UK IR35 Rules Explained: Inside vs Outside

IR35 is the UK tax framework for contractors providing services through intermediaries. Introduced in 2000 to target 'disguised employment' (a contractor functionally working as an employee but operating through a limited company for tax benefit), the rules have evolved through major changes in 2017 (public sector) and 2021 (private sector medium and large clients).

This guide covers the legislative framework (Chapter 8 and Chapter 10 of ITEPA 2003), the status tests, the determination process under off-payroll rules, and the practical implications for contractors.

Chapter 8: the original IR35 rules

Chapter 8 of Part 2 of ITEPA 2003 applies where a contractor provides services through an intermediary (PSC, partnership, or unincorporated individual) to a client. The intermediary (typically the PSC) must determine whether the engagement would be employment if the contractor worked directly for the client.

Where the engagement is 'inside IR35' under Chapter 8, the PSC must apply 'deemed payment' rules: calculate a notional employment income from the engagement, deduct employer NI and apprenticeship levy, then apply PAYE on the residual. The contractor receives the post-tax-and-NI amount; corporation tax does not separately apply to the inside-IR35 income.

Chapter 8 remains operative for engagements with 'small' private-sector clients - those that meet 2 of 3 criteria: turnover GBP 10.2m or less, balance sheet total GBP 5.1m or less, 50 or fewer employees. For these clients the contractor's PSC retains the determination obligation.

Worked example: a contractor's PSC engaging with a small private client (turnover GBP 8m, 35 employees). The PSC determines status under Chapter 8. If inside IR35, the PSC applies deemed payment rules. If outside, normal PSC operation continues with corporation tax and dividend extraction.

Chapter 10: off-payroll working rules

Chapter 10 of Part 2 of ITEPA 2003 was introduced for public sector engagements from 6 April 2017 and extended to medium and large private-sector clients from 6 April 2021. The determination obligation moves from the contractor's PSC to the end client.

The end client must issue a Status Determination Statement (SDS) for each engagement before it starts, setting out the determination of inside or outside IR35, with reasons. The SDS must be communicated to the contractor and to the immediate party in the supply chain (typically the agency).

Where the engagement is inside IR35 under Chapter 10, the 'fee-payer' (the party in the chain that pays the PSC, typically the agency or the client where direct) must operate PAYE and NI on the payments. The contractor's PSC receives net amounts; corporation tax does not apply to those receipts.

The status disagreement process allows the contractor to challenge the SDS. The client must respond within 45 days with the determination (confirmed or revised) and the reasons. The contractor cannot force a change but the response shows what evidence was considered. Persistent disagreement may lead to engagement termination or litigation.

The status tests: control, substitution, mutuality

The IR35 status tests are the same as general employment status tests from common law. Three main areas: right of substitution (can the contractor send a substitute to do the work?), control (who decides how, when and where the work is done?), and mutuality of obligation (is there continuing obligation between engagements?).

Right of substitution: a genuine business that could send any qualified worker is more likely outside IR35. A specific personal-service requirement (only this individual contractor) suggests employment-like engagement. Many contracts include substitution clauses but they must be genuine - the right must actually be exercisable in practice, not just in writing.

Control: how, when, where, what work is done. Employee characteristics include set hours, set workplace, set method, set work allocation. Self-employed characteristics include autonomy on how the work is delivered, freedom to take breaks, work from preferred location, accept or refuse specific tasks.

Mutuality of obligation: between separate engagements, is the client required to offer work and the contractor required to accept? Employment shows continuing obligation; genuine self-employment shows discrete engagements without onward expectation. The mutuality test is contested - HMRC has argued it always applies; the courts have generally supported the contractor view that mutuality between separate engagements is needed to be employment-like.

CEST tool and HMRC's view

HMRC's Check Employment Status for Tax (CEST) tool at gov.uk gives an indicative determination based on a series of questions. The tool produces 'employed', 'self-employed', or 'unable to determine' outputs. HMRC commits to standing behind a positive CEST result (where the answers given are accurate).

CEST has been criticised for not handling some scenarios well, particularly around mutuality of obligation (CEST treats some engagements as employment that the courts would treat as self-employment). The tool has been refined over multiple versions; the current version is closer to case law positions but still not comprehensive.

