Unforeseen Ground Conditions: Who Pays in NZ? (Auckland Guide)

Steve Parker
Jul 24
8 min read

Updated: 3 days ago

Unforeseen ground conditions who pays in NZ depends on how the risk was allocated at tender. Here is how rock, soft ground and extra piling costs are decided.

By Steve Parker · Trueworks · NZ construction estimation · 8 min

The cost of rock or soft ground is rarely decided when the digger hits it. It was decided months earlier, in the fine print of how the tender allocated ground risk.

What you'll learn

Who carries the cost of unforeseen ground conditions under the four common NZ contract setups
Why pile depth and pile count overruns are the most expensive thing to discover on a residential site
How a genuine ground-conditions change becomes a valid variation rather than a cost you simply absorb

Quick answer: Who pays for unforeseen ground conditions in NZ depends on how the risk was allocated at tender, not on who is surprised on the day. Under a true fixed price with no exclusions, the contractor generally carries the extra cost of rock, soft ground or deeper piling. If the contractor qualified or "tagged out" ground risk in their tender, the client carries the excess. If foundations were priced as a provisional sum, that figure is only an estimate and is reconciled to the actual cost. The deciding document is your contract and the tender it was built on, so read those before you accept any ground-conditions claim.

Why ground conditions cause more cost surprises than anything else

Ground conditions are the single most common origin of delay and variation claims on New Zealand building work, and Auckland makes that worse. A site in Titirangi can be sloping clay over papa rock; a flat section in Mangere can be soft, high-water-table ground that needs far deeper piles than the plans assumed. Nobody can see what is under the surface until they dig, and most residential contracts are signed before anyone has dug.

This matters because of how NZ jobs are priced. A large share of fixed-price contracts are let on incomplete design, often only 60–70% resolved, and the foundation detail is frequently the least resolved part of all. "Fixed price" fixes the price for the defined scope. It does not fix the price for ground that turns out different from what was assumed, and it does not turn an estimate into a guarantee.

The most expensive discovery point is piling. Pile depth and count overruns surface only after piling starts, when the rig cannot reach refusal at the design depth, or hits rock far higher than expected. By then the programme is committed and the cost of driving deeper or redesigning the foundation is no longer theoretical. This is the moment a homeowner gets a phone call asking for another $8,000–20,000 and is given an hour to decide.

Send us the variation, progress claim, or contract clause you are unsure about and we will return an independent, plain-English read on whether it stacks up, within 24 hours. No charge for your first review. NDA available, NZ-hosted processing. → Get the free check at trueworks.co.nz/contact — or email hello@trueworks.co.nz

Not sure this is fair?

Who actually pays: it comes down to risk allocation at tender

There is no single rule that says the builder pays or the owner pays. The answer is set by how ground risk was allocated when the price was agreed. There are four common setups in NZ residential and small-commercial work.

1. Fixed price, no exclusions. The contractor priced the work and made no qualification about ground conditions. They have, in effect, taken the ground risk. If they hit rock or soft ground, that is generally their cost to bear, because they offered a complete price without protecting themselves. Most competent contractors know this, which is why few leave ground risk open.

2. Fixed price with the ground "tagged out". Competent contractors qualify ground risk at tender, writing something like "price excludes rock excavation", "piling priced to assumed depth of X metres", or "ground assumed to be good bearing per the geotechnical report". The contractor is saying: I am not carrying the risk of conditions worse than this assumption. If the ground turns out worse, the client carries the excess. This is the most common cause of a genuine surprise, because owners rarely read the tender qualifications closely.

3. Provisional sum for foundations or ground works. A provisional sum is a placeholder estimate for work that could not be fully defined at signing. It is not a fixed amount. The actual cost is reconciled against it: if the real cost is higher, the client pays the difference; if lower, the client gets the saving. Prime-cost (PC) sums work the same way for supplied items. So if foundations sat in a provisional sum and the ground needed more piling, the extra is owed because the figure was always an estimate, not a cap.

4. Measure-and-value. Some earthworks are priced per unit, for example per cubic metre excavated or per metre of pile. You pay for the quantity actually done at the agreed rate. More rock, soft-ground removal or pile metres means a higher final figure, and that is the agreed mechanism rather than a dispute.

An independent Trueworks review checks a variation, progress claim, or building contract against the relevant NZ standards and the Construction Contracts Act, so you know what is fair before you commit. Most homeowners spend far less on the review than the first disputed item would cost. See how it works at trueworks.co.nz →

A second opinion before you sign or pay

How a real ground change becomes a valid variation

A claim for additional payment is not automatically valid just because the ground was hard. For it to be a legitimate variation rather than something the contractor should have allowed for, there has to be a genuine change from what the contract scope assumed, valued and instructed properly.

