Mt Eden villa basement-cut case study — the 25-series MBRW retaining argument valued at $22k
Updated: Jul 9
A Mt Eden villa basement-cut had a blockwork quote priced against 20-series block per the foundation plan note. Every individual retaining wall detail called up 25-series. NZS 4229 and NZS 4297 between them resolve which controls. The 25-series argument was worth $20-25k of under-quoted scope — caught at tender.
By Steve Parker · Trueworks · NZ construction estimation · 5 min
The foundation plan said 20-series. The individual wall details said 25-series. Both were stamped. A blocklayer pricing from the foundation plan alone underquoted by about 25% on cost per square metre of face and about 20% on grout. The argument was won by the wall details, but only if someone read both and noticed the conflict before the quote was accepted.
By Steve Parker · Trueworks · NZ construction estimation · 5 min
What you'll learn in this case study
The 20-series vs 25-series MBRW conflict pattern on Mt Eden villa basement-cut work
The NZS 4229 and NZS 4297 clauses that resolve which specification controls
The dollar effect of catching the spec conflict at tender versus catching it after the first courses are laid
Quick answer: A Mt Eden villa basement-cut had a foundation plan note specifying "all 20-series block unless noted otherwise." The individual masonry retaining wall detail sheets called up 25-series for the basement walls. NZS 4229 governs masonry construction and NZS 4297 governs reinforced concrete masonry retaining walls — together they require the controlling spec to be the wall detail (the specific) over the foundation plan note (the general). A blockwork quote priced against the foundation plan alone underquoted by about 25% on cost per square metre and 20% on grout. Caught at tender it saved $20-25k of inevitable variation.
The build
A character-villa renovation in Mt Eden involving a basement-cut under the existing ground floor to create a new garage and utility space. Build value in the $1.8-2.5M range. Basement retaining was about 180-220 m² of masonry-faced reinforced retaining wall (MBRW). The head contractor sent the blockwork quote across at tender stage as part of a multi-trade quote-check.
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What we found in the quote
The blockwork quote was priced against the foundation plan. The headline note on the foundation plan stated: "All concrete masonry units to be 20-series unless noted otherwise. Reinforcement to engineer's schedule. Grout to NZS 4229 Class A." The quote followed that lead — 20-series block, standard reinforcement, Class A grout. Pricing was within the band we'd expect for 20-series MBRW at the quoted face area.
The problem appeared when we read the wall detail sheets. The engineer's individual wall detail drawings for the basement retaining walls called up 25-series block — specifically, "200×190×190 25-series CMU with N16 vertical reinforcement at 400 mm centres, fully grouted, NZS 4229 Class A grout." The 25-series spec was on every wall detail sheet for the basement perimeter. The 20-series spec was only on the foundation plan headline note.
This is the canonical "general vs specific" conflict in construction documents. The foundation plan was the overview drawing; the wall details were the detail drawings. Standard contract documentation hierarchy resolves this conflict in favour of the more specific drawing, but only if the blocklayer reads both and only if the head contractor doesn't accept the cheaper quote on the basis of the headline note.
The cost difference between 20-series and 25-series MBRW for a basement-retaining application is substantial. The 25-series block is approximately 25% more expensive per unit at supply. Each 25-series block holds approximately 20% more grout than a 20-series. The structural performance is materially different — 25-series is the appropriate spec for retaining walls supporting more than about 1.5 m of soil, which the basement walls clearly were.
On a 180-220 m² face area, the cost gap was about $20-25k of underquoted scope. The blocklayer who priced from the foundation plan would arrive on site, read the wall details, and either: (a) issue a variation for the corrected scope, or (b) build to the cheaper 20-series spec and trigger a structural rejection at inspection.
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How the code resolves it
NZS 4229 (Concrete masonry buildings not requiring specific engineering design) provides the framework for masonry construction within defined envelopes. The standard sets out the requirements for block types, mortar, grout, reinforcement, and connections. It explicitly allows specific engineering design (SED) to override the standard's default requirements where the engineer takes responsibility for the design.
NZS 4297 (Engineering design of earth buildings) — and for our purposes the more relevant NZS 3101 (Concrete structures) which governs reinforced concrete masonry as a structural element — sets the engineering design requirements for retaining walls that fall outside the NZS 4229 envelope or that are specifically engineered. A basement-cut retaining wall holding back more than 1.5 m of soil with no buttressing typically falls outside NZS 4229's deemed-to-comply scope and requires SED.
For SED retaining walls the engineer's specification on the wall detail drawing controls. The foundation plan headline note is a general statement that applies where no more specific instruction exists. Where the wall details call up a different specification, the wall details govern.
Standard NZS 3910 documentation hierarchy under §2 (Definitions and interpretations) reinforces this: where a conflict exists between contract documents, the more specific document controls. A wall detail drawing showing a specific block size, reinforcement schedule, and grout fill is more specific than a foundation plan headline note.
The reconciliation at tender stage was straightforward: re-issue the blockwork quote against the wall detail specification (25-series MBRW), accept the $20-25k cost increase, and lock the documentation so that the conflict couldn't surface as a variation later.
