The Three IR35 Status Tests Explained

IR35 status is determined by applying a set of employment tests to the working relationship between you and your end client. The tests are not a checklist — they are weighted factors, and no single one is determinative. In practice, three tests carry the most weight in HMRC's assessment and in tribunal cases: substitution, control, and mutuality of obligation.

Why these tests exist

IR35 status is not determined by a simple checklist — it draws on decades of employment law case law that developed long before IR35 existed. The key cases (Ready Mixed Concrete, Market Investigations, Autoclenz) established the principles HMRC and employment tribunals apply. The three tests are the distillation of that case law into practical indicators, but no single test is conclusive and a contract could fail one test while passing others and still be outside IR35.

Test 1: Personal service and substitution

The most important single test. Employment involves personal service — the employer engages you specifically. If you can send a qualified substitute in your place without the client's consent based on personal preference, this strongly indicates a contract for services (outside IR35) rather than a contract of service (employment/inside IR35).

What makes a substitution clause effective

• The right is written explicitly in the contract
• Client approval is based on qualifications and capability only — not personal preference
• The substitute does not need to be known to or approved by the client in advance
• Your company bears the cost of the substitute (you pay them, not the client)
• The right is genuine and exercisable — not purely theoretical

What weakens a substitution clause

• Client approval is required for any substitute (at the client's absolute discretion)
• The contract says you 'may' substitute but in practice you never would
• The client selected you personally and would not accept anyone else
• Substitution has never been exercised and there is no practical mechanism for it

Test 2: Control

Employment involves being controlled — the employer directs how, when, and where the work is done. A contractor should deliver a result or service based on their own methods, with the client controlling only the desired outcome, not the method of delivery.

Indicators pointing toward employment (inside IR35)

• Client dictates working hours (9–5 attendance required)
• Work is supervised by a line manager at the client
• You follow the client's internal HR policies and procedures
• The client can redirect your work to different tasks at will
• You are part of the client's management hierarchy or org chart

Indicators pointing toward genuine contracting (outside IR35)

• You control how the work is done and your working methods
• Agreed deliverables or milestones, not time-based attendance
• You can work from home or remotely (even if you often work at the client's site)
• The client manages the outcome, not the process
• You set your own hours within project deadlines

Test 3: Mutuality of obligation (MOO)

Mutuality of obligation is the least well-understood of the three tests and the most important limitation of HMRC's CEST tool (which does not assess it). In employment, there is a mutual ongoing commitment: the employer provides work and the employee accepts it. For a contractor, the engagement should be for a defined scope — once the project is done, both parties are free to walk away.

Indicators of problematic MOO

• You have worked with the same client continuously for 2+ years with automatic renewals
• The client is obliged to offer you further work after each contract period
• You are expected to accept work whenever it is offered by the client
• Your contract has rolled over more than 2–3 times without a break

Indicators of healthy MOO position

• Each engagement has a clearly defined scope and end date
• You negotiate and sign a fresh contract at each renewal (not an automatic roll-over)
• You have gaps between engagements (even short ones)
• You have worked for multiple clients, demonstrating you are not exclusively dependent

Other factors tribunals weigh

Common mistakes

• Relying solely on the contract wording — HMRC investigates actual practice. A contract saying "right to substitute" means little if the reality is you have never and would never send anyone else.
• Using CEST as the only assessment tool — CEST does not assess MOO and can produce misleading results. Use it as one input, not the sole determinant.
• Letting the contract auto-renew without review — long continuous engagements look increasingly like employment over time. Renegotiate terms, take a break where possible, and document each renewal as a fresh decision.