IR35 ATS: How to Run a Compliant UK Contractor Pipeline in 2026

IR35 (off-payroll working rules) holds UK end-clients responsible for determining whether contractors are inside or outside IR35 since April 2021. HMRC enforcement is now active and back-tax assessments can be substantial per misclassified contractor, covering unpaid income tax, employee and employer NI, plus interest and penalties. UK SMBs hiring contractors need a clear ATS pipeline with documented Status Determination Statements (SDS), routing logic, and audit trail.

What IR35 actually requires from your ATS

• SDS at the role level: Status Determination Statement determining inside or outside IR35, issued before contractor starts
• Reasoning recorded: the factors considered (mutuality of obligation, control, substitution rights, etc.)
• Issued to contractor and any agency in the chain
• Audit trail: HMRC expects to see who made the determination and when
• Status disagreement process: contractor can disagree; you must respond within 45 calendar days

How Treegarden's IR35 pipeline works

The contractor pipeline includes IR35 stages distinct from PAYE roles:

1. Role classification at requisition: hiring manager flags 'IR35 in scope' and answers HMRC CEST tool questions inline
2. SDS generation: Treegarden produces an SDS PDF based on CEST answers, signed by the determination owner
3. Inside vs outside routing: inside-IR35 candidates route to umbrella company workflow; outside-IR35 to Limited Company onboarding
4. Rate uplift calculation: for inside-IR35 roles, optional rate uplift to compensate for additional tax (roughly 22-25% is needed to break even on take-home pay, though agencies in practice often offer far less)
5. SDS distribution: automatic email to contractor + agency with PDF copy
6. Disagreement handling: contractor can submit disagreement via portal; HR has 45-day SLA
7. Audit log: HMRC-grade evidence retention

Inside vs outside IR35: what changes for the contractor

Inside IR35: contractor is treated as employee for tax. End-client (or fee-payer) deducts PAYE and NI. Contractor receives net pay; can use umbrella company. Loses ability to claim business expenses, dividend efficiency, etc.

Outside IR35: contractor operates as genuinely self-employed via Limited Company. Pays own tax, can claim expenses, draws dividends. End-client pays gross.

Misclassification is what HMRC enforces. Calling someone outside IR35 when the working pattern looks like employment (set hours, supervised, no substitution) is the most common failure mode.

The three status tests that decide inside or outside

There is no single statutory test for IR35. Status comes from decades of employment case law, distilled into three factors that a tribunal weighs together. Your SDS reasoning, and any defence you mount in an HMRC enquiry, lives or dies on how honestly you assess these three against the real working pattern, not the wording of the contract.

• Control. Who decides what the contractor does, and how, when, and where they do it? This is usually the heaviest factor. Where the client controls how, when, and where the work happens, it is very hard to argue outside IR35 even with a substitution clause in place. Genuine outside-IR35 engagements look like a delivery brief with a deadline, not a managed seat in a team.
• Right of substitution. Could the contractor send a competent replacement that the client must accept, and would they actually do it? A real, unfettered right is strong evidence of self-employment. A clause everyone knows will never be used carries no weight; HMRC and tribunals look at the reality, not the paper.
• Mutuality of obligation (MOO). The most misunderstood test. In September 2024 the Supreme Court's ruling in the PGMOL football referees case ([2024] UKSC 29) confirmed that mutuality can apply within each discrete contract, the "wage-work bargain" where one party agrees to do work and the other agrees to pay for it. Crucially, there does not need to be an overarching obligation to offer or accept future work for MOO to be present. The practical upshot for 2026: do not lean on a "no obligation to provide future work" argument as your headline reason for an outside determination. It is weaker after PGMOL than many older guides suggest.

Other factors (financial risk, providing your own equipment, being "part and parcel" of the organisation, and exclusivity) tip borderline cases. The point for an ATS is that each of these should be a recorded answer attached to the requisition, not a verbal judgement that vanishes when the determination owner leaves.

