Ambiguities and Illegal Requirements

NEC clause 17 is what you turn to when the contract documents contradict themselves, or when the Scope asks for something illegal or impossible. In NEC4 ECC, ambiguities and inconsistencies are at 17.1 and illegal or impossible requirements at 17.2. In NEC3 ECC, the same topics are at clauses 17 and 18.

The address on this page mentions clause 16 as well, because the numbering catches people out. Clause 16 in NEC4 is the Contractor's proposals to change the Scope, and clause 16 in NEC3 is early warning, so neither is about ambiguity. Always check the edition. This page is part of the NEC definitions glossary.

Where to find each rule

Topic	NEC4 ECC	NEC3 ECC
Early warning	Clause 15	Clause 16
Contractor's proposals to reduce cost	Clause 16	No core clause
Ambiguity or inconsistency	17.1	17.1
Illegal or impossible requirement	17.2	18.1

The procedure for an ambiguity

1. Notify. Whichever of the Project Manager and the Contractor spots it first tells the other straight away. This duty applies to both sides, and it applies to all documents forming part of the contract.
2. Resolve. The Project Manager says how the ambiguity or inconsistency is to be resolved.
3. Assess. If that means changing the Scope, it's a compensation event. It's assessed as if the Prices and dates reflected the interpretation most favourable to the Party that didn't provide the Scope (63.10 in NEC4, 63.8 in NEC3).

A worked example

A fictional warehouse project, with illustrative figures. A floor drawing shows a 150mm slab, while the specification gives 200mm. Both come from the Client. The Contractor priced 150mm and spots the mismatch while ordering concrete.

Step	What happens
Notification	Contractor notifies under 17.1 that day
Decision	Project Manager says the slab is to be 200mm, which is what the Client needs
Assessment	Most favourable reading for the Contractor is 150mm. The change adds, for example, £60,000
Outcome	The Prices rise by £60,000 plus the resulting Fee

If the Project Manager had decided on 150mm, there'd be no increase at all, because that's the reading the Contractor priced. It's a good reason for the Client to check the documents before tender.

The rule looks at who provided the Scope. Where the Contractor wrote part of it, for instance through its own design, the favourable reading goes the Client's way.

Illegal or impossible requirements

	NEC4 ECC (17.2)	NEC3 ECC (18.1)
Who notifies?	Project Manager or Contractor	The Contractor
Trigger	As soon as either becomes aware	As soon as the Contractor considers
Result	If the requirement really is illegal or impossible, the Project Manager instructs a change to the Scope	If the Project Manager agrees, it instructs a change to the Works Information

Examples might be a Scope that requires piling at a time the planning permission forbids, or a tolerance beyond what any process can reach. A difficult requirement isn't an impossible one. Nor is it impossible just because it's more expensive than priced. If the Project Manager instructs a change, that's usually a compensation event under 60.1(1).

What people get wrong

• Staying quiet and pricing the cheaper reading. The favourable-reading rule helps the Contractor, but keeping quiet about an error you've noticed doesn't sit well with clause 10 or the notification duty. And a late notice might draw a response that the matter could've been warned about under the missed early warning rule.
• Treating every drawing question as an ambiguity. A technical query isn't the same as a conflict. Check that the documents really do contradict each other, or that a statement really can be read two ways.
• Using an order of precedence. The standard clause doesn't rank documents. If your Scope says the specification overrides the drawings, a Z clause or the document itself may change things, and that may cause problems of its own under this clause. Check how they sit together.
• Going straight to building. Without a Project Manager's decision, the Contractor takes a risk on which version it chooses.
• Applying the rule to Site Information. Conflicts within the Site Information are dealt with under 60.3, as a different rule, when judging physical conditions.

Related guides

• NEC Definitions: A Glossary of Key Terms
• Period for Reply
• Clause 14: Project Manager and Supervisor
• Clause 11: Identified and Defined Terms

This guide is general information in our own words, not legal advice. Your contract and its Contract Data always govern. NEC, NEC3 and NEC4 are trade marks of Thomas Telford Ltd; Contract Status is not affiliated with or endorsed by NEC.