Because of that, some clients assume the safest approach is to complete the engineering and architectural design first, then ask contractors to price a resolved set of drawings.
There is logic in that approach. It gives the design team time to investigate the problem and define a solution before the contractor commits to a price.
The weakness is that the most important commercial decisions may be made before the people responsible for procurement and construction have tested them.
For a technically complex project, design and build can provide more protection, not less. The engineering challenge needs to be solved by a team that can assess design performance, construction cost, availability, sequencing and programme at the same time.
On a sloping or constrained site, the civil design affects the finished floor level. That affects retaining, foundations, access, drainage and the building structure. The structural solution can then affect fire design, internal planning, construction sequencing and the amount of usable space.
Each consultant may produce a sound answer within their own discipline. The overall project can still become inefficient if one solution pushes cost or risk into another area.
A civil engineer may reduce earthworks but create a more expensive foundation system. A structural engineer may reduce steel tonnage but introduce details that take longer to fabricate or erect. A lower-cost pile type may depend on specialist availability that does not suit the programme.
These trade-offs need to be assessed as one commercial decision.
Technical design often involves several workable options.
Poor ground might be addressed through piling, ground improvement, preloading, a gravel raft or a different building platform. A suspended floor may compete with backfilling and a ground-bearing slab. Retaining could be achieved through several structural solutions.
The easiest engineering answer is not automatically the best project answer.
Cost information from a previous job may no longer be reliable. Subcontractor capacity changes. Material availability changes. Freight, lead times and site access can make a solution that once looked economical far less attractive.
A design and build team should validate the options with current supplier and subcontractor input before the project becomes committed to one path.
A recent wholesale facility in the Auckland region shows how quickly a relatively simple building can become technically complex.
The site was constrained and the project needed to move quickly to support the developer and proposed occupier. Early in the process, a transport requirement linked to parking and vehicle access meant the building platform had to be raised higher than first anticipated.
That single requirement affected the wider design.
The higher finished floor level changed the foundation and piling requirements. Existing retaining structures near the site could not be relied on as expected. The team had to compare pile types, pile sizes and depths, backfilling, precast elements and a suspended floor solution.
At the same time, the design still had to satisfy resource consent controls, setbacks, fire egress, site access and the operational brief.
No single consultant could optimise that problem alone. Civil, geotechnical, structural, architectural, fire, cost and construction advice had to be considered together.
Time became a major part of decision-making. Some options had a lower theoretical material cost but carried less certainty around subcontractor availability, pricing or programme. Other methods used products and trades that were readily available and could be priced with more confidence.
Concrete blockwork was selected in parts of the project because it offered flexibility and access to available contractors. Structural steel was used for a suspended parking area rather than a concrete beam and column solution because the team could obtain firmer pricing and move faster.
A design decision that saves money on one trade can still increase the total project cost if it adds weeks to the programme. Additional preliminaries, finance costs, delayed rent or a later operational start can quickly remove the saving.
For a developer or business owner, the useful comparison is the total cost and risk of each option, not the rate for one material.
The model only works when the right capability is involved early. You need a structural team that understands fabrication and construction cost, rather than designing in isolation. The quantity surveyor needs to obtain real market feedback from relevant subcontractors. The project manager must test how each option affects procurement, sequencing, and the wider programme.
The team also needs clear accountability for the combined result.
Before selecting a contractor for a complex project, ask:
A vague promise to “value engineer” the design later is not enough. The contractor should be able to explain the actual process, the people involved and the points where decisions will be made.
Technical complexity does not make design and build unsuitable but makes integration more important.
The projects with difficult ground, major structural demands, or tight site constraints carry the greatest risk at the interfaces between design, cost and construction. When those decisions are developed and tested by one coordinated team, the client has a better chance of reaching a buildable solution, a realistic programme and a price that can be relied on.