The five-figure item the drawings did not show: pricing the code, not just the drawings

Steve Parker
Jul 4
8 min read

Updated: Jul 9

A civils contractor asked us to build a bill of quantities and indicative price for a 9-dwelling infill development, straight off the issued civil drawings. The take-off was the easy part. The code-of-practice review we ran beside it found a code-required stormwater attenuation system that appeared nowhere in the drawn design — a five-figure item that would otherwise have surfaced after contract.

By Steve Parker · Trueworks · NZ construction estimation · 7 min

What you'll learn in this case study

Why a civil bill of quantities priced only off the drawings can be complete, accurate, and still wrong by $35–45k
How a code-of-practice review beside the take-off catches code-required-but-not-drawn items
Why the vesting question ("do these pipes become public assets?") has to be asked before pricing, not after

Quick answer: A civils contractor engaged us to build a bill of quantities and an indicative price for the in-site civil works of a 9-dwelling infill development in South Auckland, off the issued civil drawing set: cut and fill over about 1,200 m² with a modest net import, stormwater and wastewater reticulation, and a shared concrete driveway. The take-off was straightforward. The risk sat in what the drawings did not show. A review against the regional stormwater code of practice, the council's water-sensitive-design guidance, and the district plan's stormwater provisions found that on-site attenuation and treatment is code-required for an intensification of this scale in that catchment — yet no attenuation device appeared anywhere in the drawn design. We added a provisional allowance of about $35–45k, flagged that the drawn ~675 mm chambers become ~1050 mm manholes if the line vests as a public asset, and listed the network utility's off-site works as written exclusions. The indicative total landed around $300–350k excluding GST, with margin and contingency as separate lines.

The drawing set

A civils contractor asked us to price the in-site civil works for a 9-dwelling infill development in South Auckland. The engagement was a bill of quantities and an indicative price, built off the issued civil drawing set.

The drawn scope was familiar territory. Earthworks: cut and fill over about 1,200 m², with a modest net import to make the levels work. Stormwater and wastewater reticulation: mainlines, individual dwelling connections, and a run of chambers. And a shared concrete driveway serving all nine dwellings.

Measuring that is bread-and-butter work — trench metres by depth band, pipe by diameter and class, chambers by count and depth, driveway by area and build-up. The take-off reconciled cleanly against the drawings.

Which is exactly the trap. A bill of quantities built off the drawings can be complete, internally consistent — and still miss the largest single risk on the job, because the largest single risk was never drawn.

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What we found

Alongside the take-off we ran a code-of-practice review: the regional stormwater code of practice, the council's water-sensitive-design guidance (the GD01-type document), and the district plan's stormwater management provisions, read against what the drawings actually showed. Two gaps came out of it.

Gap 1: no attenuation, anywhere. Taking one suburban lot to nine dwellings pushes the impervious area well past the thresholds at which the district plan and the stormwater code require on-site stormwater management — attenuation to keep post-development discharge at or near pre-development rates (the hydraulic-neutrality principle), plus quality treatment for the new hard surfaces. That means a device: a detention tank or basin with a controlled outlet, plus treatment for the driveway runoff.

No such device appeared anywhere in the drawn design — nothing between the new hard surfaces and the public network but pipe.

Whether the omission was a staging decision or an oversight was not ours to referee. What mattered was that a consentable version of this development would almost certainly include attenuation and treatment, and the client deserved to see that cost now, not after contract. We added a provisional allowance of about $35–45k to the indicative price, labelled plainly as code-required works not shown on the issued drawings.

Gap 2: chambers that grow if the pipes go public. The drawings showed ~675 mm chambers on the drainage runs. Fine for a private system. But if the line is ever vested as a public asset, the code of practice requires full ~1050 mm manholes on maintainable public drains — a per-structure cost step across the affected run. The vesting decision had not been settled, so we priced the drawn chambers, stated the assumption, and flagged the uplift a vesting decision would trigger.

The exclusions. The network utility's off-site upgrade works — anything beyond the site boundary required to give the development capacity — went into the price as explicit written exclusions — listed, in writing, as work the price does not carry.

The indicative total landed around $300–350k excluding GST, including the provisional allowance, with margin and contingency shown as separate lines.

