The disallowed cost audit: how to defend Defined Cost under NEC4 Options C to F

The audit letter lands at month nine. The Project Manager wants backup for £180,000 of the Defined Cost you have been claiming since spring. Timesheets, delivery tickets, the reason a gang stayed on after a section reached Completion. You have most of it. The problem is where it lives: two spreadsheets, a shared drive, and a supervisor who left in June. Every cost you cannot substantiate is a candidate for disallowance, and on a target contract every pound disallowed comes straight out of your share of the gain.

This is the quiet erosion nobody warns you about on Options C, D, E and F. Not a headline dispute. A slow audit that trims your cost line after the money has been spent. This guide covers how the disallowed cost mechanism actually works, what auditors disallow first, and the records defence that keeps your Defined Cost intact.

What Disallowed Cost is, and why Options C to F change the game

Disallowed Cost is cost the Project Manager decides does not qualify as Defined Cost, defined at clause 11.2(26) in the NEC4 Engineering and Construction Contract. It is not a penalty. It is a filter. Cost you actually spent, removed from the total the contract will reimburse or count towards the target.

On a lump sum contract, Options A and B, your Defined Cost only surfaces when you price a compensation event. Everywhere else the Prices are fixed and the cost of the work is your risk. Disallowed Cost barely touches you.

Options C, D, E and F are different. Here the client pays Defined Cost plus Fee, and on the target options C and D that running cost is measured against the target to settle the pain and gain at the end. Every line of Defined Cost is live and auditable for the whole job. So Disallowed Cost stops being an occasional CE argument and becomes a standing exposure on your largest cost contracts.

The definition lists what the PM can strip out. It reads short. It bites hard.

What auditors disallow first

Clause 11.2(26) gives the Project Manager specific grounds. Three of them account for most of what gets removed on a real audit.

Cost with no records to support it. The definition explicitly disallows cost that the contract requires the Contractor to keep records of, where those records do not exist. This is the big one. It is not that the cost was wrong. It is that you cannot prove it, and unproven cost is disallowable by definition, not by argument.

Correcting Defects after Completion. Cost of putting right a Defect after Completion is disallowed. So is the cost of correcting Defects before Completion where the correction was needed because you did not follow the Works Information. The audit looks for rework, snagging gangs, and returns to finished sections, then asks whether that cost belongs to the client or to you.

Resources not used to Provide the Works. Plant sitting idle, labour booked to the job but working elsewhere, materials over-ordered and never fixed. The definition disallows resources not used, or not used to Provide the Works and retained after the PM instructed their removal. Idle time you cannot explain is idle time you cannot recover.

The list also reaches preparation and conduct of adjudication, and payments to Subcontractors for work that is itself Disallowed Cost. But the pattern across all of them is the same. The PM is not attacking your commercial judgement. They are checking whether each cost qualifies, and the burden of showing it qualifies sits with you.

The audit is an evidence test, not a cost test

Here is what changes how you should prepare. A Defined Cost audit is not really about whether the money was reasonable. It is about whether the records exist.

Picture the review. The PM's cost assurance team pulls a sample: forty cost entries across three months. For each one they want the source document and the link to the work. A labour entry needs the timesheet, the shift, and the activity it served. A plant entry needs the on-hire and off-hire dates and evidence it was working. A subcontractor payment needs the application, the assessment, and proof the work was to the Works Information.

Where the chain is complete, the cost stands. Where it breaks, the cost is queried, and a queried cost with no answer becomes a disallowed cost. On a target contract that disallowance does double damage. You lose the reimbursement, and the target stays where it was, so the gap between your cost and the target widens and your share of any gain shrinks.

The QS who treats records as an end-of-job chore walks into that audit exposed. The one who treats them as they happen walks in with the sample already answered.

The records defence, built as you go

The defence against disallowance is not a better argument at month nine. It is contemporaneous records built while the work happens.

A contemporaneous record is created at the time, by the people doing the work, and it ties cost to activity. Timesheets logged daily against the right cost code, not reconstructed from memory. Plant on and off hire captured the day it moves. Diary entries that explain why a gang stayed on a section, so idle time reads as instructed standing time rather than unexplained cost. Photographs timestamped on the day a condition changed. Delivery and materials records matched to what was actually fixed into the works.

Do that and the audit inverts. Instead of hunting for backup after a query, you hand over the record that answers it before the query is even raised. The PM's assurance team moves faster, disallows less, and trusts the rest of your cost line more because the sample held up. On a target contract that trust is worth real money at final account.

The gap between the two positions is not effort. Both teams did the work and spent the cost. One captured the proof as it happened. The other tried to assemble it after the supervisor left.

This is exactly the work the QS AI Agent is built to carry. Gather reads every site diary entry as it lands, links it to the programme activity and the cost code, and flags the entries that will matter in a Defined Cost audit long before the audit letter arrives. When the PM samples forty cost lines, the substantiation is already assembled against each one, not scattered across spreadsheets and an inbox. If you want to see a Defined Cost audit answered from live site data, book a 15-minute demo.

