What Is IR35? A Plain-English Guide
for UK Contractors 2025/26

IR35 is one of the most significant pieces of legislation affecting UK contractors and freelancers — yet many workers find it genuinely confusing. Whether you are new to contracting or have worked through a limited company for years, understanding what IR35 means and how it affects your take-home pay is essential for 2025/26.

What Is IR35?

IR35 is HMRC’s off-payroll working legislation. Its official name is the Intermediaries Legislation, and it was first introduced in 2000. The term “IR35” comes from the original Inland Revenue press release number that announced it.

The legislation exists to tackle what HMRC calls “disguised employment” — situations where a worker operates through a limited company but works in a way that is, in substance, identical to being an employee. Without IR35, such workers could avoid income tax and National Insurance by channelling income through a company and paying themselves in dividends.

IR35 determines whether a contractor is genuinely self-employed (outside IR35) or should be treated as a deemed employee for tax purposes (inside IR35). The determination is made on a contract-by-contract basis — your overall status is not fixed, and you could be outside IR35 for one contract and inside IR35 for another.

Inside IR35 vs Outside IR35 — The Key Difference

The financial impact of IR35 is significant. The difference between being inside and outside IR35 can amount to thousands of pounds per year.

How IR35 Works — The Deemed Salary Calculation

When a contract is inside IR35, your fee-payer (agency or end client) must deduct income tax and National Insurance at source using the following process:

1. The gross contract income is received by the client or agency
2. Employer NI (15% above £5,000) is deducted from the gross
3. The remaining amount becomes the “deemed salary”
4. Income tax and employee NI (8%) are applied to the deemed salary
5. The net amount is paid to your limited company

This means that inside IR35, your effective tax burden is very similar to that of a permanent employee — but without the employment benefits such as sick pay, holiday pay, or employer pension contributions.

The Three IR35 Tests

HMRC determines whether a contract falls inside or outside IR35 by examining the actual working arrangements — not just the wording of the contract. Three primary tests are applied, based on case law developed over decades of employment tribunal decisions.

Test 1: Substitution

The most powerful indicator of genuine self-employment is the ability to send a substitute. If you have the right to send another suitably qualified person to perform the work in your place — and the client must accept that substitute — this strongly points toward outside IR35 status.

The substitution clause in your contract must reflect reality. A substitution clause that exists only on paper, with the client having never agreed to accept a substitute in practice, carries little weight with HMRC or an employment tribunal.

Test 2: Control

A genuinely self-employed contractor controls how, when, and where the work is done. If the client dictates your working hours, working method, location, and day-to-day tasks in the same way they would manage an employee, this points strongly toward inside IR35 status.

Having genuine freedom over how you complete a project — even if the deliverable is specified by the client — is consistent with outside IR35. Being managed like a member of staff, attending all-hands meetings, receiving line management, and having performance reviews are all indicators of employment.

Test 3: Mutuality of Obligation

Mutuality of obligation refers to whether there is an ongoing obligation for the client to offer work and for you to accept it. In a genuine contractor relationship, each engagement is discrete — you are under no obligation to accept the next project, and the client is under no obligation to offer it.

If you have worked for the same client through multiple renewals over several years with an expectation of continuity on both sides, HMRC may argue that mutuality of obligation exists — pointing toward inside IR35.

Who Determines IR35 Status?

Since April 2021, IR35 status for medium and large private sector businesses has been determined by the end client rather than the contractor. This is known as the off-payroll working reform. The rules apply to:

• All public sector engagements (since April 2017)
• Medium and large private sector companies (since April 2021)

For engagements with small private sector clients — defined as those meeting two of three criteria: turnover under £10.2m, balance sheet under £5.1m, or fewer than 50 employees — the contractor’s own limited company still determines IR35 status.

What Happens if You Get IR35 Status Wrong?

Working outside IR35 when your contract should be inside can result in a substantial tax liability. HMRC can investigate historical contracts and demand back taxes, National Insurance, interest, and penalties. In serious cases, investigations can cover multiple years of contracting.

Since the 2021 reform, liability for incorrect status determinations now falls primarily on the end client or fee-payer rather than the contractor. However, contractors who actively misrepresent their working arrangements to obtain an outside IR35 determination can still face personal liability.

Protecting Your Outside IR35 Status

If your contract has been legitimately assessed as outside IR35, there are practical steps you can take to maintain that status throughout the engagement:

• Maintain genuine substitution rights — ensure your contract includes a real substitution clause and that you could exercise it if needed
• Avoid integration with the client team — do not use a permanent employee job title, do not attend internal HR meetings, do not receive employee benefits
• Keep clear project boundaries — work to a defined statement of work with specific deliverables
• Use separate equipment where possible — using your own laptop and software supports self-employment status
• Avoid long rolling contracts — very long engagements with the same client attract scrutiny around mutuality of obligation
• Have contracts reviewed professionally — an IR35 specialist solicitor can review both the contract wording and working practices

IR35 and Umbrella Companies

If your contract is inside IR35, many contractors choose to work through an umbrella company rather than operating a limited company. An umbrella company employs you directly and processes your pay through PAYE — removing the administrative burden of running a limited company when the tax treatment is effectively identical.

The take-home pay through an umbrella company is very similar to working inside IR35 through a limited company. Use our umbrella company calculator to see your take-home at your day rate, and compare against our IR35 calculator for the inside vs outside comparison.

Final Thoughts

IR35 remains one of the most complex areas of UK tax law for contractors and freelancers. Understanding whether your contract is inside or outside IR35 — and the financial consequences of each — is essential for planning your finances accurately.

Use our free IR35 calculator to see the exact difference in take-home pay at your day rate. For a complete view of your outside IR35 take-home including salary, dividends, and corporation tax, use our outside IR35 calculator. Always have new contracts reviewed by an IR35 specialist before starting work, and use HMRC’s CEST tool as a starting point for any status assessment.