Why an existing home changes the investigation strategy
A knockdown-rebuild property is not the same as an untouched vacant lot. Existing slabs, footings, driveways, landscaping, services and past renovations can restrict access to the proposed new building footprint or mask fill and previous ground disturbance.
The structural designer may need an AS 2870 site classification for the proposed replacement dwelling, while larger excavations, basements, slopes or retaining walls may require broader geotechnical investigation.
Investigation before or after demolition
The suitable timing depends on site access, the information needed for design, the proposed building footprint and the project's demolition programme. Pre-demolition investigation may help early planning, but existing structures can limit accessible investigation locations.
After demolition, altered surface conditions, exposed fill or changed access may justify further observations or targeted investigation. The project team should identify what can be reliably assessed at each stage rather than assume one visit answers every later design question.
Issues that may warrant a broader scope
Unknown imported fill, previous basements, buried structures, substantial cut or fill, proposed retaining walls and revised drainage can affect footing design and excavation planning. Nearby boundaries and existing neighbouring structures may also influence safe construction sequencing.
These considerations arise across established Sydney suburbs as well as Western Sydney and South West Sydney renewal areas. Actual conditions depend on the individual property's history and proposed works.
Provide the site address, demolition status, proposed new dwelling plans, site survey, access photographs, prior geotechnical reports and the structural designer's request. Explain whether the existing house is occupied and whether access is possible within the proposed footprint.
This information allows Sydney Geotechnical to recommend a proportionate investigation rather than assuming a knockdown-rebuild always requires the same field methods or report.
Common questions
Knockdown-rebuild investigations: common questions
Should the soil investigation happen before demolition?+
It depends on access, the design programme and whether meaningful investigation locations are available. Some projects benefit from early information, while demolition may reveal conditions requiring follow-up.
Is an old house's site classification automatically suitable for the replacement house?+
No. The new footprint, proposed design, demolition, fill and changes to ground conditions can affect whether earlier information is suitable.
Need site classification or 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.