Where a compensation event and a disallowed cost collide

One trap catches QS teams on every target contract. Cost that flows from a compensation event still has to survive the Disallowed Cost test.

When you price a compensation event, you assess the change to the Prices as the effect on Defined Cost plus Fee. If that Defined Cost quietly carries idle plant, rework, or resources with no records, it is disallowable whether or not it sits inside a CE. The CE gives you the entitlement. It does not launder the cost. A quotation that carries disallowed cost is a resubmission waiting to happen, and a settled CE that later turns out to include disallowed cost is money the audit can claw back.

So the two mechanisms are not separate workstreams. The same record that proves your CE quotation is the record that defends your Defined Cost at audit. Build it once, use it twice.

Common mistakes that hand the auditor an easy disallowance

• Treating record-keeping as a final account task instead of a daily one, so the sample cannot be answered.
• Booking labour and plant to the job without the timesheet or hire record the contract requires you to keep.
• Leaving idle or standing time unexplained in the diary, so it reads as a resource not used to Provide the Works.
• Carrying rework and post-Completion Defect correction in the cost line without separating it out.
• Assuming a Subcontractor's cost is safe because you paid it, when the underlying work is itself Disallowed Cost.
• Pricing a compensation event on Defined Cost that would not survive an audit in the first place.

A worked example

Take a £60m target cost contract under Option C, on a two-year highways upgrade. The Contractor's Defined Cost runs at roughly £2.4m a month. At month nine the PM commissions a cost assurance review and samples £180,000 of claimed cost across the drainage and earthworks packages.

Run it the usual way. Records live in three spreadsheets and a shared drive. Of the £180,000 sampled, the team cannot cleanly substantiate £52,000: a fortnight of an excavator that shows on the cost report but not on any off-hire record, a snagging gang whose time is not separated from productive work, and materials over-ordered and never fixed. The PM disallows £41,000 of it. On a 50/50 gain share, that disallowance costs the Contractor £41,000 of reimbursement now and around £20,500 of eroded gain at settlement. Same spend, worse outcome, entirely because of missing proof.

Now run it record-referenced. The excavator hours come from off-hire tickets logged the day the plant left. The snagging time is booked to a separate cost code and the diary explains it. The materials record is matched to what went into the works, so the over-order is identified and credited before the auditor finds it. The sample is answered in a morning. The PM disallows nothing from the £180,000 and signs off the rest of the cost line faster because the sample held. Same work. Same cost. £41,000 kept and the gain share protected, because the records were there when they were needed.

Frequently asked questions

What is Disallowed Cost under NEC4?

Disallowed Cost is cost the Project Manager decides does not qualify as Defined Cost, defined at clause 11.2(26) of the NEC4 Engineering and Construction Contract. It includes cost that is not justified by the records the contract requires the Contractor to keep, the cost of correcting Defects after Completion, and resources not used to Provide the Works. Disallowed Cost is removed from the amount the client reimburses and, on target contracts, from the cost measured against the target.

Which NEC4 Options does Disallowed Cost apply to?

Disallowed Cost matters most on Options C, D, E and F, where the client pays Defined Cost plus Fee and every cost line is auditable throughout the job. On the target options C and D it directly affects the pain and gain share. On Options A and B, which are lump sum, Defined Cost only appears when pricing a compensation event, so the exposure is far smaller.

Why does missing records mean disallowed cost?

Clause 11.2(26) disallows cost that is not justified by the records the contract requires the Contractor to keep. The test is not whether the cost was reasonable but whether you can prove it qualifies. If the record does not exist, the cost is disallowable by definition, so the audit removes it regardless of whether the money was genuinely spent.

How do I defend Defined Cost in a PM audit?

Keep contemporaneous records that tie every cost to the work it served. Log timesheets daily against the right cost code, capture plant on and off hire as it moves, explain idle or standing time in the site diary, and match materials to what was fixed into the works. When the Project Manager samples cost lines for assurance, a complete record answers the query before it becomes a disallowance.

Can a compensation event include disallowed cost?

Yes, and it is a common trap. A compensation event gives you entitlement, but the Defined Cost inside your quotation still has to survive the Disallowed Cost test. If the CE cost carries idle plant, rework, or resources with no records, the Project Manager can disallow it whether or not it sits inside a compensation event. The record that proves the CE is the record that defends the cost at audit.

The bottom line

On Options C to F, Disallowed Cost is not an occasional argument. It is a standing test that runs for the life of the contract, and it turns on one question: can you prove each cost qualifies. Get the records right as the work happens, tie every cost to an activity, explain the idle time, separate the rework, and the audit becomes a formality. Get them wrong and the disallowance is quiet, cumulative, and paid for out of your share of the gain.

The teams that keep their Defined Cost do not audit better. They keep better records, and they keep them the day the cost is incurred.

Ready to walk into a Defined Cost audit with the answers already assembled? Gather reads every site diary entry, links it to your programme and cost codes, and has the substantiation ready before the audit letter arrives. Book a 15-minute demo.

Source: Gather Insights, the AI-powered site diary and commercial record management platform for UK construction.