On contracts using the standard construct-only conditions, this runs through NZS 3910 §14, which sets a valuation hierarchy: an agreed price first, then applicable contract rates, then reasonable rates, then daywork as a last resort. The certifier values the change on that basis rather than waving through whatever number lands in the email. On the more principal-controlled NZS 3915, where no independent Engineer is appointed, the principal's representative certifies directly and is under no obligation to act independently, so an owner needs to scrutinise ground claims harder, not less.

Two things sharpen a claim or kill it. First, evidence of the original assumption: the geotechnical report, the PS1 design certificate, the assumed bearing and pile depths. If the ground genuinely differs from the documented assumption, the change is real. Second, the margin. A main contractor commonly adds 10–15% overhead and margin to a variation, a subcontractor's price already carries its own roughly 10% margin, and compounded up the chain an owner often pays 20–30% on top of the base cost of the extra work, because there is no competitive tension once the rig is on site. A ground-conditions variation deserves the same line-by-line scrutiny as any other, and arguably more.

Contract setup, who carries ground risk, and the typical outcome

| Contract setup at tender | Who carries the ground risk | Typical outcome when rock or soft ground is found | |---|---|---| | Fixed price, no exclusions | Contractor | Contractor generally absorbs the extra excavation or piling; no variation owed | | Fixed price with ground "tagged out" / qualified | Client (above the stated assumption) | Client pays the excess as a variation; check the qualification was clear and the change is real | | Provisional sum for foundations / ground | Client (reconciled to actual) | Final cost trues up against the estimate; client pays the difference, or banks the saving | | Measure-and-value (per m³ or per pile metre) | Client (by measured quantity) | Final figure follows actual quantities at agreed rates; verify the measure |

What to check before you pay a ground-conditions claim

Read the tender, not just the contract sum. Find any wording that excludes or qualifies rock, piling depth, bearing capacity or ground conditions. That single line usually decides who pays.
Identify how foundations were priced. Fixed, provisional sum, PC sum or measured. A provisional sum was always an estimate, so an overrun is not a broken promise, but it should still be reconciled with evidence.
Get the geotechnical basis. Ask for the geotechnical report and the PS1. Compare what was assumed against what was actually found. A genuine difference supports a claim; no difference undermines one.
Insist on a properly valued, instructed variation. It should follow the §14 hierarchy with rates and quantities you can check, not a round-number lump sum.
Test the margin. Strip the compounded overhead and margin back to a defensible level. Twenty to thirty percent on top of base cost is common but not automatically reasonable.
Prevent the next one early. Before signing, a thorough geotechnical investigation and a pre-tender risk register push ground risk into the open while there is still competitive tension, rather than after the rig arrives.

For the detail on how a ground change is priced once it is accepted, see how NZS 3910 §14 variations are valued.

This is general information, not legal advice — get advice on your specific contract.

FAQ — unforeseen ground conditions in NZ

Q1: Who pays for unforeseen ground conditions in New Zealand? It depends on how the ground risk was allocated at tender. Under a true fixed price with no exclusions, the contractor generally carries it. If the contractor qualified or tagged out ground risk, or if foundations were a provisional sum, the client carries the excess. The contract and its tender decide it, not who is surprised on the day.

Q2: Is extra rock excavation a variation or part of a fixed price? If the fixed price had no ground qualification, extra rock excavation is usually the contractor's cost. If the tender excluded or limited rock, or priced excavation by measured quantity, the extra is generally chargeable to the client. Check the tender wording before accepting the charge.

Q3: Who pays when piles have to go deeper than planned in Auckland? If pile depth was tagged out or assumed in the tender, the additional depth is typically the owner's cost, often $8,000–20,000 or more on a residential job. If piling was part of an unqualified fixed price, the contractor usually carries the overrun.

Q4: What is a provisional sum for foundations and can it go up? A provisional sum is an estimate for work that could not be fully defined at signing. Yes, it can go up. The actual cost is reconciled against the estimate, so the client pays any genuine difference, or receives the saving if it comes in lower.

Q5: What does it mean when a builder "tags out" ground conditions? Tagging out means the contractor has qualified their tender to exclude or limit ground risk, for example pricing piling only to an assumed depth or excluding rock. It signals they are not carrying the cost of worse-than-assumed conditions, so the client should expect to fund any difference.

Get an independent review before you pay or sign

Send the variation, progress claim, or contract clause and get a plain-English, code-cited read on whether it stacks up, within 24 hours.

No charge for your first review. No commitment. NDA available. Files NZ-hosted, deleted after 30 days unless you ask us to retain them.

→ Get the free check at trueworks.co.nz/contact — or email hello@trueworks.co.nz

About Trueworks

Trueworks is built by Steve Parker — 20 years on the analytical side of NZ construction. Variation reviews, contract advisory, programme review, and AI-augmented document workflows. Trueworks gives homeowners and builders the same defensible, independent analysis a developer's quantity surveyor would run — in plain English, at a price and pace that makes sense for a single project.

I answer every email personally during pilot phase. If you've got a quote you want a second opinion on, the easiest way to find out if Trueworks is useful is to send it.

hello@trueworks.co.nz · trueworks.co.nz/contact

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