IR35 is shorthand for the off-payroll working rules — a set of UK tax legislation designed to ensure that contractors who work in a way that resembles employment pay broadly the same Income Tax and National Insurance as permanent employees. The name comes from the original Inland Revenue press release, numbered 35, that announced the rules back in 2000. The rules themselves, however, have been significantly reformed since then, most recently with changes that took effect from 6 April 2021 for the private sector.

The core question IR35 tries to answer is straightforward: if the limited company or other intermediary were removed from the picture, would this worker be an employee of the client? If the answer is yes, HMRC considers the worker a “disguised employee” and expects them to pay tax accordingly — regardless of the trading structure they have chosen.

For contractors operating through their own limited company, understanding IR35 is not optional. A determination that you are inside the rules can have a material impact on your take-home pay, your company’s tax position, and the way your engagements are structured. This guide walks through every key element so you know exactly where you stand.

IR35 applies to workers who provide their services through an intermediary — most commonly their own limited company, often called a personal service company (PSC) — and who would be treated as an employee if they engaged directly with the client, without that intermediary in place.

The typical contractor scenario

Picture a software developer who invoices a large bank through their limited company. They work on-site five days a week, use the bank’s equipment, and have been in the same role for two years. Structurally they look like a contractor; in practice, they look very much like an employee. IR35 exists precisely for situations like this.

Who is exempt?

Not everyone working through a PSC is automatically caught by IR35. The rules only bite where the hypothetical employment test is met. A contractor who genuinely works across multiple clients simultaneously, controls how and when work is done, brings their own specialist equipment, and takes on real financial risk is likely to sit outside IR35. The substance of the working arrangement matters far more than the label on the contract.

It is also worth noting that the rules operate differently depending on the size of the engaging client:

• Small private-sector clients: Where the end client qualifies as a small company under the Companies Act 2006 (broadly, meeting two of the three criteria: turnover under £10.2 million, balance sheet under £5.1 million, fewer than 50 employees), the responsibility for determining IR35 status remains with the worker’s own company. The pre-2021 regime effectively continues for these engagements.
• Medium and large private-sector clients, and all public-sector bodies: The end client is responsible for determining status and must issue a Status Determination Statement before any work begins.

This distinction is critical. Many contractors assume the same rules apply regardless of who they work for, but your obligations — and your risks — differ significantly depending on the client’s size.

There is no single, definitive test for employment status under IR35. Instead, HMRC and the courts apply a series of well-established principles, drawing on decades of employment case law. Three factors carry the most weight.

Substitution

Can you send someone else to do the work in your place, and is that right genuine rather than theoretical? A true contractor can substitute a suitably qualified colleague without needing the client’s approval of that individual. If a contract says substitution is permitted but the reality is the client would always insist on you personally, the clause carries little weight.

Control

Who decides how, when, and where the work is done? An employee is typically directed by the employer on day-to-day matters. A genuinely self-employed contractor decides how to achieve an outcome — they might agree on deadlines and deliverables, but the method is their own. If a client dictates your hours, requires you to follow their internal processes, and manages you like a line manager manages a member of staff, that points firmly towards employment.

Mutuality of obligation

Is the client obliged to offer work, and are you obliged to accept it? In a true contractor relationship, there is no expectation of ongoing work beyond the current project, and no obligation to accept future work even if it is offered. Long-running engagements where both sides simply assume the arrangement will continue indefinitely look very much like an employment relationship.

Other relevant factors

Beyond these three, HMRC also considers whether you are financially integrated into the client’s business, whether you bear genuine financial risk (for example, you fix your own mistakes at your own cost), and whether you provide your own equipment for core work tasks. No single factor is decisive — it is the overall picture that matters.

The off-payroll working rules have gone through two significant phases of reform in recent years, and it is worth understanding both to see where things currently stand.

The April 2021 private-sector reform

Before April 2021, contractors working for private-sector clients assessed their own IR35 status. The 2021 reform shifted that responsibility to medium and large private-sector engagers — mirroring the rules that had applied to the public sector since April 2017. From that point, clients in those categories became responsible for carrying out the status assessment, issuing a Status Determination Statement to the contractor and any agency in the chain, and ensuring the correct tax treatment was applied where inside-IR35 determinations were made.

The 2026 update

As of 26 February 2026, HMRC published updated guidance on the off-payroll working rules, with certain procedural changes taking effect from 6 April 2026. If you are reading this guide after that date, it is worth checking HMRC’s current guidance directly to ensure you are working from the most up-to-date version of the rules. The substantive framework — who determines status, how the tax flows, and what constitutes a valid SDS — remains broadly consistent with the post-2021 position.