What it would have cost if caught later
| Stage caught | Cost range | Why | |---|---|---| | At tender (paper review) | $500-1,500 (re-quote fee) | Wall detail specification confirmed as controlling; blocklayer re-quotes against 25-series; one annotation added to the foundation plan to remove the conflict | | At pre-start (post-award) | $3,000-6,000 | Variation order to upgrade to 25-series; blocklayer's supply schedule adjusted; possible delay of 1-2 weeks awaiting stock availability of 25-series | | At first course (post-mobilisation) | $8,000-15,000 | First courses stripped and re-laid in 25-series; blocklayer's labour partly written off; possible damage to foundation reinforcement; 2-3 weeks of programme delay | | At inspection (mid-wall) | $20,000-35,000 | Wall rejected at engineering inspection; full wall stripped and re-laid; foundation potentially re-poured if reinforcement was set for 20-series; 4-6 weeks of programme delay; potential damages claim from waterproofer and following trades | | Post-completion (under structural review) | $40,000-80,000 | Wall identified as under-spec'd against engineer's design; possible partial demolition or external buttressing; structural re-engineering of as-built; consent re-issue at cost |
The Mt Eden case caught the conflict at tender. The variation that wasn't booked was $20-25k. The fix was a clarification email and a re-quoted blockwork scope.
The clarification we recommended
The clarification packet to the head contractor named the 20-series vs 25-series conflict, cited NZS 4229 (for the masonry envelope) and NZS 3101 (for the reinforced concrete masonry retaining design) and the standard NZS 3910 §2 documentation hierarchy, and recommended three actions: (1) confirm in writing from the engineer to the contract that 25-series controls on the basement retaining walls (a one-line confirmation); (2) re-issue the foundation plan with the headline note amended to "20-series block unless noted otherwise on wall details" — making the hierarchy explicit on the drawing; (3) re-quote the blockwork scope against the corrected specification.
The engineer confirmed within two working days. The foundation plan was annotated. The blocklayer re-quoted at about $21k above the original quote. The build proceeded with one specification controlling and no variation surfaced post-award.
The same blockwork contractor later commented that he'd quoted this conflict pattern three times in the previous year on Mt Eden basement-cut work. Two of the three had been accepted at the headline-note rate and the variation surfaced after the first course was laid. The cost of catching it at tender was zero; the cost of catching it after first course was 5-8 times the supply-cost difference.
What other Mt Eden villa basement-cut builds should check
The foundation plan headline note and the individual wall detail specs must be cross-checked at tender — the wall details are the controlling drawings under NZS 3910 §2 documentation hierarchy
20-series block is generally adequate for above-ground masonry up to 2 storeys; 25-series is typically required for retaining walls holding more than 1.5 m of soil
The cost difference between 20-series and 25-series MBRW is about 20-25% on supply and labour combined — not a trivial premium and not one to be absorbed silently as a variation
NZS 4229 covers deemed-to-comply masonry construction; reinforced concrete masonry retaining walls outside that envelope require SED and the engineer's specific wall detail controls
Any villa basement-cut in Mt Eden is structurally non-trivial because the existing villa is bearing on the retained ground above the cut — the retaining specification has to support the existing structure, not just retain the soil
FAQ — blockwork on Mt Eden villa basement-cut work
Q1: Why is 25-series block specified for basement retaining instead of 20-series? 25-series block (a 200 mm-wide block with the wider core) accommodates larger reinforcing bars and more grout per square metre, providing the bending capacity needed to retain soil pressures and surcharge from existing structures above. 20-series block is adequate for above-ground masonry and lightly-loaded retaining (typically up to 1.2-1.5 m) but is generally insufficient for villa basement-cut retaining where the existing structure surcharges the wall.
Q2: Can a 20-series wall with enhanced reinforcement match a 25-series wall structurally? Sometimes — under SED a 20-series wall with closer-spaced reinforcement, larger bars, or buttressing can achieve equivalent capacity to a 25-series wall. The choice depends on cost, available space, and the engineer's preference. The 25-series spec is typically the cheaper option overall because it reduces reinforcement quantity and labour even though the block supply is more expensive.
Q3: Which document controls when the foundation plan and the wall details disagree? Under standard NZS 3910 §2 documentation hierarchy, the more specific document controls — the wall details are more specific than the foundation plan headline note. In practice the engineer to the contract should issue a written confirmation if the conflict surfaces, but the default contract position favours the wall details.
Q4: How long does 25-series block take to source compared to 20-series? Both are readily available from Auckland masonry suppliers. 20-series is the higher-volume product so stock is typically available next-day; 25-series may have a 5-10 working day lead time on larger quantities. The lead time should be confirmed at the time of order, not assumed.
Q5: What's the consequence if the wall is built to 20-series and not detected until structural inspection? The wall is likely to be rejected and require either rebuild in 25-series, addition of structural over-blocking (an additional skin of block on the loaded face), or external buttressing. The rebuild is the cleanest and often the cheapest of the three options. The cost is typically 3-5 times what the correct 25-series wall would have cost at the original build.
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