Why duplex projects need a project-specific ground scope
A duplex or dual-occupancy proposal may involve a larger footprint, two dwelling layouts, boundary constraints, shared or separate foundations, retaining structures and changed ground levels. The right investigation should reflect the actual design rather than assuming it is equivalent to a single small dwelling.
Sydney, Western Sydney and South West Sydney include properties with varied terrain, fill histories, residual soils and local rock conditions. A relevant assessment considers the particular development footprint, proposed earthworks and available access.
Site classification versus a broader duplex investigation
An AS 2870 site classification can inform residential footing and slab design where applicable. A broader investigation may be needed if the project also involves significant excavation, slope, retaining walls, basement areas, variable fill or specific questions from the design team.
Investigation locations and depths should be selected for the proposed footprints and engineering questions. A result from one corner of the property or a neighbouring site does not automatically represent both dwellings or every proposed foundation location.
- Proposed footprints, finished levels and shared or separate footing systems.
- Variation in soil, fill, weathered material or rock across the site.
- Boundary excavation, retaining walls and neighbouring structures.
- Slope, groundwater, drainage and proposed cut-and-fill works.
- Structural designer, certifier or council information requirements.
DA, CDC and design-stage requirements
A duplex may proceed through different approval pathways depending on its location, planning controls, proposed design and eligibility. Geotechnical requirements are not determined by the words dual occupancy alone; they depend on the site, proposal and relevant approval or design professionals.
Provide any written council or certifier request rather than assuming every duplex needs a prescribed report. Where the structural engineer identifies ground-related design issues, the investigation can be scoped to address those questions directly.
Provide the property address, current architectural drawings, survey, intended floor levels, proposed retaining or basement details, site-access photographs and any written approval request. Explain whether an existing house will be demolished and whether previous reports are available.
Site classification starts from $750 when that is the appropriate service. Broader geotechnical investigations start from $2,200, with the final scope and fee confirmed against the actual duplex proposal.
Common questions
Duplex and dual-occupancy reports: common questions
Does every duplex development need a geotechnical report?+
No. The need and scope depend on the proposed design, site conditions, structural requirements, approval pathway and any project-specific request from the relevant professional or authority.
Can a single site classification cover both proposed dwellings?+
It depends on the property, the proposed footprints and how ground conditions vary across the development area. The investigation scope should reflect the actual design and site.
What if the duplex includes a basement or retaining wall?+
Basements, retaining structures, significant excavation or slopes can introduce additional questions beyond residential site classification and may require a broader geotechnical investigation.
Need duplex geotechnical investigation in Sydney?
Get a scope matched to the project
Send the site address, proposed works, current plans and any request from your builder, engineer, council or certifier. Sydney Geotechnical will review the information and confirm an appropriate scope and fee.
This guide provides general information only and is not a substitute for investigation, engineering advice, footing design or approval advice for a particular property. Project requirements and ground conditions must be assessed site by site.