CEST and reasonable care: what actually protects you

HMRC's free Check Employment Status for Tax (CEST) tool gives HMRC's view of a worker's status from the answers you supply, and HMRC will "stand by all results given by the tool, as long as the information you give remains accurate and is in accordance with our guidance." The tool was refreshed on 30 April 2025 with simpler language and clearer routing. That standing-behind guarantee is the reason to use it, but two limits matter:

• Accuracy is the whole game. The result only protects you if the inputs reflect the real engagement. Garbage in, no protection out. Capture the actual working pattern, then store the exact answers and the result with the candidate record.
• CEST alone is not "reasonable care". Reasonable care is a separate legal obligation. HMRC expects the person making the determination to understand the role, understand employment status, and have proper support, not just click through CEST. On material engagements, pair the CEST output with a documented rationale. Relying on the tool in isolation, especially where it returns "unable to determine", is widely treated as insufficient.

Inside an ATS this translates to a simple rule: the CEST questionnaire, the answers, the result, and a short written rationale all attach to the requisition before the contractor starts. That bundle, plus the SDS, is what you hand HMRC if an enquiry lands.

The 2024 offset rules changed the cost of getting it wrong

For engagements assessed after an enquiry, the penalty picture shifted in the contractor-engager's favour on 6 April 2024. Before then, a reclassified engagement could be effectively taxed twice: the contractor's PSC had already paid corporation tax, income tax, and dividend tax, yet HMRC then assessed the deemed employer for the full PAYE and NI as if nothing had been paid. The set-off (offset) rules that took effect on 6 April 2024 let HMRC offset tax and NIC already paid by the worker or their intermediary against the deemed employer's liability, removing most of that double-counting.

This does not make misclassification cheap. The deemed employer is still liable for employer NI (which the offset does not refund), plus interest, plus penalties of up to 100% of the unpaid tax for deliberate non-compliance and lower amounts for careless errors. But it does mean the headline back-tax figure on a reclassified contract is now the net shortfall after credits, not the gross PAYE bill. The compliance lesson is unchanged: a defensible SDS, a CEST result with accurate inputs, and a clean audit trail are far cheaper than an enquiry.

Common IR35 mistakes UK SMBs make

• Letting contractors self-determine when the end-client is medium or large (the client must determine since 6 April 2021; only genuinely small clients are exempt)
• Blanket inside-IR35 determinations without role-by-role analysis, which HMRC treats as a failure of reasonable care
• Not issuing the SDS before the contractor starts
• Not retaining the CEST output, the answers given, and a written rationale as evidence
• Treating contractors like employees in practice (set hours, line management, no real substitution) while claiming outside-IR35
• Leaning on a "no obligation to provide future work" argument as the main reason for an outside determination, which is weaker after the 2024 PGMOL Supreme Court ruling on mutuality of obligation

When the small company exemption applies

Small private-sector companies are exempt from IR35 determination duties. When the end-client is small, responsibility passes back to the contractor's PSC (Personal Service Company), which determines its own status under the original Chapter 8 rules. Most UK SMBs that hire contractors qualify as small, so this exemption is the single most important thing to check before you build any IR35 routing into your ATS.

A company is "small" if it meets at least two of three conditions in the relevant financial year. The Companies Act size thresholds were raised by statutory instrument and now read: annual turnover not more than £15M, balance sheet total not more than £7.5M, and no more than 50 employees (up from £10.2M and £5.1M previously, with the employee figure unchanged).

Watch the timing. The raised thresholds apply to financial years beginning on or after 6 April 2025, but a transitional two-year rule governs when a company actually changes status. Because a company must satisfy the size test across two consecutive financial years, and the earliest qualifying accounts only get filed in 2027, the new thresholds first bite for off-payroll purposes in roughly the 2027 to 2028 tax year, as specialist analysis of the transitional provisions confirms. In practice this means a growing SMB does not suddenly inherit IR35 duties the moment it crosses a threshold; the duty follows once two consecutive qualifying years are on file. Re-check your size status at each year end, and do not assume the exemption is permanent once headcount and turnover climb.

Frequently asked questions