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Pricing the code: how the review runs beside the take-off

The method is a second pass through a different set of documents, asking a different question. The take-off asks "what is drawn, and how much of it?" The code review asks "what does an approvable version of this development have to contain, and is all of it drawn?"

For an infill civil package in this region, three documents do most of the work.

The regional stormwater code of practice sets the engineering requirements for anything that might become a public asset — pipe materials, minimum grades, manhole sizing, connection standards. This is where the 675-versus-1050 chamber question lives.

The water-sensitive-design guidance (GD01-type) sets the expectations for treatment and attenuation devices — adequate treatment for driveway runoff, detention sizing, maintenance access.

The district plan's stormwater management provisions set the trigger: at what scale of intensification, and in which catchments, on-site attenuation and hydraulic neutrality become requirements rather than preferences.

Read together against the drawings, they generate a short list of code-required-but-not-drawn items. Each item then gets one of three treatments in the price: a provisional allowance with a stated basis, a stated assumption with a flagged uplift, or a written exclusion. What no item gets is silence.

What it costs when it's caught late

| Stage caught | Cost range | Why | |---|---|---| | At pricing, via code review | About $500–1,500 of review time | The gap becomes a labelled provisional allowance the client can see and plan around | | At consent processing | About $5,000–15,000 | Redesign, resubmission, and programme slip while the attenuation design catches up | | After contract award, before construction | About $35–45k unfunded | The allowance that was never priced becomes a variation argument with no drawn baseline | | During construction, network connection refused | About $50,000–100,000 | Retrofitting a detention system and treatment train into a part-built site, plus standing time | | After vesting decision, chambers rejected | About $4,000–8,000 per structure | Rebuilding ~675 mm chambers as ~1050 mm manholes on a live drainage run |

The five-point checklist for pricing infill civil works

List the governing documents before scaling a single drawing. Regional stormwater code of practice, water-sensitive-design guidance, district plan stormwater provisions. The list tells you what the second pass has to check.
Ask what the code requires that the drawings do not show. Attenuation, treatment, overflow paths, maintenance access. If intensification thresholds are crossed and no device is drawn, that is a pricing item, not someone else's future surprise.
Provisional-sum the code-required-but-not-drawn items — and say so. A labelled allowance with a stated basis protects the client and you. An unpriced gap protects nobody.
Ask the vesting question early. Whether the drainage becomes a public asset changes chamber sizes, materials, standards, and inspection regimes. It is one email to the designer before pricing, or a per-structure rebuild after.
Put exclusions in writing. The network utility's off-site works, consent conditions not yet issued, anything beyond the boundary. Exclusions in writing beat assumptions in silence, every time.

FAQ — stormwater attenuation and civil pricing on infill developments

Q1: What is stormwater attenuation and when is it required? Attenuation is on-site detention that stores runoff during rain events and releases it slowly, so post-development discharge stays at or near pre-development rates. District plans and regional stormwater codes typically require it once an intensification crosses impervious-area thresholds in constrained catchments — nine dwellings on one former lot is comfortably past most of those triggers.

Q2: What does hydraulic neutrality mean in practice? That the development should not increase peak stormwater discharge to the receiving network compared with the pre-development site. In practice it means detention storage sized to the design storm plus a controlled outlet.

Q3: Why price a provisional allowance instead of just excluding undrawn work? Because an exclusion hides the cost and an allowance reveals it. A $35–45k item left as an exclusion makes the price look sharper while making the client's decision worse.

Q4: Who is responsible when a code-required item is missing from the design? Design responsibility sits with the designer, but the cost consequence lands on whoever noticed last. A contractor who prices the drawings in silence inherits an argument; a contractor who flags the gap in writing at pricing time has converted it into the designer's question and the developer's budget line.

Q5: Does the chamber-versus-manhole question really change the price that much? Per structure, the step from a ~675 mm chamber to a ~1050 mm manhole is real — more excavation, more concrete, different access requirements — and it repeats across every structure on the vested run. An unsettled vesting decision is a five-figure swing hiding behind a design note.

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About Trueworks

Trueworks is built by Steve Parker — 20 years on the analytical side of NZ construction. Variation reviews, contract advisory, programme review, and document-heavy estimation work. Trueworks is the productisation of that practice for NZ trades and builders: the same defensible analysis, at a price and pace a working contractor can actually use.

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