Understand what the reports do, when they are needed and how they influence foundation costs.
Quick Facts – At a Glance
Small buildings still need suitable foundations: The footing design must respond to the actual ground and building loads.
A soil report is not pass or fail: Different results call for different foundation designs.
Test the proposed building area: An old report for the main house may not cover the new footprint.
Engineering and approval are separate: Structural design does not replace certification or construction inspections.
Price known conditions early: Ask what the quotation includes and which site costs remain allowances.
Indicative guidance only: the requirements and costs must be confirmed for the actual property and design.
Why the soil report matters
A granny flat may be small, but its footings still have to respond to the soil underneath it. The soil test and structural engineering are what turn a concept floorplan into a building that can be properly supported on the actual site.
The test is not simply a pass or fail. A stable sand or rock site behaves differently from reactive clay, uncontrolled fill or a site affected by unusual moisture conditions. The aim is to choose and detail foundations for those conditions.
What does a soil investigation do
A residential soil investigation assesses the ground at the proposed footing area and provides information used to classify the site. The National Construction Code’s Housing Provisions require foundation sites for the relevant footing and slab pathway to be classified in accordance with Australian Standard AS 2870.
The engineer then uses the classification, investigation details and building loads to select or design the foundations. Soil information does not replace the need to check levels, nearby structures, underground infrastructure or slope stability where relevant.
Understanding the result in plain English
Class A generally covers many sand and rock sites with little expected moisture-related movement. Class S indicates slight reactivity, while Class M indicates moderate movement potential.
Classes H and E describe higher and extreme reactivity. Class P identifies problem conditions such as some soft soils, abnormal moisture conditions, erosion or other issues that require particular assessment. The report may use more detailed subclasses.
A higher or problem classification does not automatically stop the project. It means the design must respond to the identified conditions. Do not select a footing system or compare prices from the class letter alone.
What engineering is commonly needed
Structural engineering commonly includes the footing or slab design and the bracing and structural details required for the chosen building system. Bracing helps the structure resist sideways forces such as wind.
Additional design may be needed for raised floors, retaining walls, substantial excavation, nearby buildings or building over or near services. A separate geotechnical or flood specialist may be needed for issues beyond an ordinary footing design.
A survey measures site features and levels. A soil test investigates the supporting ground. A utility location investigates buried services. They answer different questions and may all be needed on a constrained property.
When should testing happen
The useful time is once the likely building location is known and before final foundation pricing relies on assumptions. A test in a different part of a large property may not describe the proposed granny-flat footprint.
An old report can be useful background, but the engineer must decide whether it is suitable. Changes to fill, drainage, trees, the building position or site conditions can affect that decision.
QBCC guidance says appropriate foundations data generally needs to be obtained before the relevant building contract, unless suitable existing data can reasonably be relied on or another applicable exception applies. Confirm the project’s requirements with the builder and certifier.
What to check in each council area
The council checks below sit alongside the common building requirements. A planning scheme is the council’s rulebook; its zones and precincts identify areas with different controls.
Brisbane
The basic structural requirements come from the building framework, not a different Brisbane soil-class system. Check local flood, landslide and infrastructure constraints as well. These can add design investigations beyond the ordinary soil report.
Moreton Bay
The same building standards apply. Check the zone, values-and-constraints provisions, landslide/flood mapping and any Unitywater assets. Council planning requirements or an asset-owner assessment can need extra documents; a soil report alone does not resolve them.
Ipswich
The same structural framework applies. Review City Plan 2025 hazard controls, particularly difficult topography, landslide and flood/overland flow where mapped. Water/sewer and council stormwater constraints need their own check before foundation details are finalised.
Logan
The same building standards apply, with site-specific planning checks for steep-slope, flood and other overlays. If the project needs slope-stability or infrastructure assessment, the engineer and certifier should identify that scope early rather than treating it as part of every basic soil test.
Redland
The same structural framework applies. Landform, landslide/flood constraints and council infrastructure can require additional assessment. Confirm the reports for the actual design; an ordinary site classification does not automatically settle slope stability or an asset-clearance issue.
What can increase footing costs
Deep or uncontrolled fill, reactive soil, difficult founding material, slopes, nearby trees and moisture issues, and infrastructure conflicts can affect foundation design and price.
Ask whether the final quotation reflects the available soil report and engineering. If there is a provisional sum - an allowance for work not yet fully defined - ask what it covers and how it will be adjusted. A soil class alone cannot give a reliable extra-cost figure.
Engineering does not replace inspections
The building still needs the required certification and construction-stage inspections. Arrange the relevant inspections before work is covered, and retain the approved plans, engineering and inspection records.
After completion, maintain drainage and follow the engineer’s site-maintenance advice. Avoiding large changes in soil moisture can be important for the long-term performance of foundations on reactive ground.
Request Your Free Site Assessment Report
Start with Hive Small Homes’ Free Site Assessment Report. This preliminary desktop review identifies potential setbacks, mapped underground infrastructure and easements, overlays and other feasibility issues, subject to verification. It helps identify what needs further investigation before detailed plans or specialist work.
The report is not a boundary survey, on-site utility location, soil investigation or formal approval. Necessary specialist investigations are separate. Hive coordinates feasibility, design, engineering and approvals; construction contracts are directly between homeowners and independent QBCC-licensed builders.
Click here to request your Free Site Assessment Report
Related Articles
Do I Need a Survey Before Building a Granny Flat?
Granny Flats on Sloping Blocks
Retaining Walls and Granny Flats
Benefits of a Site Visit Before Building a Granny Flat
Important Information
This article is general information, not building, planning, energy assessment, engineering or legal advice. Eligibility, floor area, approval pathways, specifications and pricing must be confirmed for each project by the relevant professionals. Published October 2026.
Official Sources and Further Reading
NCC Housing Provisions site classification
QBCC soil movement and footing design