Where CEST is inconclusive or contested, professional employment status review is the standard approach. Firms like IR35 Shield, Qdos, and Markel provide review services that produce detailed determinations with case law reasoning. The reviews are typically GBP 50-200 per engagement.

Worked example: a contractor signs a 12-month engagement with a major bank. The bank uses CEST and gets 'employed' (inside IR35). The contractor disputes through the status disagreement process. The bank reviews with professional input and revises to outside IR35 based on the substitution clause and project-based delivery. The engagement proceeds outside IR35 with the corrected SDS.

Inside IR35 financial impact

Inside IR35 increases the contractor's tax burden materially. The dividend efficiency of the PSC route is lost; the income is effectively treated as employment income with PAYE and NI deductions plus the employer NI cost (passed through to the contractor in most umbrella arrangements).

Take-home comparison: a GBP 500/day contract outside IR35 through PSC typically delivers GBP 75,000-85,000 net per year (assuming reasonable expenses and pension contribution). The same rate inside IR35 through umbrella typically delivers GBP 60,000-65,000 net. The difference is GBP 15,000-20,000 per year.

Contractors facing inside-IR35 status may negotiate a higher rate to compensate. A 20% uplift to a GBP 500/day rate (to GBP 600/day) approximately restores the take-home position. Negotiation success depends on the market for the contractor's skills and the client's willingness to pay.

Edge case: blanket determinations (where a large client unilaterally classifies all PSC contractors as inside IR35 without individual assessment) are technically non-compliant with Chapter 10. The client must make individual SDS determinations. In practice, some clients use blanket approaches; contractors can challenge through the status disagreement process.

Small client exemption under Chapter 8

Small private-sector clients are exempt from Chapter 10 off-payroll rules and operate under Chapter 8 instead. Small means meeting 2 of 3 criteria: annual turnover GBP 10.2m or less, balance sheet total GBP 5.1m or less, 50 or fewer employees. The criteria are tested at the client level, not the parent group.

For small clients, the contractor's PSC retains the determination obligation. The status tests are the same; the practical effect is that the contractor takes the risk of the determination rather than the client. Where status is genuinely outside, the contractor benefits from continuing dividend efficiency.

Aggressive HMRC enforcement of Chapter 8 has historically focused on high-profile or high-value cases. The volume of enforcement is lower than the public/large-private Chapter 10 regime, partly because the small-client universe is large and each engagement is small in revenue terms.

Practical action: small-client engagements still need careful contract drafting and operational discipline to support an outside-IR35 position. Genuine right of substitution, lack of personal-service requirement, project-based delivery and absence of mutuality between engagements all support the contractor's determination.

Insurance and protection against retrospective IR35

IR35 insurance products provide protection against the cost of an HMRC IR35 enquiry plus the potential tax liability if the enquiry succeeds. Two main components: legal costs cover (paying for professional defence of the case, typically GBP 50,000+ of legal fees in a contested case) and tax liability cover (paying the additional tax, NI and penalties if HMRC succeeds).

Major providers: Qdos Contractor Insurance, IR35 Shield Premier, Markel IR35 Shield. Premiums vary GBP 200-600/year depending on cover level and the contractor's risk profile. The cover typically excludes pre-existing engagements where status review was not undertaken.

IR35 contract review services examine specific engagement contracts and working practices, producing a formal opinion on status with reasoning. Reviews are typically GBP 50-200 per engagement. Where the review supports outside IR35 and the contractor's working practices match, the position is defensible if HMRC enquires later.

Worked example: a PSC contractor pays GBP 400/year for IR35 insurance covering legal costs and potential tax liability up to GBP 100,000. Across a 5-year contracting period the total cost is GBP 2,000. A successful HMRC IR35 enquiry on a single year of outside-IR35 income (say GBP 100,000) would typically attract GBP 30,000-50,000 of additional tax + penalties. The insurance pays out where the enquiry succeeds, making the GBP 2,000 over 5 years a worthwhile risk mitigation.

