The IR35 status determination guide for 2026/27

IR35 status determination is the single most consequential financial decision most UK contractors face. Get it right and you operate outside IR35 — retaining access to the Ltd company tax advantages. Get it wrong, or have it determined incorrectly by a risk-averse client, and you face paying income tax and NI on your full contract value as if you were an employee — at a cost of £10,000 to £30,000+ per year at typical contractor rates.

This guide covers the three core legal tests, the 2026 small-company threshold change and what it means for 14,000 contractors, the CEST tool's limitations, and practical steps to manage your status risk.

A brief history of IR35

IR35 (officially the Intermediaries Legislation, Chapter 8 of ITEPA 2003) was introduced in April 2000. Its purpose: to prevent employees from incorporating a personal service company and contracting back to their former employer, gaining the tax advantages of a Ltd without genuinely running an independent business.

For the first 17 years, the contractor (or their Ltd company) was responsible for determining their own status.

2017 reform (public sector): HMRC shifted responsibility for status determination to the public-sector engager. If you contracted with an NHS trust, a local council, or a government department, that body now assessed whether you were inside or outside IR35 — and if inside, deducted PAYE and NI from the engagement payment.

2021 reform (private sector, medium and large companies): The same rules extended to medium and large private-sector clients. Now, if you contracted with a company meeting at least two of: £10.2M+ turnover, £5.1M+ balance sheet, 50+ employees, that client was responsible for your status determination and must issue a Status Determination Statement (SDS).

Small companies remained exempt — the contractor continued to determine their own status for engagements with small clients.

The 2026 small-company threshold change

In April 2026, the company size thresholds were revised upwards across UK company law (principally affecting the Companies Act 2006 definitions). The small-company upper threshold increased from £10.2M turnover to £15M turnover.

The off-payroll working rules link directly to these company size definitions. The consequence: clients previously classified as medium (turnover £10.2M–£15M, potentially responsible for SDS) reverted to "small" status. For contractors working with those clients, status determination responsibility shifted back to the contractor from April 2026.

HMRC estimates approximately 14,000 contractors are affected by this reclassification — moving from client-determined to self-determined status.

For those 14,000 contractors, this means:

• No longer receiving an SDS from the client
• Needing to determine their own status (and document that determination)
• Taking on the risk of HMRC challenge if their self-assessment is wrong

This is not necessarily bad news. Many contractors who were blanketed as inside IR35 by risk-averse medium-sized clients may legitimately be outside IR35 when assessed properly. But it requires active attention, not passive acceptance.

The three core tests

Employment law has no single definition of "employee" — courts and tribunals have developed a multi-factorial test over decades of case law. IR35 applies the same framework. Three tests are central:

1. Substitution

The question: Can you provide a substitute to perform the work, without the client's approval, at your own expense?

Why it matters: An employee cannot send someone else to do their job. An independent contractor running a business can. Genuine substitution rights are the most powerful single indicator of outside-IR35 status.

What HMRC looks at — reality over paperwork: Most modern contractor contracts include a substitution clause. HMRC knows this. HMRC will look beyond the contract wording to the reality of the working arrangement:

• Has a substitute ever been provided, or been actively considered?
• Does the client implicitly or explicitly expect you personally to do the work?
• Would the client in practice refuse a substitute (even if the contract allows it)?
• Is the substitution right genuinely unfettered, or conditional on the client's approval?

A well-drafted substitution clause is necessary but not sufficient. If a substitute has never been used, HMRC may argue the right is not genuine. If your contract says you can substitute but the client's onboarding requires specific DBS checks or security clearances that only you hold, the right is arguably hollow.

Practical steps:

• Have a genuine substitution clause — not conditional on client consent
• If possible, actually use a substitute, even once
• Document any occasion where substitution was considered or offered
• Ensure the substitute would be paid by your company, not by the client

2. Control

The question: Does the client control what you do, when you do it, where you do it, and how you do it?

Why it matters: Employees work under their employer's direction. Independent contractors agree to deliver outcomes and retain discretion over methods and timing.

The four control dimensions:

• What: Is the scope of your work defined by detailed instructions, or by a deliverable/outcome specification?
• When: Must you work fixed hours set by the client, or do you choose your hours?
• Where: Must you work on-site, or can you work remotely? Is the on-site requirement driven by a business need (access to a data centre, physical equipment) or by supervision preference?
• How: Does the client direct your working methods in detail, or do you apply specialist expertise independently?

Modern knowledge work blurs these lines. A contractor attending daily stand-ups, working core hours in the client's office, and having their tasks allocated through the client's project management tool looks very much like an employee on the control dimension — even if the contract says otherwise.

Lower-control indicators (outside IR35 friendly):

• Output-based contract (deliver X, not "work 9-5")
• Freedom to work remotely
• Specialist expertise that the client defers to on methodology
• No requirement to follow client's employee policies (leave procedures, internal appraisals)
• No access to client HR systems or employee benefits

3. Mutuality of Obligation (MOO)

The question: Is the client obliged to offer you work, and are you obliged to accept it?

Why it matters: Employees have a continuous employment relationship — the employer must provide work (or pay when none is available), and the employee must show up and do it. Independent contractors are engaged for specific projects; when the project ends, so does the obligation.

