Newmarket mixed-use conversion case study — the G6/AS1 acoustic separation upgrade negotiated mid-build

Steve Parker
Jun 29
8 min read

Updated: Jul 9

A Newmarket mixed-use commercial-to-residential conversion's inter-tenancy wall did not meet G6/AS1 STC requirements once the use class was reviewed. Caught at the partition framing stage, the upgrade was negotiated mid-build before linings closed up.

By Steve Parker · Trueworks · NZ construction estimation · 5 min

Acoustic separation is one of the few details where the building code's performance number is the design number. Get G6/AS1 wrong on a mixed-use conversion and the inter-tenancy wall has to be opened up after PC. Negotiate the upgrade while the partitions are still open and the cost is a fraction of the rebuild.

By Steve Parker · Trueworks · NZ construction estimation · 5 min

What you'll learn in this case study

The acoustic separation ambiguity on a mixed-use conversion
The G6/AS1 STC and IIC resolution that closed it
The cost-of-catching-it-later table from framing to PC inspection

Quick answer: On a Newmarket mixed-use commercial-to-residential conversion, the inter-tenancy partition between a ground-floor retail unit and a first-floor residential apartment was framed to a generic 90 mm timber-stud detail that the head contractor's quote priced against. G6/AS1 requires STC 55 minimum at habitable-space inter-tenancy walls, with IIC 55 minimum at the floor between different occupancies. The framed partition would have delivered approximately STC 45-48 once lined — well below the §G6 performance threshold. Caught at framing stage the upgrade cost $4-7k. Caught at PC acoustic test it would have run $22-35k in strip-back, refit, and re-test.

The build

A Newmarket mixed-use conversion: a 1980s two-storey corner building, ground floor retail (existing café tenancy continuing), first floor converted from office space to two residential apartments. Total floor area 240 m² across the two levels. Head contract NZS 3910 lump sum in the $1.4-1.8M envelope. Standard RMBA head contractor.

The conversion design required new inter-tenancy partitions between the retail use below and the residential use above, plus new partitions between the two residential apartments on level 1. The architectural set called up "acoustic-rated partitions per G6/AS1" without specifying the assembly. The head contractor's quote read this as a standard 90 mm timber-stud partition with single layer of 13 mm plasterboard each side and R1.8 sound-rated batt insulation in the cavity.

The carpenter and plasterer scopes had just started on the level-1 partitions when the quote crossed our desk for a sanity check.

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What we found in the quote

The head contractor's wall-build-up specification §0610 named "90 mm timber stud at 400 cc, 1 x 13 mm gib each side, R1.8 sound-rated insulation. Acoustic-rated per G6/AS1." That assembly is a common acoustic partition for inter-room separation within a single tenancy — it typically delivers laboratory STC 38-42 and on-site about STC 35-40 once installation imperfections, services penetrations, and flanking transmission are factored in.

G6/AS1 sets the performance requirements for sound transmission, dictated by use-class adjacency. For habitable-space inter-tenancy walls between different occupancies the requirement is STC 55 minimum (laboratory rating) on the building element, plus an apparent in-situ requirement that translates roughly to STC 50 on completed construction. For floors between different occupancies the requirement adds IIC 55 minimum (impact-isolation) on top of the STC requirement.

The framed assembly was 10-15 STC points below the G6/AS1 inter-tenancy threshold. The floor-ceiling assembly between the retail below and the residential above had a parallel deficit on IIC: the existing concrete-on-steel-joist floor structure delivered around IIC 45 before any carpet or floor-finish improvement, against the IIC 55 requirement.

The architectural specification's reference to G6/AS1 was correct in intent but the head contractor's pricing assumed the wrong assembly. The cost-of-not-asking was a gap between what was specified (G6/AS1 compliant) and what was being framed (sub-standard).

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How the code resolves it

G6/AS1 Airborne and Impact Sound is the Acceptable Solution for compliance with the Building Code G6 Airborne and Impact Sound clause. The Acceptable Solution sets the STC and IIC performance numbers and provides reference assemblies that meet the numbers.

For inter-tenancy walls between residential and commercial use classes, G6/AS1 Table 1 sets STC 55 minimum. Reference assemblies in §3 of the Acceptable Solution include double-stud 70 mm + 70 mm with a 25 mm cavity, two layers of 13 mm plasterboard each side, and R3.6 sound-rated insulation — laboratory STC 60+. Single-stud equivalents with resilient channel and double plasterboard achieve STC 55 in laboratory.

For floor-ceiling assemblies between different occupancies, G6/AS1 §3 sets both STC 55 and IIC 55 minimum. Achieving IIC 55 on an existing concrete-on-steel-joist floor requires either a floating-floor system, an underlay-and-carpet finish meeting the IIC contribution, or a ceiling-side resilient-mount lining system.

The Auckland Unitary Plan E25 (noise) and the building consent conditions for mixed-use conversion both reference G6/AS1 as the compliance standard. A council inspector or an independent acoustic engineer can require an in-situ acoustic test at PC to evidence compliance — particularly common where the use class change from commercial to residential is recent.