What a Status Determination Statement must contain

Where the rules place responsibility on the client, they must provide a written Status Determination Statement that sets out their conclusion (inside or outside IR35) and the reasons for reaching it. A determination without reasoning is not valid. Contractors have the right to raise a dispute with the client if they disagree, and clients must respond within 45 days.

If an engagement is determined to be inside IR35, the tax treatment changes significantly. Rather than your limited company receiving the full fee and you extracting income through a combination of salary and dividends, the income is treated as employment income — even though you are not actually employed.

Who deducts the tax?

For medium and large-client engagements, the deemed employer — which is typically the fee payer in the chain (often a recruitment agency, or the client itself if there is no agency) — is responsible for deducting Income Tax at source and employee National Insurance Contributions from the fees paid to your company, before those fees are transferred. The fee payer must also pay employer’s National Insurance on top.

In practical terms, this means your company receives a net amount after these deductions. Because this income has already been taxed as employment income, you cannot then extract it as dividends without risking double taxation. HMRC does allow an offset mechanism to avoid that outcome, but it requires careful calculation — this is one of the areas where getting the accounting right genuinely matters.

What your company can still deduct

Inside IR35, the notional salary calculation allows for a small number of deductions before the tax liability is computed — including pension contributions to an employer scheme — but the list is narrower than the expenses a genuinely self-employed contractor could claim. Travel, subsistence, and home-office costs are generally not deductible within an inside-IR35 engagement.

Student and postgraduate loan repayments

One easily overlooked point: where a contractor is inside IR35, the deemed employer does not deduct student loan or postgraduate loan repayments from the fees. The contractor must account for those repayments through Self Assessment instead. If you have an outstanding loan balance, this is an administrative responsibility that falls back to you personally.

HMRC provides an online tool called Check Employment Status for Tax — universally known as CEST — which works through a series of questions about a specific engagement and produces a determination of the employment status for tax purposes.

What CEST actually tells you

The tool can return one of four results: employed for tax purposes, self-employed for tax purposes, the off-payroll working rules apply, or the off-payroll working rules do not apply. In some cases it returns an inconclusive result — essentially telling you that the facts are too finely balanced for an automated tool to resolve.

Does HMRC actually stand behind it?

HMRC has stated publicly that it will stand by a CEST determination, provided the information entered is accurate and consistent with the actual working arrangements. That commitment is meaningful — if you run a CEST assessment honestly and it returns an outside-IR35 result, HMRC should not then challenge that engagement on IR35 grounds.

The caveat is important, though. CEST is only as reliable as the inputs. If the answers entered describe the contractual position rather than the real-world working practices, any protection the determination might offer evaporates. HMRC investigates the reality of how a contractor works, not just what the contract says.

CEST’s limitations

The tool has been criticised by tax professionals for not adequately weighing mutuality of obligation — one of the core employment tests under case law — and for producing inconsistent results in edge cases. It is a useful starting point, but for high-value or long-running engagements, a formal written assessment supported by a qualified tax adviser is worth the investment. CEST results can be saved and printed, which is good practice for your records regardless of the outcome.

Being outside IR35 means the rules do not apply to a particular engagement. Your limited company receives the full fee, and you continue to extract income in the most tax-efficient way available — typically a combination of a modest salary (often set around the National Insurance secondary threshold to preserve contribution credits without triggering large NI bills) and dividends from post-tax company profits.

Outside IR35 does not mean risk-free

A determination that you are outside IR35 is not a permanent guarantee. HMRC can investigate historic engagements and challenge a status determination if the working practices suggest the conclusion was wrong. Engagements lasting several years with the same client, or where working practices have gradually drifted towards something resembling employment, can be vulnerable even if the original assessment looked solid.

Maintaining your outside-IR35 position

Contractors who want to protect an outside determination should take practical steps to ensure the reality of their working life matches the contractual position. That means:

• Exercising the right of substitution where genuinely possible — and keeping records when you do
• Avoiding the use of the client’s equipment for core deliverables where you can provide your own
• Working across multiple clients rather than relying on a single engager for all your income
• Ensuring your contract reflects the actual working arrangements, not just a set of favourable-sounding clauses that bear no relation to reality
• Keeping contemporaneous records — emails, timesheets, project correspondence — that support your status if it is ever queried

IR35 disputes are won and lost on evidence. The more clearly you can demonstrate that your working arrangements reflect genuine self-employment, the stronger your position in any challenge.

Whether you are starting a new contract or reviewing an existing one, these steps will help you approach IR35 methodically and reduce your exposure to unexpected tax bills.

These are the errors that cause real problems in practice — and the ones that a generic content article is unlikely to flag clearly enough.