Unforeseen Physical Conditions Under NZS 3910:2023 — Is It a Variation? (NZ)

Quick answer: Under NZS 3910:2023, unforeseen physical conditions are covered by clause 9.5.4 and are treated as a Variation, valued under clause 9.3. The test is objective: the condition must be one an experienced contractor could not reasonably have foreseen at the time of tendering. Late or missing notice under clause 5.21 can reduce the entitlement.

You excavate for foundations and hit rock, a buried fuel tank, contaminated fill or a live service that no drawing showed. Work stops. The immediate question on a New Zealand site is always the same: is this the Principal's cost, or the Contractor's problem? Under NZS 3910:2023 the contract has a specific mechanism for it, and whether you are the contractor claiming or the principal's representative assessing, the entitlement turns on three tests, not on who shouts loudest.

Clause 9.5.4 — unforeseen physical conditions are "treated as a Variation"

NZS 3910:2023 groups a set of compensable events under clause 9.5 and applies a single formula to them — each "shall be treated as a Variation." Clause 9.5.4 is the unforeseen-physical-conditions limb. That drafting is deliberate: rather than create a separate loss-and-expense regime, the 2023 edition routes physical-condition surprises into the ordinary Variation machinery. The consequence matters. Once a condition qualifies under 9.5.4, it is priced under the Variation valuation rules in clause 9.3, and it can also support an extension of time under the contract's time provisions where the condition delays the works.

The scope of 9.5.4 covers physical conditions and artificial obstructions encountered during the works — subsurface ground, water, existing structures, buried services and the like — that differ materially from what the Contract documents and a reasonable site inspection would have led the Contractor to expect. Weather itself is handled separately; 9.5.4 is about the ground and what is in it.

The experienced-contractor test — an objective bar

The entitlement is not triggered simply because the Contractor did not expect the condition. The test is objective. The condition must be one that could not reasonably have been foreseen by an experienced contractor at the time of tendering. That has three practical edges:

• It is measured at tender, not at excavation. Foreseeability is judged against the information available when the price was fixed — the drawings, the specification, any geotechnical or site-investigation reports issued with the tender, and a competent site walkover.

• Provided information sets the baseline. If a geotechnical report flagged variable fill or a high water table, conditions consistent with that report are foreseen — even if the specific severity was not spelled out. Principals who issue thorough site-investigation data narrow the Contractor's 9.5.4 window; those who issue little widen it.

• Experience is imputed. The benchmark is a contractor experienced in that type of work in that locality, not the actual crew on site. Local knowledge — reclaimed land in parts of Auckland and Christchurch, volcanic rock on the Auckland isthmus, liquefiable soils in the eastern suburbs — is assumed.

This is where most disputes actually live. The condition is real and documented; the fight is whether an experienced contractor should have priced for it.

Sitting on a variation, claim or ground-condition surprise you are not sure about? Send it with the contract — a written, code-cited answer back within 24 hours.

Clause 5.21 — the notification trap that quietly discounts claims

The 2023 edition added an advance-notification duty at clause 5.21: the Contractor (and the Engineer or Contract Administrator) must give early notice of any matter likely to affect Cost or the completion date. This is the clause that catches otherwise-valid claims. Where the Contractor fails to notify a matter it reasonably ought to have notified, the contract allows the entitlement to be discounted to the extent the late notice caused avoidable cost or lost the other party a chance to mitigate.

For an unforeseen-condition claim the sequence is unforgiving: stop, record the condition (photographs, survey, dates), notify promptly under 5.21, and keep contemporaneous cost records before you backfill or build over the evidence. A strong 9.5.4 entitlement can be trimmed at valuation purely because notice was late and the other side could have designed around the condition more cheaply.

How the Variation is valued — clause 9.3

Because a 9.5.4 event is treated as a Variation, it is valued through clause 9.3's hierarchy, not by open-book negotiation:

1. Schedule rates where the work is the same character and executed under similar conditions;

2. Rates derived from the schedule where the work is similar but the conditions differ;

3. Fair and reasonable rates where no comparable schedule rate exists; and

4. Cost-based valuation / daywork as the fallback for genuinely one-off work.

On-costs — overhead and profit — are added using the percentages set in the Contract, not invented after the fact. For rock, dewatering, contaminated-spoil disposal or redesigned footings there is rarely a clean schedule rate, so most 9.5.4 valuations land in the fair-and-reasonable or daywork tiers. That makes the quality of the Contractor's cost records, and the assessor's reasoning, the whole game.

Check the Special Conditions before you rely on 9.5.4

The single most important step is also the one most often skipped. Clause 9.5.4 is a default that the Special Conditions can amend or delete. Principal-drafted contracts — councils, larger developers, some infrastructure clients — frequently narrow the physical-conditions risk or transfer it to the Contractor outright, sometimes by deeming certain conditions foreseeable or by requiring the Contractor to have satisfied itself as to the ground. Read the Special Conditions and the Schedule 1 particulars first. If 9.5.4 has been modified, the standard-form position you remember may no longer apply to your contract.

A related point on the 2023 edition: the traditional single "Engineer" role can be split into a Contract Administrator who runs the contract and an independent reviewer who certifies. Knowing who values your 9.5.4 claim — and whether they are contractually independent — shapes how you present it.

A valid Variation still has to meet the Building Code

Entitlement and compliance are separate questions. Even where a condition is a legitimate 9.5.4 Variation, the redesigned solution — deeper footings, piling, ground improvement, a retaining change — still has to satisfy the Building Code (B1 structure, B2 durability, E2 external moisture where relevant) and may need a consent amendment. Pricing the Variation and clearing the compliance pathway are two workstreams; treating them as one is how a funded Variation still ends in a build-stop.

Where a second opinion pays for itself

Unforeseen-condition claims are decided on documents: the tender information, the notice trail, the cost records and the clause wording as amended. A neutral, written read against clause 9.5.4, the 9.3 valuation hierarchy and your actual Special Conditions tells you whether the claim is strong, where the experienced-contractor argument is exposed, and whether a 5.21 notice gap is about to cost you. Trueworks provides that as an analyst — not as your lawyer or your engineer of record — so the assessment is defensible on its own terms.

For the broader picture, see our guide to an independent building variation review. For the contract-type angle, see what a fixed-price contract does and doesn't lock in on variations, and for the time side of a claim, extension of time and time bars under NZS 3910.

Sitting on a variation, claim or contract clause you are not sure about? Send it with the contract — a written, code-cited answer back within 24 hours. Send us the drawings and the quote, tender or variation. You get a written, code-cited check back within 24 hours. No charge for your first check. No card, no obligation. NDA available. → Get the free check at trueworks.co.nz/contact — or email hello@trueworks.co.nz

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