The client-led disagreement process is the statutory route under Chapter 10 by which a worker (or the fee-payer) can challenge a Status Determination Statement (SDS) they believe is wrong. It applies where a medium or large end client has determined an engagement to be inside or outside IR35 and the worker disagrees.
The worker makes representations to the client setting out why they think the determination is wrong. The client must then consider those representations and respond within 45 days, either confirming the original SDS with reasons or issuing a new SDS reaching a different conclusion. The process is "client-led" because the client makes the final decision, but it is obliged to respond and to give reasons.
The disagreement process is the contractor's main formal tool against a blanket determination, where a client has applied an "inside" label across a category of roles without individual assessment. Such a determination is likely to be a failure of reasonable care, and the disagreement process gives the contractor a route to demand an individual assessment. The statutory hook is ITEPA 2003 s.61T, with HMRC guidance at ESM10015.