Specific contract terms that support outside IR35

Contract terms that support an outside-IR35 position: a clear right of substitution that is genuinely exercisable (not merely on paper), absence of personal-service requirement, project-based deliverables rather than time-based engagement, control over how the work is performed, absence of mutuality between separate engagements, business indicators (own equipment, own insurance, multiple clients).

Right of substitution is the most important factor. A genuine right means the contractor can send a qualified substitute to do the work without the client's specific approval; some level of basic vetting (security clearance, qualifications) is acceptable but not personal preference. Substitution clauses must be genuine - HMRC and the courts examine whether substitution would actually be permitted in practice.

Control: the contractor decides how to do the work, when (within reasonable project deadlines), and where (subject to security and operational requirements). A contract that requires the contractor to follow client procedures step-by-step, attend specific hours, and work from a specific location suggests employment-like control.

Worked example: a contract for a software development project specifies the deliverables (working application meeting specified requirements), the deadline (project end date), and the right of substitution (any qualified developer with similar skills can substitute). The contract does not specify daily attendance, working hours, or method. This supports an outside-IR35 position.

Defending IR35 status at HMRC and tribunal

HMRC may open a Status Determination enquiry into a contractor's PSC, examining historical engagements to determine whether they were correctly classified as outside IR35. The enquiry process: opening letter from HMRC, request for documents (contracts, working practices evidence, communications with clients), HMRC's view on each engagement, contractor's response, escalation to tribunal if disputed.

Successful defence typically requires evidence of: contract terms supporting outside-IR35, actual working practices matching the contract (HMRC and the tribunal can look behind paper terms to actual practice), business operations (own insurance, equipment, marketing), and a substitution clause that was at least theoretically usable.

Tribunal cases provide guidance. Jensal Software Ltd v HMRC (2020), Atholl House Productions v HMRC (Kaye Adams case, 2022), Susan Winchester Ltd v HMRC (2017) are commonly cited. Each turned on specific facts but established principles around control, substitution, and mutuality.

Frequently asked questions

What is IR35?

UK tax framework determining whether a contractor providing services through an intermediary (PSC, partnership, individual) should be taxed as if directly employed by the client. Introduced 2000 to target 'disguised employment'. Chapter 8 ITEPA 2003 puts determination on intermediary; Chapter 10 (off-payroll, from April 2017 public / April 2021 private medium-large) puts determination on end client.

How do I know if I'm inside or outside IR35?

The client (under Chapter 10) or your PSC (under Chapter 8) determines status based on factual tests: right of substitution, control over how/when/where work is done, mutuality of obligation between engagements. Tools like HMRC's CEST give indicative results. Professional review services (IR35 Shield, Qdos, Markel) provide detailed determinations. Inside IR35 means PAYE-style deductions apply; outside means normal PSC operation.

What's a Status Determination Statement?

Under Chapter 10 off-payroll rules, the end client must issue an SDS for each engagement before it starts, stating inside or outside IR35 with reasons. The SDS goes to the contractor and the immediate party in the supply chain. The contractor can challenge the SDS through the status disagreement process; the client must respond within 45 days with confirmed or revised determination.

Can I challenge an inside IR35 determination?

Yes through the status disagreement process under Chapter 10. The contractor writes to the client raising specific points of disagreement. The client must respond within 45 days with the determination (confirmed or revised) and the reasons. The contractor cannot force a change but the response shows what was considered. Persistent disagreement may lead to engagement termination or, in extreme cases, court action.

Are small clients exempt from IR35 changes?

Small private-sector clients (meeting 2 of 3 criteria: turnover GBP 10.2m or less, balance sheet GBP 5.1m or less, 50 or fewer employees) are exempt from Chapter 10 off-payroll rules. For these clients, Chapter 8 still applies and the contractor's PSC retains the determination obligation. The status tests are the same; the practical risk shifts from client to contractor.

How much does inside IR35 cost?

Materially more in tax. A GBP 500/day contractor outside IR35 through PSC typically takes home GBP 75-85k a year. The same rate inside IR35 through umbrella typically delivers GBP 60-65k. Difference around GBP 15-20k per year. Contractors with inside-IR35 status may negotiate higher rates (typically 15-20% uplift) to compensate; success depends on skills market and client willingness.