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Disallowed cost is one of those NEC4 ECC ideas that sounds simple until it hits your payment certificate. When you are working under cost reimbursable contracts like NEC4 Option C, Option D or Option E, one challenged line item can turn “defined cost” into a cost you carry yourself. We unpack what disallowed cost really means, why it exists, and how it acts as a commercial control to keep cost-based payment honest without turning the contract into a blame game.

We talk through the real mechanics of payment assessment under clause 50: the Contractor’s application, the Project Manager’s duty to assess the amount due, and how disallowed cost fits into the calculation because defined cost is the Schedule of Cost Components less disallowed cost. From there we go deep on the most common trigger we see on projects: costs not justified by accounts and records. Open-book accounting only works when the evidence is clear, consistent, and mapped to the cost components, with enough narrative to explain context, decisions, and timing.

The conversation also covers supply chain and process traps: subcontractor costs that should not have been paid, procurement or acceptance procedures stated in the Scope, missed early warnings, and dispute notifications that can make otherwise valid expenditure unrecoverable. We explore defects after Completion and why the Scope definition of Completion is commercially critical, plus “reasonable wastage” and “reasonable utilisation” when you are deciding whether to keep people and plant on site in the real world.

If you manage NEC4 cost options, this will sharpen how you set expectations pre-contract, run collaborative commercial conversations during delivery, and keep records that stand up to audit. Subscribe, share it with your Project Manager or QS, leave a review, and tell us: which disallowed cost category causes the biggest arguments on your jobs?

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