Under NZS 3910 §6, the architect or QS as CA can issue an instruction under §10.3 to clarify the partition specification before the framing is closed up. The instruction is a §14 variation if the upgrade exceeds the original specification, or a clarification if the original specification (read against G6/AS1) was already binding.

What it would have cost if caught later

| Stage caught | Cost range (ex GST) | Why | |---|---|---| | Pre-framing (where we caught it on a later partition) | $4-7k | Partition specification upgraded before framing complete, double-stud or single-stud-with-resilient-channel detail substituted, R3.6 insulation, double plasterboard, programme effect 2-4 days | | Mid-framing (some partitions framed, others not) | $8-13k | Variation order to carpenter and plasterer, partial re-frame of completed partitions, programme effect 1 week | | Linings closed up | $15-22k | Strip-back of completed plasterboard, re-frame, re-line, paint, programme effect 2-3 weeks | | PC acoustic test failure | $22-35k | Independent acoustic test commissioned, fails G6/AS1, strip-back and rebuild of failing partitions and floor-ceiling, re-test, CCC delayed | | Post-PC complaint from new tenant | $30-50k+ | Tenant moves in, complains of sound transmission, owner-funded remediation, potential dispute with head contractor on responsibility, retention released or held in dispute |

The bottom row also carries a tenancy-management issue. A residential tenant in a sound-failing apartment can claim under the Residential Tenancies Act 1986 for breach of the landlord's obligation to provide premises in a reasonable state of repair.

The clarification we recommended

A three-line CA instruction under §10.3 to the head contractor before further partitions were framed. First, the inter-tenancy wall assembly between the level-1 apartments and the ground-floor retail upgraded to a G6/AS1 §3 reference assembly meeting STC 55 minimum — specifically, single-stud 90 mm with resilient channel one side, two layers of 13 mm plasterboard each side, R3.6 sound-rated batt. Second, the inter-apartment walls (level 1 between the two residential units) upgraded to a G6/AS1 STC 50 reference assembly. Third, the floor-ceiling assembly between the retail tenancy and the residential apartments upgraded with a resilient-mount ceiling lining and a floating-floor underlay on the level-1 side to deliver IIC 55 minimum.

The head contractor priced the upgrade at $4-7k in materials and labour with a 2-4 day programme effect. The CA instruction was issued as a clarification rather than a §14 variation (the architectural set had specified G6/AS1 compliance from the start; the head contractor had priced an assembly that did not meet it).

The CCC issued on schedule. No PC acoustic test was required because the assemblies on completion matched G6/AS1 reference details.

What other Newmarket mixed-use conversion builds should check

The G6/AS1 STC and IIC performance numbers tied to specific partition and floor-ceiling assemblies, not just referenced in the spec
The use-class adjacency at every partition — different occupancies trigger the STC 55 / IIC 55 threshold
The reference assemblies in G6/AS1 §3 named on the architectural details, not left to the head contractor to interpret
The PC acoustic test requirement set in the head contract — independent in-situ testing where the council inspector requires it
The services-penetration acoustic seals at every penetration through an acoustic partition (electrical boxes, plumbing, ductwork) — flanking-transmission paths defeat a good assembly

FAQ — G6/AS1 acoustic separation on Newmarket mixed-use conversions

Q1: Why doesn't a standard 90 mm timber stud partition meet G6/AS1 inter-tenancy? A single-stud, single-layer partition has direct mechanical coupling between the two sides of the wall. The studs conduct vibration. The single layer of plasterboard has limited mass per square metre. Together the assembly delivers around STC 38-42 — fine for inter-room within a tenancy, well below the STC 55 inter-tenancy threshold.

Q2: What's the cost difference between a standard partition and a G6/AS1 inter-tenancy partition? Material and labour combined, a G6/AS1 inter-tenancy assembly costs roughly 1.6-2.2x a standard partition per lineal metre. On a small conversion the absolute dollar difference is $30-50 per lineal metre over the standard partition.

Q3: Is an in-situ acoustic test always required, or only on inspector's request? Not always. The Acceptable Solution route under G6/AS1 §3 deems compliance through reference assemblies — no test is needed if the assembly built matches the reference. Tests are typically commissioned where the as-built assembly departs from a reference, or where the inspector has specific cause to test.

Q4: How does IIC differ from STC and why does it matter for the floor-ceiling assembly? STC measures airborne sound transmission (speech, music). IIC measures impact sound transmission (footsteps, dropped objects). Floor-ceiling assemblies need both. STC compliance does not guarantee IIC compliance — a heavy concrete slab is STC-strong but IIC-weak without a resilient layer or floor-finish improvement.

Q5: Who owns the acoustic compliance under NZS 3910 — the head contractor or the architect? The architect designs to the performance standard. The head contractor builds to the design and the specification. If the specification names G6/AS1 compliance but the head contractor prices a non-compliant assembly, the head contractor has misread the documents — that is not a variation, it is a quote error against the contract.

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