Why the proposed granny-flat location matters
A secondary dwelling is commonly located behind an existing home, where ground conditions, access, buried services, trees, landscaping and previous filling can differ from the front of the property. A geotechnical assessment should therefore relate to the actual proposed building footprint.
The structural or footing designer may request an AS 2870 site classification to inform the granny-flat slab or footing system. Whether additional geotechnical information is needed depends on the site, design and approval pathway.
Common access and investigation constraints
Narrow side passages, retaining steps, existing fences, paving, swimming pools and established gardens can limit equipment access. Compact or hand-based investigation methods may be appropriate in some situations, but the method must still answer the engineering question.
Provide photographs and approximate gate or pathway widths when enquiring. Existing utility information, sewer locations and any recent excavation should also be identified before fieldwork.
When more than a site classification may be needed
A sloping rear yard, proposed retaining wall, significant excavation, soft fill, nearby trees or unusual drainage may require additional ground-related consideration. If the granny flat includes a basement or is close to neighbouring structures, a broader investigation may be more appropriate than a basic classification alone.
Council and certifier requirements vary with the proposal and approval pathway. Do not assume every granny flat requires the same report; forward the exact request and plans.
Send the full address, architectural drawings, proposed granny-flat position, site survey, access photographs and any builder, certifier or council wording. If an existing site report is available, include it so the project team can assess whether it remains relevant to the proposed footprint.
Sydney Geotechnical's AS 2870 site classification service starts from $750 where the project is suitable for that scope. More complex investigation requirements are quoted separately.
Common questions
Granny-flat geotechnical reports: common questions
Can I use the soil report for the existing house?+
An existing report may provide background information, but its suitability depends on the location, age, site changes, proposed granny-flat footprint and the requirements of the footing designer or certifier.
Does every granny flat require a full geotechnical investigation?+
No. Some projects may need an AS 2870 site classification, while others require broader advice because of slope, retaining structures, excavation, fill or specific approval requests.
Need AS 2870 site classification 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.
