IR35 is the off-payroll working legislation that determines whether a contractor working through their own limited company should be taxed as a genuine business (outside IR35, with normal corporation tax and dividends) or as a deemed employee of the end-client (inside IR35, with PAYE-style tax deduction at source). Since April 2021, in the private sector, the responsibility for determining IR35 status has moved from the contractor to the end-client (or fee-payer in the supply chain), with the financial risk of getting it wrong sitting on whoever is responsible.
For a UK limited company contractor, the IR35 determination is the single largest commercial issue affecting take-home pay. Outside IR35 leaves the standard contractor economics intact: corporation tax on profits, salary at the personal allowance, dividends from post-tax profit, pension contributions through the company. Inside IR35 collapses this: revenue is taxed as deemed employment income with PAYE and NI deducted before it ever reaches the company, eliminating most of the structural advantage of operating as a limited company at all.
Blanket determinations are everywhere, and challengeable
Many large end-clients defaulted to blanket "inside IR35" determinations across their entire contractor population from April 2021 to avoid the administrative burden of case-by-case assessment. A blanket determination is not a defensible Status Determination Statement (SDS) under the rules; the contractor has the right to appeal. Many appeals succeed where the underlying contract and working practices genuinely support an outside-IR35 position.
How the IR35 test actually works
IR35 is determined by considering whether the contractor would be an employee of the end-client if the limited company intermediary did not exist. The test is fact-specific and considers multiple factors:
- 1Mutuality of obligation (MOO): is the engager required to provide work and the contractor required to accept it? An employment hallmark.
- 2Right of substitution: can the contractor genuinely send a substitute to do the work? A contractor hallmark.
- 3Control: who decides what is done, when, where, how? More client control suggests employment.
- 4Equipment: who provides the tools and materials? Self-provided suggests contractor.
- 5Financial risk: does the contractor bear cost overruns, pay for rework, lose money on poor performance? Suggests contractor.
- 6Integration: is the contractor part of the engager's organisation or genuinely external?
- 7Length and exclusivity: long, exclusive engagements look more like employment.
The financial impact: inside vs outside IR35
For a contractor on a £600/day rate working 220 days per year (£132,000 gross):
| Position | Take-home (approximate) | Effective rate |
|---|---|---|
| Outside IR35 via Ltd company (salary + dividends) | £90,000 to £95,000 | ~30% effective tax |
| Inside IR35 via Ltd company (PAYE deduction) | £75,000 to £80,000 | ~40% effective tax |
| [PAYE umbrella](/guide/limited-company-vs-umbrella-strategic-choice/) (employed by umbrella) | £72,000 to £77,000 | ~42% effective tax (with employer NI passed through) |
| Direct PAYE (employed by client) | £75,000 to £80,000 | Standard employment |
The £15,000-20,000 annual gap between outside and inside is the IR35 stake. Over a 10-year contracting career, the cumulative difference is £150,000-200,000.
The Status Determination Statement and your right to appeal
Since April 2021, the end-client must issue a Status Determination Statement (SDS) for each contractor engagement. The SDS must:
- State whether the engagement is inside or outside IR35.
- Provide reasons for the determination, referring to the specific facts of the engagement.
- Be issued before the contractor starts work or as soon as is reasonably practicable.
- Be passed down the supply chain to the agency and ultimately to the contractor.
The contractor has 45 days to appeal the SDS. The end-client must consider the appeal and issue a written response within 45 days. If the appeal is rejected and the contractor disagrees, the contractor can escalate to HMRC or the Tax Tribunal in due course. Many appeals succeed at the first stage where the SDS was a blanket determination or based on incomplete information.
Why the HMRC CEST tool is flawed
HMRC publishes the Check Employment Status for Tax (CEST) tool as the official IR35 determination engine. Many end-clients use CEST as their sole determination mechanism. The tool has well-documented limitations:
- 1Mutuality of Obligation: CEST does not specifically test for MOO, despite tribunal cases showing MOO is the most discriminating factor.
- 2Substitution: CEST asks about substitution but treats hypothetical rights too generously, often producing an outside result on a substitution clause that has never been tested.
- 3Tribunal misalignment: tribunal decisions (Atholl House, Kickabout Productions, Lineker, others) have consistently produced different outcomes from CEST on the same fact patterns.
- 4No documentation of reasoning: CEST produces a determination but limited reasoning, making it hard to defend on appeal.
For high-value engagements, specialist review supplements CEST. End-clients increasingly use both: CEST for the routine baseline plus specialist review for the borderline cases.
Fee-payer responsibilities
Where a contract chain runs end-client → agency → contractor PSC, the agency is the "fee-payer" responsible for deducting PAYE/NI when the engagement is inside IR35. If the end-client fails to issue a valid SDS, the fee-payer falls back as the default determination party. HMRC can pursue any of: end-client, agency, or contractor depending on where the procedural failure sits. Each party in the chain has its own compliance interest.
How to defend an outside IR35 position
Where an engagement genuinely is outside IR35, the contractor strengthens the position through:
- A written contract with a substitution clause that has been used (where realistically possible).
- Documented evidence of project-based deliverables rather than time-based deliverables.
- Evidence of multiple concurrent engagements (the contractor running a portfolio).
- Equipment provided by the contractor (own laptop, software licences, professional tools).
- Operational autonomy: the contractor decides how the work is done, not the client.
- Financial risk evidence: rework done at the contractor's expense, fixed-price elements, contractual penalties.
- Lack of integration with the client's organisation (no manager line, no internal performance reviews, no employee benefits).
These factors do not eliminate IR35 risk on their own, but the cumulative effect is what HMRC and tribunals weigh.
IR35 status under review or appealed?
A contractor accountant specialising in IR35 will review your SDS, contract, and working practices, and prepare the appeal where the position supports it. Free initial assessment.