IR35 Complete Guide for Contractors 2025: Inside vs Outside Explained

IR35 — officially the "off-payroll working rules" — is one of the most important pieces of legislation for UK contractors. Getting your IR35 status wrong can result in significant tax bills and penalties. This guide explains everything you need to know in plain English.

What is IR35?

IR35 targets contractors who work for clients through an intermediary (usually a personal service company or PSC) but whose working arrangements are so similar to employment that HMRC considers them "disguised employees." The rules were introduced in 2000 and significantly reformed in 2017 (public sector) and 2021 (private sector).

If you are "inside IR35," your income from that contract is treated as employment income for tax purposes — you pay income tax and National Insurance as if you were employed, but without employment rights like sick pay or redundancy pay. If you are "outside IR35," you can operate your Ltd company normally and benefit from the tax efficiency of salary + dividends.

Who Determines IR35 Status?

Since April 2021, for medium and large private sector clients, the end client is responsible for determining IR35 status and issuing a Status Determination Statement (SDS). The client must take reasonable care in making this determination.

Small companies are exempt from these rules — the contractor determines their own status for engagements with small clients. A company is small if it meets two of three criteria: turnover under £10.2m, balance sheet under £5.1m, or fewer than 50 employees.

The Three Key IR35 Tests

HMRC and the courts use several tests to determine IR35 status. Three are particularly important:

1. Substitution

Can you send a substitute to do your work? If your contract requires you personally to perform the services, that points toward employment. A genuine contractor should be able to send a qualified substitute (at their own cost) to fulfil the contract.

Importantly, HMRC looks at whether there is a genuine unfettered right of substitution — not just whether the right exists on paper. If the client must approve any substitute or has never accepted one in practice, HMRC may treat the right as notional.

2. Mutuality of Obligation (MOO)

In a true employment relationship, the employer is obliged to offer work and the employee is obliged to accept it. For a contractor, there should be no such mutual obligation beyond the current project.

Red flags include: the client automatically offering new contracts at the end of each engagement, no gap between contracts, or a history of near-continuous work with the same client over many years.

3. Control

An employed worker is subject to the control of their employer — being told what to do, when to do it, where to do it, and how to do it. A contractor should have significant autonomy over how they deliver the contracted outcomes.

Working set hours in a client's office alongside employees, using their equipment, and being managed through the same processes as staff all point toward inside IR35.

Financial Impact of IR35

The financial difference between inside and outside IR35 is substantial. For a contractor billing £500/day for 220 days (£110,000/year):

CEST Tool and Independent Reviews

HMRC provides the Check Employment Status for Tax (CEST) tool at gov.uk/guidance/check-employment-status-for-tax. HMRC commits to stand by CEST results provided you answer accurately — but the tool has been criticised for not fully capturing mutuality of obligation.

Many contractors and clients use independent IR35 review services from specialist employment law firms or accountants. A professional review costs £200–£500 but provides much stronger protection if HMRC investigates.

Protecting Yourself

• Get a written contract that accurately reflects your working practices
• Ensure the right of substitution is genuine and documented
• Use your own equipment where possible
• Work for multiple clients, not just one continuously
• Have proper business indemnity insurance in place
• Keep records showing you operate as a genuine business
• Consider an independent IR35 review for each contract