose to work through an umbrella company rather than a PSC. The umbrella employs the contractor, handles PAYE, and passes on the net pay after deductions. This seemed like the tidy solution to IR35 — if you’re inside anyway, let an umbrella handle the admin.

The problem is that the umbrella company market has significant compliance failures. Some umbrella companies have used non-compliant schemes — disguised remuneration, loan arrangements, and other structures — to artificially inflate the contractor’s take-home pay by reducing the PAYE actually paid to HMRC. The contractor receives what looks like a normal payslip, but the tax deducted is either minimal or channelled through a tax avoidance arrangement.

Who is now responsible

Under the 2026 rules, the agency or end client in the labour supply chain is formally responsible for ensuring the umbrella company they use is operating PAYE correctly. If the umbrella fails to pay the correct tax and National Insurance, HMRC can recover the underpaid amounts directly from the agency or end client. The liability no longer sits solely with the umbrella itself.

These rules apply to both new and existing supply chain arrangements — there is no grandfathering for contracts already in place before April 2026. One notable exception covers workers employed through their own PSC under specific conditions, where the off-payroll rules rather than the umbrella framework determine the tax treatment.

For contractors, the practical implication is simple: the umbrella company you use matters more than ever, because a non-compliant one now creates direct exposure for the businesses that engaged it — and those businesses will want to push that liability back onto the contractor where they can.

HMRC’s Check Employment Status for Tax (CEST) tool is the official route for getting a determination of whether a contractor engagement falls inside or outside IR35. It asks a series of questions about the nature of the working arrangement and produces one of three outcomes: employed for tax purposes, self-employed for tax purposes, or unable to determine.

What CEST considers

The tool works through several key areas of employment status case law:

• Substitution: Can the contractor send a substitute to do the work, and would the client accept that substitution?
• Control: Who decides how and when the work is carried out — the client or the contractor?
• Mutuality of obligation: Is the client obliged to offer work, and is the contractor obliged to accept it? (Note: CEST does not test mutuality of obligation directly, which has been a long-standing criticism.)
• Part and parcel: Is the contractor integrated into the client organisation — attending staff meetings, using the company email address, described as an employee in practice?
• Financial risk: Does the contractor bear genuine financial risk, or are they guaranteed payment regardless of the outcome?

How much weight to give CEST

HMRC states that it stands behind all CEST determinations where the information entered is accurate and complete. That means an outside-IR35 result from CEST carries genuine evidential weight if your working arrangements later come under scrutiny — provided you answered honestly and the actual working arrangement matches what you described.

However, CEST has limitations. Employment tribunals and courts do not use the tool, and a CEST result does not guarantee protection if the reality of the engagement differs from what was entered. The tool is also anonymous — it doesn’t record your answers or produce a timestamped record automatically, so it is worth saving or printing the result alongside the questions you answered.

Experienced IR35 advisers often recommend that a carefully drafted contract and contemporaneous evidence of actual working practice — IR35-friendly clauses in force in reality, not just on paper — matters more than any tool result in isolation.

For most contractors, the practical financial difference between an inside and outside determination is significant enough to shape how they structure their work and which engagements they take on.

Outside IR35

Working outside IR35 means the engagement is genuinely self-employed in character. You operate through your PSC, invoice the client or agency, and receive the fee gross. You pay yourself a combination of salary and dividends — typically a modest salary up to the NI threshold and dividends on the remainder — which is substantially more tax-efficient than employment income. You claim allowable business expenses. Corporation Tax is paid on the company’s profits, and you complete a self-assessment return as a director.

The key point is that outside IR35 does not mean you can ignore the rules. You need the working arrangement to genuinely reflect self-employment: a right to substitute, no obligation on either side to continue the relationship, and control over how the work is done. If the reality is that you are working exclusively for one client, on their premises, under their supervision, for years on end, the fact that your contract says otherwise will not save you.

Inside IR35

Working inside IR35 means your fee-payer deducts Income Tax and National Insurance before paying your PSC. You take home a net amount similar to employment income. You lose the salary/dividend efficiency, the benefit of the employment allowance on NI, and the ability to retain profits in the company to draw down in a future lower-tax year.

There are some allowable deductions — travel to and from the client’s site in certain circumstances, and some professional subscriptions — but the overall tax position inside IR35 is meaningfully more expensive than outside. Many contractors in this position question whether operating through a PSC still makes sense, or whether working directly on a client’s payroll or through a compliant umbrella removes the administrative burden without much extra cost.

HMRC has consistently described IR35 compliance as a priority area. Its 2024/25 annual report on compliance yields covers the returns from off-payroll working enforcement, and the direction of travel is clear: investigations are ongoing, and the sums recovered run into the hundreds of millions of pounds across the public and private sectors combined.

Where HMRC focuses

HMRC’s compliance teams look at several patterns. Public-sector bodies that issued blanket outside determinations without proper assessment. Private-sector medium and large businesses that failed to issue Status Determination Statements at all. Contractors whose PSC invoiced a single client for several years with no substitution ever exercised, no financial risk, and no work for any other client — the hallmarks of disguised employment rather than genuine contracting.

It also targets non-compliant umbrella schemes specifically. Following the 2026 rule changes, agencies and end clients that cannot demonstrate due diligence on their umbrella supply chain face direct recovery action.

What a dispute looks like

Contractors and HMRC have not always agreed on how the law should be applied. There is a history of tribunal cases going both ways, and the courts have not always accepted HMRC’s interpretation. There has been notable contractor criticism following specific legislative debates where proposed IR35 amendments were defeated in Parliament — demonstrating that the legislation and HMRC’s attitude towards it remain contested in some quarters.

That said, the practical risk of getting things wrong is real. An HMRC enquiry into a contractor’s IR35 position — particularly one involving several years of engagements — can result in significant back tax, interest, and penalties, particularly if HMRC concludes that the contractor knew the arrangement was caught and chose not to account correctly. Early, documented status reviews are a straightforward way to reduce that risk.

Whether you are setting up a new contract or reviewing an existing one, these steps reduce your IR35 risk and ensure your position is documented.

These are the errors that cause the most trouble in practice — and the ones a generic guide is least likely to flag.