IR35 is the shorthand name for the UK’s off-payroll working legislation. At its core, it addresses a straightforward question: if someone works in a way that looks and feels like employment — same desk, same hours, same supervision — should they be taxed like an employee? HMRC’s answer, since 2000, has been yes. The IR35 changes explained throughout this guide concern how that question gets answered, and who has to answer it.

The rules exist to prevent what HMRC calls “disguised employment”: a situation where a worker and a business choose to structure an arrangement through a limited company primarily to reduce Income Tax and National Insurance contributions, without the working relationship genuinely reflecting self-employment. If IR35 applies to a contract, the contractor pays broadly the same tax as a permanent employee would on the same income.

Understanding the off-payroll working rules matters whether you’re an IT contractor deciding how to price your next contract, an engineering consultant reviewing your existing arrangements, or a finance director at a medium-sized business responsible for your contractor workforce’s compliance. The stakes on all sides are real: unpaid tax, penalties, and — since 2026 — a significantly more data-driven HMRC enforcement regime.

The legislation applies when a worker provides services to a client through an intermediary — typically their own personal service company (PSC) or limited company. The central test is straightforward in principle, complex in practice: would this worker be classed as an employee if they were engaged directly, without the intermediary in the middle?

The key employment status factors

HMRC and the courts have developed a set of indicators that together paint a picture of whether a working relationship is genuinely self-employed or functionally employed. The main factors include:

• Control: Does the client control how, when, and where the work is done? Greater client control points toward employment.
• Substitution: Can the worker send a suitably qualified substitute to do the work in their place, without the client’s approval? A genuine right of substitution is a strong indicator of self-employment.
• Mutuality of obligation: Is the client obliged to offer work, and is the worker obliged to accept it? If both are true, it leans toward employment.
• Integration: Is the contractor embedded in the client’s team and operation, using their equipment and following internal processes, or do they remain clearly distinct?
• Financial risk: Does the worker bear genuine commercial risk — for example, by fixing their price for a project and absorbing cost overruns?

No single factor is decisive. Employment status under IR35 is always a holistic judgement based on the real-world facts of each engagement, not just what the contract says on paper.

The CEST tool

HMRC provides the Check Employment Status for Tax (CEST) tool online to help workers and clients reach a status determination. It’s a useful starting point, but practitioners treat it with caution: it has faced criticism for not adequately handling the mutuality of obligation question, and HMRC will only stand behind its output if the information entered is accurate and complete. An answer from CEST is not a guaranteed safe harbour.

One of the most significant IR35 changes in recent years was the shift in who carries the responsibility for determining employment status. Getting this right matters because the liability for unpaid tax can fall on whichever party in the chain failed to fulfil their obligations correctly.

Public sector — from April 2017

The off-payroll rules were reformed for the public sector first. From April 2017, responsibility for determining a contractor’s IR35 status moved away from the contractor and onto the public-sector client. NHS trusts, local authorities, government departments, and other public bodies became responsible for assessing each contractor engagement and communicating that decision clearly.

Medium and large private-sector businesses — from April 2021

Following a delay caused by the pandemic, the same reform extended to medium and large private-sector organisations from 6 April 2021. If your client meets at least two of the following three criteria, they are responsible for your IR35 status determination:

• Annual turnover of more than £10.2 million
• Balance sheet total of more than £5.1 million
• More than 50 employees

When these rules apply, the client must carry out a proper assessment of each engagement and issue a written Status Determination Statement (SDS) to both the contractor’s intermediary and any agency in the chain. The SDS must include reasoned conclusions — a blanket determination applied across all contractors without individual assessment is not compliant.

Small private-sector clients

Where the end client is a small business (below the thresholds above), the old rules still apply: the contractor’s own intermediary is responsible for making the status determination and accounting for any tax due. This exemption remains in place as of June 2026, though contractors in this position should not treat it as an invitation to ignore the substance of the rules.

When a medium or large client concludes that IR35 applies to a particular engagement, they must produce a Status Determination Statement. This document is more than a formality — it is the trigger for the correct tax treatment to flow through the supply chain, and it gives contractors a right of challenge.

What the SDS must contain

The SDS must set out the client’s conclusion on employment status, along with the reasons that support it. A statement that simply says “inside IR35” without explanation fails to meet the requirement. The reasoning should address the actual facts of the engagement — control arrangements, substitution clauses, integration into the client’s workforce, and so on.

The contractor’s right to dispute

If a contractor or their intermediary disagrees with an inside-IR35 determination, they have a statutory right to raise a dispute with the client. The client must respond within 45 days, either confirming their original view with a fuller explanation or revising their determination. Clients cannot simply ignore a dispute — doing so makes them liable for any resulting tax.

Fee-payer liability

In a supply chain involving an agency, the “fee-payer” — typically the agency paying the contractor’s company — is responsible for deducting Income Tax and NICs under PAYE if the engagement is inside IR35. However, liability can travel back up the chain to the client if the client issued a non-compliant SDS or failed to pass the determination on correctly. This is an area where both clients and agencies need robust processes rather than assumptions.

