Understanding IR35: The key factors which determine IR35 status

What is IR35 and how does it work?

IR35 is a set of UK tax regulations designed to identify deemed employment. It ensures that contractors who operate through an intermediary, such as a personal service company (PSC), but work in the same way as permanent employees, pay broadly the same income tax and National Insurance contributions.

When an engagement is deemed inside IR35, the contractor is treated as an employee for tax purposes, and tax must be deducted at source. If the engagement falls outside IR35, it is recognised as a genuine business-to-business relationship, allowing the contractor's PSC to be paid gross.

Who is responsible for determining IR35 status?

In the public sector and for medium and large private-sector organisations, the responsibility for determining IR35 status lies entirely with the end client. The hiring business must assess the contractor's working practices and contractual terms, and then issue a Status Determination Statement (SDS) detailing the decision.

If the client qualifies as a small business under the Companies Act 2006, or is based entirely overseas with no UK presence, the statutory off-payroll rules do not apply. In those specific scenarios, the responsibility remains with the contractor's personal service company.

What are the key factors that determine IR35 status?

IR35 status is determined by looking at the operational reality of the working relationship alongside the written contract. Case law has established three primary tests of employment status: the right of substitution, the level of control over the worker, and the mutuality of obligation between both parties.

HMRC and tax tribunals assess these main tests together with secondary business indicators to form an overall picture of the engagement. Relying on a single test or focusing solely on contract clauses will not suffice; working practices are important for properly construing the contractual terms and understanding how the engagement operates.

What is the right of substitution under IR35?

The right of substitution refers to a contractor's contractual and practical ability to send a qualified substitute to perform the services in their place. A genuine right of substitution can demonstrate that the client is purchasing a service from a business rather than hiring a specific individual.

To support an outside IR35 position, the substitution clause must not be unreasonably restricted by the client. If the hiring business can reject a substitute for reasons other than basic qualifications, experience, or security vetting, the right may be viewed as invalid. Hiring organisations should keep records of any instances in which a substitute has been proposed or successfully utilised.

How does control affect IR35 compliance?

Control looks at who determines how, what, when, and where the contracted work is executed. If a hiring organisation exercises sufficient control over the contractor, the relationship may resemble employment rather than a business-to-business engagement.

For a contract to remain safely outside IR35, the contractor must retain autonomy over their deliverables. Managers should avoid treating contractors like permanent staff. This means organisations should not enforce fixed working hours, dictate exact day-to-day processes, or include contractors in internal line management structures and performance appraisals. Engagements should remain project-based with clearly defined milestones.

What does mutuality of obligation mean for IR35?

Basic mutuality of obligation simply means payment is made for work done. A reciprocal agreement in which an employer is required to provide continuous work, and the worker is obligated to accept it, can be a pointer towards employment, but its absence is not determinative.

To maintain compliance, contracts should have explicit start and end dates or be strictly tied to the completion of a specific deliverable. Once the project concludes, the hiring company has no obligation to offer further assignments, and the contractor has no obligation to accept them. Rolling contracts with lengthy notice periods can sometimes compromise this boundary.

What secondary factors impact an IR35 determination?

Beyond the main principles, several other indicators help establish whether a contractor operates as a genuine independent business. These secondary factors include the provision of equipment, the level of commercial risk, the payment terms, integration into the client's company, and the exclusivity of the service.

• Provision of equipment: Genuine contractors typically invest in and use their own specialised tools, software, and hardware rather than relying on equipment provided by the client.
• Financial risk: An independent business assumes commercial risk. If a contractor makes an error, they should be contractually required to rectify the defect in their own time and at their own expense.
• Basis of payment: Paying a contractor according to project milestones or a fixed job price points toward self-employment, whereas standard hourly or daily rates require closer inspection to ensure they do not mimic a salary.
• Integration into the business: Contractors must not become part and parcel of the client's organisation. They should not receive company perks, access staff facilities, or participate in internal corporate events meant exclusively for employees.
• Exclusive service: Operating a real business usually involves marketing services to multiple concurrent or consecutive clients rather than working indefinitely for a single hirer.

How can businesses ensure robust IR35 compliance?

Businesses can ensure IR35 compliance by implementing an objective, evidence-based assessment process that carefully examines both contract terms and actual working practices. This process must demonstrate reasonable care to avoid penalties and the transfer of tax liabilities to the end client.

Relying entirely on basic checklist tools (like the HMRC CEST tool) can expose an organisation to inaccuracies and subsequent tax disputes. Instead, forward-thinking businesses implement specialised assessment technology and secure expert oversight to ensure every determination is accurate, consistent, and legally defensible.