IR35 in 2026 — what changed since the CEST update

HMRC's Check Employment Status for Tax (CEST) tool was significantly updated in March 2026. The update is the largest revision since the original tool launched in 2017 and changes the IR35 status determination for a meaningful subset of UK contractors.

What changed in March 2026

The CEST update made four substantive logic changes:

1. Mutuality of obligation re-weighted. The previous version under-weighted MoO; the new version applies more emphasis. Contractors who relied on "I can refuse work" as their outside-IR35 anchor need to verify their working pattern actually demonstrates that refusal.
2. Substitution rights stricter. CEST 2026 now requires evidence of substitution that's genuinely exercisable in practice — not just a clause in the contract. Contracts that allow substitution "with client consent" are now treated as substitution-restricted.
3. Equipment-and-control bundled. The previous separate equipment and control questions are now combined into a single composite question with higher weighting.
4. Length-of-engagement triggers. Engagements over 24 months now trigger additional review questions about exclusivity and integration.

Who's affected most

From the engagements we've reviewed since the March update, three contractor profiles have flipped from outside to inside:

• Long-engagement IT contractors who have been with one client for 18+ months. The new length-of-engagement triggers plus the substitution scrutiny moves many into inside status.
• Fractional senior leaders who serve multiple clients but spend the majority of their time with one. The "majority client" pattern is now treated more like employment.
• Specialist consultants embedded in client teams attending standups, using client equipment, reporting to a client manager. The bundled equipment-and-control question catches this pattern more aggressively.

What doesn't change

The actual IR35 legislation hasn't changed — CEST is HMRC's assessment tool, not the law itself. Genuine self-employed engagements remain outside IR35. The change is that more borderline cases now lean inside under the tool's assessment.

It's still worth running CEST: it's HMRC's own tool, and a "Determined as Outside" CEST result is reasonable evidence (though not conclusive) of correct treatment. A "Determined as Inside" or "Unable to Determine" result is a flag to seek specific advice.

Practical responses

For contractors and clients facing the changed determinations:

• Re-run CEST on every active engagement, especially anything 12+ months old. Document the result.
• Review contracts for substitution clauses — make sure substitution is genuinely exercisable without client consent.
• Document the actual working pattern — refused work, multiple clients, project-shaped engagement boundaries. Behavioural evidence is what HMRC reviews if challenged.
• Consider umbrella for borderline cases. Yes it costs ~5% in fees, but it eliminates the IR35 risk entirely for the contractor.

For clients (medium / large UK companies)

You're still responsible for status determination under the off-payroll working rules (IR35 in the public sector since 2017, private sector since 2021). The CEST update means previous SDS (Status Determination Statements) may need re-issuing. Specifically:

• Re-issue SDS for any engagement where the working pattern has materially changed since the original determination
• For long-running engagements (18+ months), re-run CEST and re-issue if status changes
• The penalty for getting it wrong is the income tax + NIC the contractor would have paid, plus interest and penalties

The CEST update isn't a legislative change. It's HMRC's tool getting stricter, in line with HMRC's broader enforcement posture in 2026. Behavioural evidence is what matters; CEST is the proxy.

Where Incorpwise fits

For contractors operating through their own Ltd, Incorpwise handles the formation, payroll, and statutory filings. We don't provide IR35 status advice (that's specifically a regulated activity in some contexts), but we can refer to two specialist firms we work with on borderline cases. If you've just had a status flip you weren't expecting, that's the right call to make.