Inside vs Outside IR35: The Complete 2025/26 Guide for UK Contractors

IR35 — the off-payroll working rules — is the single most consequential piece of compliance most UK limited company contractors will deal with. Get it right and you operate efficiently and legitimately. Get it wrong and you can face back-taxes, interest and penalties. Yet the rules are widely misunderstood, partly because "inside" and "outside" sound like a simple binary when the reality is a judgement based on the specific facts of each engagement.

This guide explains what the two statuses mean for your money, how status is actually decided, who is responsible for the decision, and — most importantly — how to evidence your position so it holds up.

What "inside" and "outside" IR35 actually mean

IR35 exists to stop someone working like an employee but routing their pay through a limited company (a Personal Service Company, or PSC) purely to reduce tax. The test is whether — if you stripped away the company in the middle — the relationship between you and the end client would look like employment.

	Outside IR35	Inside IR35
HMRC's view	Genuine business-to-business service	"Disguised employment"
How you're taxed	Company pays Corporation Tax; you take salary + dividends	Income taxed like employment — PAYE & NI, little left to take as dividends
Typical take-home	Higher — tax-efficient salary/dividend split available	Lower — similar to being a PAYE employee, but usually without employee benefits
Decided by	How you actually work — on a contract-by-contract basis

It is entirely normal for the same contractor to be outside IR35 on one engagement and inside on another. Status attaches to the engagement, not to you as a person.

Why it matters: the money and the risk

The financial difference is significant. Outside IR35, a contractor can pay themselves a modest salary and draw the rest as dividends, which are not subject to National Insurance — see our guide on the salary vs dividends split. Inside IR35, most of that efficiency disappears: the income is taxed close to how an equivalent salary would be.

The risk side matters just as much. If HMRC reviews an engagement you treated as outside and disagrees, you can be liable for the difference in tax and National Insurance, plus interest, and penalties if they decide you did not take "reasonable care". That is why evidence — not just a well-worded contract — is the thing that actually protects you.

How IR35 status is decided

There is no single rule. Status comes from decades of employment case law, and HMRC weighs a number of factors together. Three are widely treated as the most important:

1. Control

How much does the client direct what you do, how, when and where you do it? A genuine contractor decides how to deliver the work. An employee is told. The more the client controls the detail of your day, the more it points inside IR35.

2. Right of substitution (personal service)

Could you send a suitably qualified substitute to do the work in your place, at your cost, without the client's veto? A genuine business can. If the client is really buying you specifically and would not accept a substitute, that points inside.

3. Mutuality of obligation (MOO)

Is the client obliged to offer you ongoing work, and are you obliged to accept it? That ongoing obligation is a hallmark of employment. A contractor is engaged for a defined piece of work with no expectation of more.

Beyond the big three, HMRC also weighs secondary factors:

• Financial risk — do you bear the cost of putting faulty work right, buy your own insurance, or risk not being paid?
• Equipment — do you provide your own tools and kit?
• Part and parcel — are you embedded in the client's organisation (line-managed, on the org chart, staff perks)?
• Exclusivity — are you free to work for other clients?

HMRC publishes a free tool, Check Employment Status for Tax (CEST), but it does not capture mutuality of obligation well and a result is only as good as the answers you give it. Treat it as one input, not the final word.

Who decides — and the small-company exemption

This is where contractors most often get caught out. Under the off-payroll working rules (Chapter 10 of ITEPA 2003):

• If your end client is a public sector body or a medium or large private-sector company, the client is responsible for determining your status and giving you a Status Determination Statement (SDS).
• If your end client is a "small" private-sector company, your own company remains responsible for assessing status under the original IR35 rules (Chapter 8).

From April 2026, reforms changed how the Employer National Insurance liability flows through the supply chain for contractors working via intermediaries — pushing more of that exposure toward recruitment agencies. For a genuine outside-IR35 contractor the core picture is unchanged, but agencies and end clients now have more at stake in your status, so expect them to scrutinise your determination more closely. A documented, defensible assessment is increasingly something they want to see too.

How to evidence your status (the part that actually protects you)

A contract that says "outside IR35" is worthless if your day-to-day working practices look like employment. HMRC looks at the reality, not the paperwork alone. Two things need to line up:

• The written contract should reflect genuine B2B terms — a real right of substitution, no obligation to provide or accept future work, and control over how you deliver.
• Your working practices must match. If your contract grants substitution but everyone knows the client would never accept a substitute, the clause carries little weight.

Then, for every engagement, keep a dated, documented assessment of why you believe the status is what you say it is — with the specific facts and the reasoning. That record is what demonstrates "reasonable care" and gives you something concrete to rely on if an engagement is ever reviewed. This is exactly what Nebula's IR35 Hub is built to produce: a scored, factor-by-factor assessment with a versioned audit trail. You can see how it works without signing up.

The contractors who sleep well at IR35 enquiry time are not the ones with the cleverest contract. They're the ones who can show a clear, dated, factual record of how each engagement actually worked.

A simple per-engagement checklist

This guide is general information for UK Ltd company contractors, based on rules and thresholds for the 2025/26 tax year, and is not regulated tax or legal advice. IR35 is fact-specific; for complex or disputed engagements, consult a qualified tax adviser. Always check current figures on gov.uk.