For contractors:

• Is there a continuous stream of work automatically provided, or are individual engagements explicitly agreed?
• If the client has no work for three weeks, do you stop billing and they stop paying? (Outside-IR35 friendly: genuinely project-based engagement)
• Are you automatically offered the next project when the current one ends, or is there a re-engagement process?

MOO is the test that HMRC's CEST tool arguably handles least well. Courts have repeatedly held that MOO is relevant and can be determinative, but CEST does not include an MOO question. This is one of the primary criticisms of CEST from employment lawyers and contractor advocacy groups.

Short, discrete contracts with no automatic rollover are lower-MOO-risk. Long-running engagements that roll month-to-month, where the contractor is effectively a permanent fixture, accumulate MOO risk.

Other relevant factors

The three core tests are the most important, but courts also consider:

Financial risk: Does the contractor risk their own money? Quoting for projects, bearing the cost of corrections, providing professional indemnity insurance, and investing in their own equipment all point toward genuine business operation.

Equipment and tools: Employees are provided with tools. Independent contractors typically provide their own specialist equipment (though this is less relevant for software roles where the client's systems must be used).

Integration: Is the contractor embedded in the client's team, attending HR briefings, included in the org chart? Or are they a distinct external service provider?

Basis of payment: Fixed fee for a project, or an hourly/daily rate with no deliverable definition? Fixed-fee project contracts look more like genuine business contracts.

Exclusivity: Can you work for multiple clients simultaneously? If a contract prevents you taking any other work, that looks more like employment.

Intention: Courts give limited weight to stated intention ("the parties agree this is not an employment relationship") — because if the reality is employment, the label does not change the tax treatment. But it is not entirely irrelevant; it is one factor among many.

The CEST tool: useful, limited, and controversial

HMRC's Check Employment Status for Tax (CEST) tool is available at check-employment-status-for-tax.service.gov.uk. It asks a series of questions and delivers one of three outcomes: "employed", "self-employed", or "unable to determine".

CEST's utility:

• It is free and quick
• HMRC has said they will stand behind an "outside IR35" CEST result if the information entered was accurate
• Many medium/large clients use it to produce their SDS

CEST's limitations:

• It does not ask about Mutuality of Obligation at all — a significant gap given MOO's importance in case law
• Its questions are sometimes binary where the reality is nuanced
• An "unable to determine" result provides no protection
• It has been criticised by employment lawyers for producing results inconsistent with established case law in some scenarios

What to do with "unable to determine": If CEST cannot determine your status, you need a professional status review. Several firms — QDOS Contractor, Markel Tax, and others — offer status reviews for £200–£500. They will provide a written opinion with reasoning, which, while not binding on HMRC, demonstrates reasonable care in your status determination.

IR35 insurance

Given the stakes (an inside-IR35 determination on a £500/day contract over three years could mean £50,000+ in back taxes, interest, and penalties), some contractors buy specialist IR35 insurance.

Investigation cost insurance (approximately £100–£150/year): Covers the professional fees of defending an HMRC IR35 investigation — accountant, tax advisor, and legal costs. Does not cover the underlying tax liability.

Full liability insurance (approximately £200–£350/year): Covers both investigation costs and the actual tax bill if HMRC determines you were inside IR35 and you are found liable. QDOS and Markel are the main providers.

The value of full-liability insurance depends on your risk profile. Contractors in higher-risk sectors (public sector, large financial services clients, historically scrutinised roles) may find it worth carrying.

Practical steps for 2026/27 status management

1. Review your contract for the three tests. Does it have a genuine substitution clause? Does it define deliverables or just time? Does it avoid language suggesting MOO (automatic rollover, notice of absence)?

2. Document the working reality. Keep records that evidence outside-IR35 working practice: emails showing you declined an engagement, records of working remotely, invoices for equipment you provided.

3. Run CEST and document the result. Even if CEST gives "unable to determine", the fact that you ran it demonstrates due diligence.

4. If in any doubt, get a professional status review. At £200–£500, it is inexpensive relative to the risk.

5. Check whether your client is now "small" under the 2026 thresholds. If your client's turnover is between £10.2M and £15M, they may have reverted to small-company status, shifting responsibility back to you.

6. Consider IR35 insurance if you are operating in a high-risk sector or if your contract has characteristics that a professional review identifies as borderline.

HMRC source

The off-payroll working rules for 2026/27 are documented at gov.uk — Understanding off-payroll working (IR35). The Check Employment Status for Tax tool is at gov.uk — CEST.

Try the calculator

Our contractor calculator shows the take-home difference between outside-IR35 Ltd, inside-IR35 umbrella, and permanent employment at your day rate. The financial stakes of inside vs outside IR35 are large enough to make careful status management worthwhile. For background on the comparison, see our inside vs outside IR35 guide.

Summary

IR35 status turns on three core tests — substitution (can you genuinely send a replacement?), control (does the client direct how you work?), and mutuality of obligation (is ongoing work automatically provided and accepted?). The April 2026 small-company threshold rise from £10.2M to £15M shifted status determination responsibility back to approximately 14,000 contractors whose clients reclassified as small. CEST is useful but omits MOO and produces "unable to determine" in borderline cases. Professional status reviews (£200–£500) and IR35 insurance (£200–£350/year for full liability) are rational hedges for higher-risk contracts. The difference in annual take-home between inside and outside IR35 at £500/day is typically £15,000–£25,000 — making status management one of the highest-value activities a contractor can engage in.