An extension introduces new structural loads and may involve footings located beyond the footprint investigated for the original house. Soil, fill, slope, drainage and vegetation can vary across the property, making older or general site information insufficient for the new design question.
The structural engineer may request an AS 2870 site classification, targeted investigation of new footing locations or additional information about existing foundations, depending on the proposal.
A rear or side addition can involve narrow access, previously landscaped areas, stormwater lines, mature trees, imported fill or ground disturbed during earlier renovations. These factors can influence investigation locations and the interpretation of reactive movement or founding conditions.
The assessment should address how proposed new footings relate to the existing structure. Structural connection and footing design remain the responsibility of the structural engineer.
Second-storey additions and existing foundations
A second-storey addition can change loads on an existing building. Whether additional geotechnical investigation is needed depends on the structural engineer's assessment, available existing footing information, observed movement and the project drawings.
The appropriate scope may include review of prior reports, targeted assessment or construction-stage inspection. A new site classification alone may not establish the capacity or condition of every existing footing.
What to provide with your enquiry
Send architectural and structural plans, the original house plans if available, any prior geotechnical report, photographs of the proposed addition, access details and the structural engineer or certifier's exact request. Mention existing cracking, retaining walls, fill or planned excavation without assuming these observations establish their cause.
Sydney Geotechnical can identify whether the project is suitable for a site classification, a broader investigation or a targeted construction-stage inspection.
Common questions
Extensions and additions soil testing: common questions
Does a second-storey addition always need a new soil report?+
Not always. The need depends on the structural design, available existing information, site conditions and the specific request from the engineer, builder, certifier or approval authority.
Can an older report be reused for an extension?+
An older report may be useful, but its suitability depends on whether the investigated area, site conditions and proposed works still match the current development.
Need site classification or ground assessment 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.