Contractors caught inside IR35 receive a deemed salary payment. They can still draw a salary and any remaining funds as dividends, but the tax benefit of operating through a limited company is substantially reduced when the engagement is treated as employment for tax purposes.

The IR35 changes explained so far reflect legislation that is now several years old. What is genuinely new in 2026 is the way HMRC is enforcing those rules. The picture that has emerged is of an enforcement regime that is becoming more data-led, more targeted, and harder to fly under the radar of.

AI-driven compliance checking

HMRC has been developing and deploying artificial intelligence tools to cross-reference data across payroll records, Companies House filings, contracts, and other information sources. The goal is to identify patterns that suggest contractors who should be inside IR35 are operating outside it — or that clients are issuing blanket outside determinations without genuine individual assessments. This is not speculation: HMRC confirmed in 2026 guidance that AI-assisted compliance is now part of their operational toolkit for off-payroll working.

What this means for contractors

For contractors, the practical implication is that arrangements which might previously have avoided scrutiny are now more likely to attract an enquiry. HMRC can now match the nature of the work, the client, the duration of the engagement, and the tax paid by the contractor against expected patterns far more efficiently than manual case selection ever allowed.

What this means for hirers

For medium and large businesses engaging contractor workforces, 2026 is the year to audit your IR35 processes rather than assuming last year’s approach is still adequate. HMRC has been explicit that organisations need to be audit-ready — able to demonstrate that each SDS was issued with proper reasoning, that disputes were handled correctly, and that PAYE was deducted where required.

Investigation risk and penalties

Where HMRC finds unpaid tax resulting from incorrect off-payroll determinations, they can seek to collect the liability from the responsible party — whether that’s the contractor, the agency, or the client. Penalties apply on top of the underpaid tax and interest. In cases where HMRC determines the non-compliance was deliberate, penalties can reach 100% of the tax owed. The risk of getting this wrong has never been higher.

For a working contractor, the distinction between inside and outside IR35 is largely a financial one, though it also affects how you run your company and how you interact with clients.

Outside IR35

When an engagement is outside IR35, the contractor’s limited company receives the contract income, the contractor can take a combination of salary and dividends in a tax-efficient way, and the company pays Corporation Tax on its profits. This is the structure most contractors set up their PSC to take advantage of, and it is entirely legal where the working arrangement genuinely reflects self-employment.

Inside IR35

When an engagement falls inside IR35, the fee-payer must treat payments to the contractor’s company as if they were employment income. Income Tax and National Insurance contributions are deducted at source under PAYE before anything reaches the contractor’s company. The contractor ends up with a “deemed payment” at the end of the tax year, and the tax efficiency of the limited company structure largely disappears for that engagement.

Some contractors respond to inside determinations by working through an umbrella company instead of their own PSC. An umbrella company employs the contractor directly, handles PAYE, and passes on the net pay. This is a legitimate option, but contractors should choose umbrella companies carefully — the market contains schemes that make aggressive claims about tax efficiency that HMRC does not accept, exposing contractors to future liability.

Multiple contracts, mixed status

It is perfectly possible to have some contracts that sit inside IR35 and others outside simultaneously. Each engagement must be assessed on its own facts. A software developer might work for a large insurer (inside) and separately for a small start-up (outside) in the same tax year. Keeping clear records for each engagement is essential.

The IR35 changes have made umbrella companies a more prominent part of the contractor market, and 2026 brings additional context worth understanding here too.

What an umbrella company does

An umbrella company employs contractors directly. The contractor works at the client site, the agency or client pays the umbrella, and the umbrella deducts PAYE, employee NICs, employer NICs, and its own margin before paying the contractor a net wage. Umbrella employment satisfies PAYE requirements where a contract is inside IR35, which is why their use has grown substantially since the 2021 private-sector reforms.

Regulatory tightening in 2026

The umbrella market has faced significant regulatory attention in 2026. Non-compliant umbrella schemes — particularly those promising take-home pay that significantly exceeds what straightforward PAYE would produce — have been a persistent problem. HMRC has made clear that contractors cannot rely on an umbrella’s assurances to escape liability if the scheme is found to be non-compliant. If the arrangement doesn’t work, the contractor is the one left with a tax bill.

From a practical standpoint, contractors using umbrella companies should request a clear breakdown of how their take-home pay is calculated, verify that the umbrella is fully PAYE-compliant, and be sceptical of any arrangement promising materially higher take-home pay than a straightforward employed calculation would suggest.

Choosing the right structure

Whether you operate through your own limited company, an umbrella company, or some combination of both depends on your specific contracts, your appetite for administration, and the nature of your engagements. There is no universally correct answer — the right structure is the one that fits your situation accurately and sustainably, not the one that simply minimises tax on paper.

Whether you’re a contractor reviewing your status or a business managing an off-payroll workforce, these steps will help you approach IR35 correctly in 2026.

These are the errors that most frequently cause problems in practice — and that generic guidance rarely addresses directly.